1

2
TABLE OF CONTENTS
CHAPTER PAGE#
ABSTRACT 2
LIST OF RIGURES 5
ACKNOWLEDGEMENTS 7
INTRODUCTION 8
GEOLOGIC SETTING
Lithostratigraphy 13
Moodys Branch Formation 15
Yazoo Formation
North Twistwood Creek Clay Member 17
Cocoa Sand Member 19
Pachuta Marl Member 24
Shubuta Clay Member 25
Red Bluff Formation 26
SEQUENCE STRATIGRAPHY 27
BIOSTRATIGRAPHY 31

3
CHAPTER PAGE#
METHODS
Grain Size Analysis 34
Well Log Analysis 37
RESULTS
Grain Size 41
Sand Distribution 44
DISCUSSION 58
CONCLUSIONS 67
REFERENCES 69

4
LIST OF FIGURES
FIGURE PAGE #
Figure 1 Type Location of the Cocoa Sand Member in Alabama 9
Figure 2 Study areas in Mississippi and Alabama 10
Figure 3 Lithostratigraphy and Biostratigraphy of the Yazoo Formation. 14
Figure 4 Location map of Test hole 18, #1 Young Core, #1 Ketler Core 16
Figure 5 Upper Part of the North Twistwood Creek Member in the #1 Young Core 18
Figure 6 Outcrops location map of the Cocoa Sand Member in Alabama 20
Figure 7 Outcrop of 6 miles of type location of the Cocoa Sand Member in Alabama 21
Figure 8 Photomicrograph of lithic intraclasts (LC) from the #1 Young core in thin section 23
Figure 9 TAGC sequence stratigraphy of the Upper Eocene 29
Figure 10 Sequence stratigraphy of the Upper Eocene and Lower Oligocene strata 30
Figure 11 P: B ration of the upper Yazoo Formation in the #1 Young Core 32
Figure 12 # 1 Young Core sample in Wayne County, Mississippi 35
Figure 13 The Wentworth grain-size scale for sediments 36
Figure 14 Spontaneous and Resistivity electric wireline Log of #1 Ketler Core 39
Figure 15 Base map of 70 wells in Wayne County, Mississippi 40
Figure 16 Grain size analysis of #1 Ketler Core samples 42
Figure 17 Grain size analysis of #1 Young core samples 42

5
FIGURE PAGE #
Figure 18 Electric wireline log of the #1 Young Core 43
Figure 19 Isopach map of the Cocoa Sand Member in Mississippi and Alabama 45
Figure 20 3D wireframe surface thickness map of the Cocoa Sand Member 47
Figure 21 Base map of the six cross sections of the Cocoa Sand Member 48
Figure 22 A-A’ (SW1) cross section of the Cocoa Sand Member 50
Figure 23 B-B’ (SW2) cross section of the Cocoa Sand Member 51
Figure 24 C-C’ (SW3) cross section of the Cocoa Sand Member 52
Figure 25 D-D’ (NE2) cross section of the Cocoa Sand Member 53
Figure 26 E-E’ (NE4) cross section of the Cocoa Sand Member 54
Figure 27 F-F’ (NE3) cross section of the Cocoa Sand Member 55
Figure 28 Regional paleogeographic reconstruction map 60
Figure 29 Distribution of the mid-shelf sand ridges in Korean Strait 62
Figure 30 Depositional model of the Cocoa Sand Member 66

6
ACKNOWLEDGMENTS
I would like to thank the Department of Geological Sciences at Ball State University for giving me the opportunity to pursue my master degree in United States. I would like to thank my family for their biggest support since I was in the United States. I greatly appreciate the help from Dr. Jeffry D. Grigsby on my thesis, guiding me to be a better graduate student. I am very glad to have him as my advisor, and to work with him over these two years. I thank Dr. Richard
H. Fluegeman for his rich knowledge on the Yazoo Formation, and his willingness to serve on my thesis committee. I also wish to thank Dr. Kirsten Nicholson for serving on my thesis committee. I would also like to thank the support from the Mississippi Department of
Environmental Quality for allowing access to their well log database. I also wish to thank IHS for their grant to the Department of Geological Sciences that allowed me access to IHS Kingdom
Software. This software was essential to my analysis of the well log data used in this study. I especially give my thanks to Mike Kutis for helping me solve the problems within all the software. I would like to thank my best friend He Du in the United States for her sweet heart and support to help me go through all the things. In the end, to all the giants whose shoulder I stand on in doing this research, thank you in all!

7
INTRODUCTION :
The Cocoa Sand Member, which was named after an abandoned post office in Alabama, is part of the Yazoo Formation of the Upper Eocene. The type location of the Cocoa Sand
Member is 3.5 miles east of Melvin on the road to Gilbertown, Choctaw County, Alabama
(Figure 1). The focus of this study is on the Cocoa Sand Member found in Clarke County and
Wayne County, Mississippi and Choctaw County and, Washington County, Alabama (Figure 2).
The Cocoa Sand Member has been described as an isolated sandstone unit deposited between the Pachuta Marl Member above and the North Twistwood Creek Clay Member below (Mancini et al.1987). The deposition of this sandstone unit within the predominantly mudstone Yazoo
Formation raises questions about deposition and the sequence stratigraphic setting of this sandstone member.
The Cocoa Sand Member is a yellowish to greenish, fine grained, well to moderately sorted, poorly cemented quartz arenite (Brissette 2004). Surface exposures of the Cocoa Sand are relatively poor, but have been mapped within Clarke County and Choctaw County, Alabama
(Gilliland et al. 1980). In the subsurface, the Cocoa Sand Member can be mapped from western
Choctaw County, Alabama to eastern Jasper County, Mississippi (Dockery et al.1994). Although the Yazoo Formation has been the focus of many previous studies (Mancini 1979; Waters and
Mancini 1982; Keller 1985; Mancini 1986; Mancini and Copeland 1986; Mancini and Water 1986;
Mancini et al.1987; Pasley and Hazel 1995, Tew and Mancini 1995; Fluegeman 1996; Mancini
2000; Posamantier and Henry 2001; Fluegeman 2003), the Cocoa Sand Member has been

8
Figure 1 Type Location of the Cocoa Sand Member in Alabama (Adapted after Krutak 1961)

9
Figure 2 Study area in Mississippi and Alabama (Adapted from May 1974)

10 rarely studied, especially in relation to its depositional environment and sequence stratigraphic setting. Climatic transitions from greenhouse to icehouse conditions at the Eocene-Oligocene boundary (Miller et al.2008; Fluegeman et al.2009), make for complicated facies changes across the U.S Gulf Coastal plain. This has resulted in variable sequence stratigraphic interpretations for the Cocoa Sand Member of the Yazoo Formation of the Upper Eocene. As reported in
Brissette (2004) samples of the Cocoa Sandstone from the #1 Young core recovered from a well drilled in Wayne County, Mississippi, contain 94% quartz, Miller et al. (2008) described the
Cocoa Sand Member in the St. Stephen Quarry, located in Washington County, Alabama, as a sandy micrite or chalk with up to 50% sand fraction consisting almost exclusively of carbonate
(mostly large foraminifera). Although the sandy micrite described by Miller et al. (2008) is likely time equivalent to the Cocoa Sandstone described here, the depositional setting is quite different and adds to the difficulty in interpreting the sequence stratigraphy of the Yazoo
Formation. In this study, only the quartz-dominated sandstone of the Cocoa Sand Member will be considered.
Two previous interpretations of the deposition and sequence stratigraphic setting of the
Cocoa Sand Member have been suggested. The first interpretation, supported by the studies of
Baum and Vail, (1988), Dockery et al. (1994), Tew and Mancini, (1992), and Tew and Mancini,
(1995), is that the Cocoa Sand Member was deposited as part of a lowstand system tract or shelf margin system tract (Echols et al.2003). In terms of the paleoenvrionmental and paleobathymetric settings of North Twistwood Creek and Cocoa Sand Members, Tew and
Mancini (1992) inferred the North Twistwood-Cocoa contact as a regressive sequence boundary at the deposition shoreline break, which separates the highstand system tract and lowstand

systems tract. They argue that the contact of Cocoa and Pachuta is a transgressive surface representing the first significant marine flooding event associated with rapid relative sea-level rise. The transgressive surface in a lowstand system tract is considered as the upper bounding
11 surface (Coe et al.2003). Combined with the regressive sequence boundary and the transgressive sequence boundary, the Cocoa Sand member is considered as being deposited in a neritic marine to middle shelf lowstand systems tract during sea-level rise.
The second interpretation explains the deposition of the Cocoa Sand Member as part of transgressive system tract during sea-level rise (Mancini et al.1987; Mancini et al.2000; Hurley and Fluegeman 2003; Miller et al.2008; Fluegeman and Grigsby 2009). These authors suggest that the North Twistwood Creek-Cocoa contact is a significant unconformity associated with transgressive surface or sequence boundary. Lower Pachuta-Cocoa Sand contact is inferred as a sequence boundary associated with transgressive surface during significant increasing rate of relative sea-level (Miller et al.2008). Both interpretations are possible to explain the mechanism of how the Cocoa Sand Member formed during relative sea-level change.
However, the goal of this study is to perform a detailed subsurface geological analysis of the Cocoa Sand Member of the Yazoo Formation in order to define a depositional model of the
Cocoa Sand Member in relation to a sequence stratigraphic setting. This study will be helpful to further clarify the depositional history of the Cocoa Sand Member of the Yazoo Formation.

12
GEOLOGICAL SETTING
Lithostratigraphy
In the upper Eocene of Mississippi and Alabama, the Jackson Group, is defined by the
Yazoo and the Moodys Branch Formations. The Yazoo formation, which is composed of four distinct lithologic members, is conformably overlying the Moodys Branch Formation in eastern
Mississippi and western Alabama. The four different lithologic members are, in ascending order, the North Twistwood Creek Member, the Cocoa Sand Member, the Pachuta Marl
Member and the Shubuta Clay Member (Murray 1947) (Figure 3). In the lower Oligocene, the
Red Bluff Formation of the Vicksburg Group overlies the Yazoo formation in most of eastern
Mississippi, and the Bumpnose Limestone Formation in western Alabama (Mancini et al.1994).
More than 160 feet (53.33 m) of the exposed Yazoo Formation is heavily weathered because of the subtropical climate in Mississippi and Alabama resulting in about 60 feet (20 m) of good exposures in the southern part of Clarke County in Mississippi (Gililland et al.1980). Upon observation of the limited exposures, the Yazoo Formation in western Mississippi is described as fossiliferous with abundant planktonic and benthonic foraminiferas specifically in the
Pachuta member (Bybell et al.1982). Additionally, several distinct bentonite beds have been identified in western Mississippi and have provided radiometric ages (Obradovich et al. 1993).

Mississippi Alabama Biozones
13
Figure 3. Lithostratigraphy and Biostratigraphy of the Yazoo Formation. “E zones” are planktonic foraminiferal biozones of Berggren and Pearson (2006) (Adapted from Fluegeman
2008)

14
Moodys Branch Formation
The Moodys Branch Formation was originally recognized from Louisiana, across
Mississippi, and into Alabama (May 1974). It is correlated with the lower part of the Ocala
Limestone of Florida and Georgia, and the lower part of the Jackson Group of Texas (Moore
1974). The type locality for the Moodys Branch is near the intersection of Peachtree Street and
Poplar Boulevard in the City of Jackson, Hinds County, Mississippi (Bybell 1982). It is composed mainly of olive to yellowish-gray, fossiliferous, glauconitic marl and generally indurated at the top. At the site of test hole AM-18 (Figure 4) in the southwest of section 12, T. 9N, R. 6W,
Wayne County, Mississippi, the Moodys Branch consists of four feet of indurated marl ledge with fourteen feet of glauconitic, fossiliferous sand beneath, which is the thickest part of the
Moodys Branch Formation encountered in the Wayne County, Mississippi (Moore 1974). An outcrop of the Moodys Branch Formation is located in the Northwest of section 3, T.10 N., R.5
W., about 3000 feet (1km) west of the Alabama state line. This outcrop is badly weathered and contaminated with terrace materials (Turnbull 1948). Upon close observation, glauconite and fossil fragments are recognized (Dockery 1984). Good outcrops of the Moodys Branch are rare in Mississippi. Very few fossils are found in eastern Mississippi and western Alabama. The contact between the Moodys Branch and the overlying North Twistwood Creek Clay member of the Yazoo Formation is conformable and gradational (Mancini et al. 1994).

15
Figure 4 Location map of Test hole AM-18 , #1 Young core and #1 Ketler core in Wayne county,
Mississippi (Adapted from Dockery et al. 1994)

16
Yazoo Formation
North Twistwood Creek Member:
The North Twistwood Creek Member of the Yazoo Formation is described as light to medium greenish-gray, fossiliferous clay, with chalk shells and sandy beds in the uppermost part (Dockery et al.1994). In eastern Mississippi, where the North Twistwood Creek Clay
Member is about 25 to 50 feet (8.33 to 16.67 m) thick according to the well log data, it is a bluish-gray, micaceous, sandy clay that is calcareous at the base and containing some clay concretions in the upper part, which can be observed in the # 1 Young Core (Figure 5). In western Alabama, the North Twistwood is a greenish-gray, plastic, calcareous, sparsely fossiliferous, and blocky to massive clay (Moore 1974). There is a diverse kind of foraminiferas, and macrofossils are usually fragile and poorly preserved. Bentonites within the North
Twistwood Creek Clay have been studied by Dockery et al. (1994) for Sanidine radiometric dating with 34.32±0.05 Ma in age. Angular quartz grains were noted within these bentonites.
The upper sandy marl section of the North Twistwood Member with kind of clay lithology facies could also be observed in the #1 Young Core.

426 feet
17
425 feet
Figure 5. Upper Part of the North Twistwood Creek Member in the #1 Young Core starting from
425’ feet

18
Cocoa Sand Member
The Cocoa Sand Member is well developed throughout eastern Mississippi and western
Alabama, and the thickest part is in the vicinity of the Mississippi-Alabama state line. The type locality of the Cocoa Sand Member is in Alabama, the southwest of section 13, T.11 N., R. 5 W.,
Melvin County, Alabama (Cooke 1933). The thickness is about 27 feet (9 m), and has been reported to be extensively covered by terrace materials on the private property (Turnbell
1948). Eastern Mississippi and Western Alabama represents challenges for study of the Yazoo
Formation because of the rarity of outcrops of the Cocoa Sand Member. In Mississippi, the overall lithology of the Cocoa Sand Member can be described as yellowish to greenish fine grained argillaceous clay with more than 95% quartz (Brissette 2004). Electrical well logs are the main tools available in Mississippi to study the Cocoa Sand Member.
In Alabama, there are four significant outcrops containing the Cocoa Sand Member
(Figure 6). Approximately 4 miles from the type locality of the Cocoa Sand Member in Melvin,
Alabama, there is outcrop exposed near the southwest corner of section 20 ., T. 11 N., R.4 W in the Choctaw County (Figure 7). The exposed section of the Cocoa Sand Member is about 20 feet
(6.67m) thick (Cooke 1926). The lithology at this location is yellowish fine-grained, angular quartz arenite with highly cemented clay stratum, and there are abundant ostracode fauna containing the Cocoa Sand Member (Krutak 1961). Another isolated exposure is in the Valley of the Willow Branch along U.S highway 84 in western Choctaw County, Alabama (Tew and
Mancini 1992). At this location, as reported by Tew and Mancini (1992) the contact between

19
Figure 6 Outcrops location map of the Cocoa Sand Member in Alabama (Adapted from Tew and Mancini
1995)

20
Outcrop
Figure 7. Outcrop of 6 miles of type location of the Cocoa Sand Member in Alabama (Adapted from Krutak 1961)

21 the North Twistwood Creek and the Cocoa Sand Member is very distinct with light-brown, argillaceous sand of the Cocoa overlying medium-brown, weathered, blocky clay of the North
Twistwood Creek Member. They report that the uppermost part of the North Twistwood Creek clay member near the North Twistwood-Cocoa contact of this exposed outcrop is weathered and burrowed, with the burrows partially filled with sand from the Cocoa Sand Member above.
The lithology is similar to that encountered in eastern Mississippi with the exception of conglomeratic quartz pebbles in the lower part of the Cocoa Sand Member (Tew and Mancini
1995). An abrupt section showing nearly the entire Jackson Formation and much of the
Claiborne group is exposed in the valley of willow branch, Choctaw County, Alabama (Cooke
1933). The Cocoa Sand of this exposure is fine-grained argillaceous yellowish sand with about
30 feet (10m) thick. The other outcrop including the Cocoa Sand Member of well-preserved
Yazoo Formation is in 3.5 miles southeast of Cullomburg, Alabama. The lithology of the Cocoa
Sand Member is similar to the other outcrops with yellowish calcareous sand and poorly preserved shells. The thickness of the Cocoa Sand is around 18 feet (6 m) (Cooke 1933).
Dockery et al. (1994) reports that the contact between the Cocoa Sand Member and the
North Twistwood Creek Member in Mississippi is fairly conformable, however the contact is reported to be a subaerial unconformity in southwestern Alabama by Tew and Mancini (1995) and Echol et al. (2003). Rip-up clasts are reported to be found at the base of the Cocoa Sand
Member by Brissette (2004) (Figure 8), which supports that the

A.
B.
Figure 8. Photomicrograph of lithic intraclasts (LC) from the #1 Young core in thin section
(Adapted from Brissette 2004)
22

contact between the Cocoa Sand Member and the North Twistwood Creek member is an erosive surface, therefore, it cannot be conformable as mentioned above by Dockery et al.(1994).
23
At Little Stave Creek, in southeast Clarke County in Alabama, the Cocoa Sand Member has also been described. Mancini and Waters (1986) described the Cocoa Sand as 5 feet (1.46 m) thick, medium gray, sandy and glauconitic marl. Miller et al. (2008) also described 5 feet
(1.46 meters) of the Cocoa Sand Member at the St. Stephen Quarry, Washington County,
Alabama. As mentioned earlier, they described the lithology of the Cocoa Sand Member as a sandy micrite and/or chalk, with the sand percent never exceeding 50 %. They described the sand sized fraction as almost exclusively carbonate composed largely of foraminifera.
Therefore, based on the previous work (Brissette 2004), it is unlikely that the Cocoa Sand
Member, as described at the type section, is exposed at either Little Stave Creek or St. Stephens
Quarry.
Pachuta Marl Member
The Pachuta Marl Member is pale greenish gray colored, glauconitic, fossiliferous clay interlaminated with both indurated and soft intervals (Dockery et al.1994). Calcareous nannofossils are abundant and poorly preserved (Bybell 1982). The type locality of the Pachuta
Marl Member is on the south side of the Clarke County, Mississippi. In southern Alabama, it has a similar lithology as in Mississippi, which is light greenish-gray clay, glauconitic, fossiliferous calcareous with sandy limestone layers (Toulmin 1977). The Pachuta is about 10 feet (3.33 m)

thick at the Mississippi and Alabama state line and thins toward southern part of the Choctaw
24
County, Alabama. There are glauconitic, condensed, sandy intervals at the Pachuta Marl-Cocoa
Sand contact (Miller et al.2008). Some authors (Mancini and Waters 1986) think that Pachuta
Marl conformably overlies the Cocoa Sand Member in eastern Mississippi but unconformably overlies the North Twistwood Creek clay at Little Stave Creek in Alabama. However, some people (Baum and Vail, 1988; Tew et al.1995) think that lower Pachuta-Cocoa Sand contact is considered as an erosive surface defined by Baum and Vail (1988) as part of the transgressive system tract sediments into the offshore associated with the first significant marine flooding event during relative sea-level rise. The upper contact of the Pachuta Member with Shubuta
Member is conformable, even though the change in lithology is immediately abrupt where the
Shubuta Clay overlies the indurated marl of the Pachuta Member.
Shubuta Member:
The Shubuta Member is well preserved throughout eastern Mississippi and western
Alabama. It is the youngest member of the Yazoo Formation, and thins from eastern Mississippi to western Alabama (Dockery et al.1984). The lithology in Mississippi is pale grayish green, blocky to massive, glauconitic clay (Moore et al.1974). Calcareous nanofossils are abundant in the Shubuta Member, but exhibit fair to poor preservation. Most of the microfossils observed in the Shubuta Member within Wayne County, Mississippi, are fragile and difficult to identify in drill cuttings. The Shubuta becomes thinner and more calcareous eastward towards southwestern Alabama, where the member is light green to white calcareous, argillaceous,

microfossiliferous containing many small irregular white calcareous nodules. In southwestern
25
Alabama, the Shubuta contains 25 to 35 feet (8.33 to 11.67 m) of light greenish-gray, fossiliferous, calcareous clay, and massive to blocky weathering marl at St. Stephens Quarry, and 12 feet (4 m) of similar lithology at Little Stave Creek. The lower contact of the Shubuta is gradational with the Pachuta at the localities along the Chickasawhay River, Mississippi; Little
Stave Creek, and St. Stephens Quarry in Alabama (Mancini and Waters 1986). The observations identified from the two cores: #1 Ketler and #1 Young in Wayne County, Mississippi, are consistent with the gradational contact between the Shubuta Member and Pachuta Marl (Echol et al.2003). As mentioned previously the Shubuta thins toward the southeast in the Washington
County, Alabama, and there is a large difference in thickness in western Mississippi where it is considered as a major composition of the Yazoo Formation. (Moore et al. 1974). In the upper member of the Yazoo Formation, the contact between Red Bluff and Shubuta Clay in eastern
Mississippi is reported as sharp and burrowed by Dockery (et al.1994), although Mancini and
Tew considered that the Shubuta-Red Bluff contact as conformable in western Alabama.
Red Bluff Formation:
The Red Bluff Clay Formation is dark greenish gray, glauconitic, usually calcareous clay interlaminated with sand beds. The type location of the Red Bluff is along the Chickasawhay
River, Mississippi, of which the thickness ranges from 11 to 30 feet (3.67 to 10m) (Hilgard, 1860;
Macneil 1940; Mancini and Waters 1986; May 1974). At the most eastern locality at St.
Stephens Quarry, Choctaw County, Alabama, the Red bluff Formation is directly above the

26
Bumpnose Limestone Formation, which is the equivalent facies of the Red Bluff Clay in
Alabama. Further west in Mississippi, the Red Bluff is replaced by its western equivalent facies, the Forest Hill Formation, which is dark cross-bedded marine clastic sediments containing clay and lignite beds (Bybell et al. 1982). To the east, the Red Bluff is gradually substituted by the
Bumpnose Formation in western Alabama. The lower contact of the Red Bluff Clay is disconformably overlying the Yazoo Formation in eastern Mississippi and western Alabama, which is well exposed at Little Stave Creek, Clarke County and St. Stephen Quarry
,
Washington
County, Alabama (Waters and Mancini 1982).
Sequence Stratigraphy
The Yazoo Formation has been the focus of many studies on the sequence stratigraphy of the Gulf Coast area for many years (Baum 1986; Baum and Vail 1987, 1988; Loutit et al.1983;
Mancini and Tew 1986, 1988, 1991; Tew and Mancini 1987, 1990; Vail et al. 1987; Mancini et al.
1987; Dockery 1990; Mancini and Tew 1991). There is general agreement on the number of depositional sequences within the Yazoo, but quite a few differences in the placement of sequence boundaries. Several unconformities or correlative conformities in the Yazoo
Formation are recognized by Mancini and Tew (1991). The Tejas A Gulf Coast (TAGC) is assigned to name the sequence boundaries by Tew and Mancini (1995) to replace the earlier terminology of TE by Mancini and Tew (1991) (Figure 9). TAGC4.3 and TAGC 4.2 are mainly discussed in this thesis and includes the four members of the Yazoo Formation and the Moodys
Branch Formation. The North Twistwood Creek member represents the highstand systems tract

27 of depositional sequence TAGC 4.3 (Figure 10). The Cocoa Sand Member represents the transgressive systems tract of depositional sequence TAGCE 4.2 with the Pachuta Marl Member and Shubuta clay member both being associated with the transgressive systems tract of TAGC
4.2. The upper most Shubuta clay member is considered as a condensed section at the maximum flooding surface of sequence TAGC 4.2. The North Twistwood-Cocoa Sand contact is interpreted by Tew and Mancini (1995) as a type two sequence boundary now termed as a regressive sequence boundary at the depositional shoreline break, which separates the highstand systems tract of the TAGC 4.2 sequence from the transgressive system tract of the
Cocoa Sand Member that is part of the TAGC 4.3. This interpretation is supported by the paleoenvrionment and paleobathymetric settings of the North Twistwood Creek Clay Member and the Cocoa Sand Member (Tew and Mancini 1995).

28
Figure 9 TAGC Sequence stratigraphy of the Upper Eocene (Adapted from Tew et al. 1995)

29
Figure 10 Sequence Stratigraphy of the Upper Eocene and Lower Oligocene strata in southeast Mississippi and Southwest Alabama
(Adapted from Mancini et al.1992)

30
Biostratigraphy
In southeastern Mississippi and southwestern Alabama, the biostratigraphy of the Yazoo
Formation has been comprehensively studied by DeBoo (1965), Bybell (1982), Siesser (1983),
Mancini and Waters (1986), Fluegeman (1996, 2008, 2009), Echols et al. (2003). The planktonic foraminiferal biostratigraphy of the Yazoo Clay in the much thicker sections of western
Mississippi generally corresponds to the results obtained in western Alabama and eastern
Mississippi (Mancini and Waters 1986).Mancini and Waters (1986) studied the Planktonic foraminiferas of the Yazoo Formation and concluded that the Yazoo Formation is in the late
Eocene Age. Van der Zwann et al. (1990) established a relationship between Planktonic to
Benthic foraminifera ratios (P:B) and water depth. It is useful to apply P: B ratio values to the paleoenvrionment and paleobathymetry settings of the Yazoo Formation. Fluegeman et al.
(1996, 2003, and 2008) has been focused on the benthic foraminiferas and P: B ratios of the
Yazoo Formation for many years. The P: B ratio study of the Yazoo Formation completed by
Fluegeman (2008) is based on two cores: #1 Young core and Mossy Grove Core, which are both in Mississippi. The P: B ratios calculated from the # 1 Young core and Mossy Grove Core have an apparent cyclicity in the upper Yazoo Formation that indicates the important role of the glacioeustacy in changing sea level from the late middle Eocene (Fluegeman et al. 2008). Figure
11 shows the P: B ratios for the upper Cocoa Sand, Pachuta Marl and Shubuta Clay members of the Yazoo Formation in the #1 Young core. There are few planktonic foraminiferas in the Cocoa
Sand Member of the Yazoo

Figure 11 P:B ration of the upper Cocoa Sand, Pachuta Marl, Shubuta Clay Members in the #1
Young Core, Wayne County, Mississippi (Adapted from Fluegeman 2008)
31

Formation upon the observation of the #1 Young Core. The P: B ratio obtained from the Yazoo
32
Formation in the Young core is consistent with the sequence stratigraphic interpretation of
Mancini and Tew (1991) (Fluegeman et al. 2008). Fluegeman (2008) suggests that the benthonic foraminiferal assemblages obtained from the Cocoa Sand Member in the # 1 Young Core in
Mississippi are characterized by species of Nonion and Nonionella, which is associated with inner neritic conditions. The lack of planktonic foraminifera in the Cocoa Sand Member followed by gradually increasing P: B ratios in the Pachuta Marl Member suggest the transition from transgressive system tract to early highstand system tract. The fluctuations of the P: B ratio in the lower Shubuta likely represents influxes of clastic sediments during late transgression or early highstand, while on the other hand, the more stable, lower values of the
P: B ratio in the upper Shubuta Member may reflect eustatic sea level fall during the latest
Eocene (Fluegeman et al. 2009).
Calcareous nanofossils from the Yazoo Formation have been studied by Bybell (1982) and Siesser (1983) and concluded that the Yazoo Formation is also late Eocene in age. They placed the Yazoo Clay in the Globorotalia cerroazulensis interval zone as that of Stainforth et al.
(1975). The results of the paleomagnetic studies in a series of outcrops and cores in eastern
Mississippi and western Alabama by Echols et al. (2003) indicated that the Yazoo Formation is late Eocene in age, which is consistent with Mancini and Waters (1986).

33
METHODS
Grain Size Analysis:
Grain size analysis is a fundamental attribute of sedimentary rocks and reflects the weathering and erosion process in a particular deposit. Due to various sizes of the particles, grain size analysis describes the subsequent transport processes in order to reveal the sedimentary history (Boggs 1995). In this study, grain size analysis is considered as a useful tool to aid in supporting the interpretation of the Cocoa Sand Member depositional environment.
Two cores were collected from wells drilled in Wayne County in 1993 by Mobil Exploration
Center: The #1 Young and the # 1 Ketler (Figure 4 &12). The #1 Young is located in longitude-
88.65186, latitude 31.66777 of Wayne County, while the #1 Ketler is in -88.68816, 31.80801 of
Wayne County. There is 26 feet (12 m) of the Cocoa Sand Member preserved in the # 1 Young
Core, and 12 feet (4 m) in the Ketler core. Six samples were obtained from #1 Young core from the following depths: 396’8’’, 401’5’’, 406’5’’, 411’5’’, 416’5’’ and 421’5’’, and four samples from the # 1 Ketler core from these depths: 136’, 134’, 132’ and 128’.
Grain Size is measured by sieving through a set of nested, wire-mesh screens with different phi sizes. The sieve numbers of U.S standard sieves that correspond to phi sizes are shown in table associated with the grain size classification ( Boggs 1995) (Figure 13). Both graphical and mathematic data-reduction methods were used during data analyses.

425 feet
423 feet
424’6’’ feet
422’6’’ feet
34
Figure 12 # 1 Young Core sample in Wayne County, Mississippi

35
Figure 13. The Wentworth grain-size scale for sediments, showing Wentworth size classes, equivalent phi ( Φ ), units, and sieve numbers of U.S Standard Sieves (Adapted from Boggs 1995)

36
Well Log Analysis
As the exposure of the Cocoa Sand is poor, subsurface information is more valuable to investigate the distribution and stratigraphy of the Cocoa Sand Member. Well logs were obtained from the Mississippi Department of Environmental Quality. The database obtained from the Mississippi Department of Quality office, are wells drilled mostly by Sohio Petroleum
Company in the 1940s. As they are drilled in 1940s, the well locations are still in a township, range format within the public land survey system instead of the longitude and latitude system.
The Microsoft Research Map was used as the main tool to make the conversion from public land survey system into longitude to latitude. The key part of data collection was to convert the old township/range format into the longitude/latitude format in order to simulate the Cocoa
Sand distribution based on the thickness in an isopach map.
Most electrical well logs are spontaneous and resistivity logs that are useful to separate the sand from the other lithology facies like clay and marls. The Spontaneous (SP) log is sensitive to changes in shale and sandstone content within relatively thick beds, which can indicate different grain size trends like fining-upward or coarsening upward. At positions of thin, permeable beds, the amplitude of the SP deflection will be dampened and virtually suppressed opposite very thin beds (Doveton 1944). In this study, the Cocoa Sand Member is identified both by Spontaneous Potential logs and Resistivity logs. The resistivity log is sensitive to different characteristics of the bedding so that shales that are composed of mostly clay minerals (high conductivity characteristics) have very low resistivity values, while sandstones
(low conductivity characteristics) have higher resistivity values. Based on these features of the

shale and sandstones applied to the SP and Resistivity logs, the Cocoa Sand Member can be
37 identified with high resistivity values and low spontaneous values on the well logs (Figure 14).
For instance, the #1 Ketler well log, there is a very high resistivity value change from 120 feet
(40 m) to 130 feet (42.33 m) going back to normal value of the shale base line, which is defined as the solid line with high peaks in SP and low values of resistivity log. Corresponding to the high value of the Resistivity log, at the same depth position, the Spontaneous log shows a lower value than the above sections (Figure 14). Therefore, from 120 to 130 feet (40 to 42.33 m) the
Cocoa Sand Member is defined based on the features observed from the SP and Resistivity logs, and define the thickness of the Cocoa Sand Member. There are 70 wells selected by their good log profiles and relatively thick intervals (Figure 15). A detailed isopach map of the Cocoa Sand
Member was constructed with Surfer Software in order to define the thickness distribution across the study area. Cross sections were created with the EarthPack package of the Kingdom
Software supported by IHS Corporation. There are five members in the correlation lithology, including Red Bluff Formation, Shubuta Member, Pachuta Member, Cocoa Sand Member and
North Twistwood Creek Member. Among the five units, Red Bluff Formation is chosen as the datum section to make the stratigraphic correlation across the Wayne County, Mississippi. In terms of the sequence stratigraphic settings documented by previous work, the cross section profiles will be helpful to determine the helpful to determine the depositional model associated with thickness distribution of the Cocoa Sand Member.

Figure 14 Spontaneous and
Resistivity electric wireline
Log of #1 Ketler Core
38

39
Figure 15 Base map of 70 wells in Wayne County, Mississippi

40
RESULTS
Grain Size
The Grain size scale used in this study is Udden-Wentworth scale associated with the phi size units, which is divided into four major size categories: clay, silt, sand and gravel. As said in the previous section, the standard sieve numbers of the wire-meshes correspond to the phi size.
Figure 16 shows the relationship between the phi size and the depth of four samples: KT-128,
KT-132, KT-134, and KT-138 with increasing depth. KT 136 sample has the biggest phi size around 2.63, and phi size of the KT-132 and KT-134 samples are very close to each other around
2.3, while the sample KT-128 has the smallest phi size around 1.8. The uppermost sample KT-
128 of the Ketler core belongs to the coarse sand, KT-132 and KT-134 belongs to fine sand, while sample KT-136 is relatively fine-grained sand on the basis of the Wentworth grain-size scale (Figure 13). Therefore, there is a coarsening upward trend of the grain size derived from the #1 Ketler Core samples. Grain Size data of six samples (S1, S2, S3, S4, S5 and S6) from the #1
Young core is shown in Figure 17. Sample 4 (S4) has the lowest phi size value, and except sample 4, there is a trend of decreasing phi size values with increasing depth in subsurface.
Therefore, there is a similar coarsening upward trend in the #1 Young Core as that of the #1
Ketler core. The depth of sample 4 (S4) is located around 411’ feet (137 m) where there is a significant low resistivity value in the #1 Young Core electronic wireless log indicating a mud or clay lithology facies (Figure 18). Two small coarsening-upward sequences are separated at the depth 411’ feet (137 m) on the #1 Young Core

131
132
133
134
127
128
129
130
135
136
137
0 0.5
1
Phi Size
1.5
2
KT-134
KT-128
2.5
KT-136
3
KT-132
#1 Ketler
Linear (#1
Ketler)
Figure 16 Grain Size Analysis of #1 Ketler Core Samples with relationship of Depth and Phi Size
393
396
399
402
405
408
411
414
417
420
423
426
0 1
Phi Size
2
S4
3
S1
S2
S3
S5
S6
4
#1 Young Core
Figure 17 Grain Size Analysis of #1 Young Core samples with relationship of depth and Phi size
41

Grain size change in
Sample 4
Figure 18 Electric wireless log of #1 Young Core, Wayne
County, Mississippi
42

log by some mudstone or clay. Although the phi size of sample 4 is quite coarse-grained, it
43 consists of clay balls or small clumps of clay as observed from the sample 4 of the #1 Young core, which might have formed in quiet water resulting in mud drapes separating the sandstones packages. One stacking sequence is from 396 to 410 feet (132 to 136.7 m) with 11 feet (3.67 m) thickness as showing in grain size data from sample 1(S1) to sample 3(S3), and another one starts from 411 to 426 feet (137 to 142 m) with 15 feet (5 m) thickness as the grain size data from sample 4(S4) to sample 6(S6).
Sandstone Distribution
Previous work has reported that the thickness of the Cocoa Sand Member ranges from
28 to 62 feet (9.34 to 20.67 m) in southeastern Mississippi (May 1974). Detailed log analysis in this study has found that the thickness of the Cocoa Sand Member varies from 5 feet (or less) to
65 feet (1.67 to 21.67 m) in southeastern Mississippi. An isopach map constructed on five foot contour intervals and built from sandstone thicknesses derived from well logs (Figure 16) in the
Wayne County, Mississippi and associated (five) outcrops from Choctaw County, Alabama helps to describe the distribution of the Cocoa Sand Member. Figure 20 indicates that the quartz-rich
Cocoa Sand Member, as defined in this study, extends from the southern part of the Clarke
County, Mississippi to the western part of Choctaw County, Alabama. Figure 20 illustrates the
Cocoa Sand Member of the Yazoo Formation thickens from 5 up to 65 feet (1.67 to 21.67 m) from northwest to southeast direction around the state line of Mississippi and Alabama, where

44
Figure 19. Isopach map of the Cocoa Sand Member in southeast Mississippi and southwest Alabama

the thickest part of the Cocoa Sand Member is found (longitude 88.500 west and Latitude
31.5500 to 31.6500 North).
45
Two sand body trends can be identified on the isopach map (Figure 19). These trends form two blocky sand bodies or sand ridges and tend to elongate parallel to each other in the northwest to southeast direction (Figure 19). The entire package of sand tends to thin more gradually to the north-northeast direction. The southern sand slope shows a steeper profile than the northern sand body (latitude from 31.7500 to 31.8500). A 3D surface thickness map
(Figure 20) illustrates thickness changes and the subsurface distribution of the Cocoa Sand
Member, which supports the steep trend profile to the south-southwest in both the southern and northern ridge. It also provides some evidence that the two ridges developed while sand was being transported towards the northeast. The X-Y axis is longitude by latitude rotated by a certain angle different from the surface level. The Z axis represents the thickness scale, which is the subsurface thickness of the Cocoa Sand Member.
The 3-D thickness wire-frame map of the
Cocoa Sand Member shows the morphological change in subsurface thickness, which is consistent with the isopach map in relation to thickness changes. Indeed, it is difficult to tell the sand shape profile in the subsurface based only on the isopach map.
Six cross sections (Figure 21) were constructed in the main distributed area of the Cocoa
Sand Member, Wayne County, Mississippi. They are divided into two groups in two directions.
Group 1 cross sections, SW1, SW2, SW3 (corresponding to A-A’, B-B’, C-C’), are in the southeastnorthwest direction across the base map (Figure 21), while Group 2 cross sections, NE2, NE3,

Figure 20. 3D wireframe surface thickness map of the Cocoa Sand Member in southeast Mississippi and southwest Alabama
46

D’
C
B
D
E
A F
E’
F’
C’
B’
A’
A
Figure 21 Base map of the six cross section of the Cocoa Sand Member in southeast Mississippi and southwest Alabama
47

48
NE4 (corresponding to D-D’, E-E’, F-F’), are located in a southwest-northeast direction almost orthogonal to the other group. Each cross section is represented in Figures 22-27.
The paleo shore-line reconstructed in study of Tew et al. (1995) is in the paleo coastal plain going across northwest of Mississippi and southeast of Alabama, which is regarded as the basic trend to make the cross sections of Cocoa Sand Member of Yazoo Formation. Therefore, three cross sections A-A’, B-B’, C-C’ are constructed parallel to the paleo-shoreline. There are six wells in the A-A’ cross section (Figure 22), which are 17-7N-8W-R2, 26-7N-8W-R3, 33-7N-7W-
S11, 19-7N-6W-Y14, 4-6N-6W-Y1, and 8-6N-5W-Z4. The thickness of the Cocoa Sand Member in each well is quite variable, for example, the well 17-7N-8W-R2 only contains at most 5 feet of the Cocoa Sand, while the well 8-6N-5W-Z4 includes about 60 feet. Observed from A-A’ Cross section, there is a thickening trend toward the southeast direction of the Wayne County,
Mississippi. The B-B’ cross section has the same trend as that of the A-A’ but more obvious thickness changes (Figure 23). There is no big change in the C-C’(Figure 24) , but the overall thickness of the Cocoa Sand Member of each well becomes thinner than the two previous cross sections in the southern part of the Wayne County. Abrupt changes in thickness give the sandstone a significant wavy appearance in the C-C’ cross section, and there is some small fluctuating morphology with thickness changes in the wells: 16-10N-7W-C4 and 25-10N-7W-C9, although the thickness of the Cocoa Sand Member in these two wells

49
Red Bluff Formation
Cocoa Sand Member
Shubuta Member
North Twistwood Creek Member
Pachuta Member
Conformity
Unconformity
Figure 22. A-A’ (SW1) cross section of the Cocoa Sand Member of the Yazoo Formation, Wayne County, Mississippi

50
Red Bluff Formation
Cocoa Sand Member
Shubuta Member
North Twistwood Creek Member
Pachuta Member
Conformity
Unconformity
Figure 23. B-B’ (SW2) cross section of the Cocoa Sand Member of the Yazoo Formation, Wayne County, Mississippi

51
Red Bluff Formation
Cocoa Sand Member
Shubuta Member
North Twistwood Creek Member
Pachuta Member
Conformity
Unconformity
Figure 24. C-C’ (SW3) cross section of the Cocoa Sand Member of the Yazoo Formation, Wayne County, Mississippi

52
Red Bluff Formation
Cocoa Sand Member
Shubuta Member
North Twistwood Creek Member
Pachuta Member
Unconformity Conformity
Figure 25. D-D’ (NE2) cross section of the Cocoa Sand Member of the Yazoo Formation, Wayne County, Mississippi

53
Red Bluff Formation
Cocoa Sand Member
Shubuta Member
North Twistwood Creek Member
Pachuta Member
Unconformity
Conformity
Figure 26. E-E’ (NE4) cross section of the Cocoa Sand Member of the Yazoo Formation, Wayne County, Mississippi

54
Red Bluff Formation
Cocoa Sand Member
Shubuta Member
North Twistwood Creek Member
Pachuta Member
Conformity
Unconformity
Figure 27. F-F’ (NE3) cross section of the Cocoa Sand Member of the Yazoo Formation, Wayne County, Mississippi

55 almost equivalent. 16-10N-7W-C4 contains about 30 feet (10 m), while the thickness of the
Cocoa sand member in the 25-10N-7W-C9 well is about 28 feet (9.33 m).
There are three cross sections created in the study area: D-D’, E-E’, and F-F’ (Figure 21), which are perpendicular to the paleoshoreline in a northeast-southwest direction. All three sections have a thickening trend toward the northeast. Cross section D-D’ shows similar morphological changes in thickness as that of Cross Section C-C’. The third well 8-9N-7W-H26 in
D-D’ Cross section (Figure 25) with 21 feet (7 m) thickness of the Cocoa Sand member is relatively shallower in the depth of the Cocoa Sand Member probably because of the difficulty in picking the datum point on the log compared to the other wells in the Cross section, which causes a wavy appearance in sand shape. Cross sections E-E’ and F-F’ (Figure 26 & 27) tend to be more stable variations without showing wavy appearance in thickness change compared to the other Cross sections associated with thickening toward northeast direction. Thickness of the Cocoa Sand Member in E-E’ ranges from 26 to 61 feet (12 to 20 m). In the F-F’ Cross profile
(Figure 27), the thickness of the Cocoa Sand Member varies from 33 feet (11 m) up to 60 feet
(20 m).
Associated with the cross sections, the randomly changing thickness on each cross section is very obvious, but they are prone to thicken in a northeast and southeast direction on the Isopach map. As a result of there being limited subsurface data from Alabama, there are several possibilities for the continued tend of the sand body. It is possible the sand continues to

thicken into the northeast part of Choctaw County, Alabama or the sand bodies might extend
56 into the southeast part of Washington County, Alabama.

57
DISCUSSION
Shelf sediments form thick and laterally extensive shallow marine successions constituting most offshore sandbodies (Johnson and Baldwin 1986). One of the most widely quoted depositional models of shelf sandstone is that of the “offshore bars” or “offshore sand ridges”, which is an interpretation of a distinctive type of elongated-shallow marine clastic body encased in shelf mudstones (Swift and Hudelson 1987). The main features of these sand bodies are well-sorted, glauconitic and quartzose sandstones with physical and biogenic structures such as hummocky cross-stratification, tabular or trough bioturbated beddings (Johnson and
Baldwin 1986). These sandbodies are unidirectional and trend parallel or slightly obliquely to paleocurrents orientation in shallow marine mudstones which physically separate these offshore sandbodies from contemporaneous shoreline sand bodies (Tillman 1985).
Posamantier (2001) suggested that shelf ridges in modern settings seem to be associated primarily with two end-member processes: wave and tidal energy (Off, 1963;
Kenyon et al. 1981; Huthnance, 1982; Yang and Sun, 1988; Yang, 1989; Snedden et al. 1994;
Berne 1996; Liu et al.,1998; Snedden and Dalrymple, 1999). Shelf ridges formed predominantly by tidal process have been described for the East China Sea, Korean Strait, and Northwest Java
Sea (Liu et al.1998; Posamentier 2001; Park et al.2003). Generally, the sand ridges are influenced by the physical oceanographic conditions, such as wave, storm and tidal currents, and the regional sea-level changes associated with sediments supply and accommodation space
(Diaz and Maldonado, 1990; Snedden et al., 1994; Wagle and Beerayya, 1996).

58
The late Quaternary sediments in the Korean strait reported by Park et al. (2003) are composed of linear and elongated offshore sand ridges formed by high energy tidal dominated currents on the middle shelf ( ~ 60 m water depth)of the Southern Korean Strait during transgression (Figure 28). The sand ridges are oriented nearly parallel to the present shore line.
The surface of the ridges is smooth without any large bedforms such as sand waves or dunes.
Core through the sand ridges found that the sand is highly bioturbated with no preserved sedimentary structures. Park et al. (2003) proposes that the source of the sand for these ridges is from the reworking of sand rich paleo-valleys that formed during periods of low relative sealevel (Figure 30). Although, sand ridges such as those found on the Korean Strait are commonly found on modern shelves, their equivalent in the subsurface or outcrops have been rarely described (Posamentier 2001).
However, Posamentier (2001) described sand bodies preserved in the Miocene of Northwest Java and determined that these sands are preserved ancient shelf sand ridges deposits. The Miocene sediments in the northwest Java Sea consisted of long linear ranging from 0.3 to 2.0 km wide sand ridges formed by high tidal energy in the Miocene shallow marine shelf during transgression (Posamentier 2001). The deposits are intensely burrowed so that no primary sedimentary structures are preserved. Posamentier (2001) described the source of the sand for these ridges as being from lowstand alluvial bypass channel deposits, and winnowing process of underlying silty mudstones. He determined that the northwest Java sand ridges formed, same as modern shelf sand ridges, in terms of in situ erosion and subsequent reworking of immediately underlying sand rich deposits during transgression.

3km
Figure 28 Distribution of the mid-shelf sand ridges in Korean Strait (Adapted from
Park et al. 2003)
59

60
The Cocoa Sand Member has some similarities with both the Northwest Java shelf ridges and Korean Strait ridges. The Cocoa Sand bodies based on the isopach map (Figure 20) are generally orientated parallel to the paleo-shoreline direction along the long shore face, which is similar to the Korean Strait ridges. The direction of the paleo-shoreline reconstructed by
Mancini et al. (1995) is quite equivalent to the direction of the thickening trend of the Cocoa
Sand Member of Yazoo Formation (Figure 29). Rip-up clasts found on the bottom of the Cocoa
Sand Member and identical quartz grain textures of the North Twistwood and the Cocoa Sand with similar fossil assemblages (Mancini et al. 1987; Brissette 2004), supports that the Cocoa
Sand Member was originated from reworking and winnowing of the North Twistwood Creek
Clay Member. This is quite similar to the sources sands of the northwest Java shelf originated from underlying silty mudstones. There are no cross-bedding features found so far in the Cocoa
Sand Member, which is also consistent with no structures preserved in both northwest Java shelf and Korean Strait ridges.
Very angular to subrounded quartz shape from the thin section of the Cocoa Sand Member (Brissette 2004), indicates that the reworked sediments from North
Twistwood Creek are not transported very far , which supports the interpretation that rapid sea-level rise during the early transgression leading to the Cocoa Sand Member deposited close to the shore face in the shallow water. There has been observed variations in thickness in the cross section A-A’, B-B’, C-C’ (Figure 22, 23 &24) and the isopach map (Figure 19) previously.
This supports the transportation and stacking of winnowed sand as a result of tidal and wave currents associated with transgression.

61
Figure 29 Regional paleogeographic reconstruction (Adapted from Mancini et al. 1995)

processes during transgression, the Cocoa Sand bodies are thought to be a couple of isolated
62 transgressive sand ridges. All three of the discussed sand ridges (Cocoa Sand ridges, northwest
Java shelf ridges and Korean Strait ridges) formed at the time when the shoreline was migrating landward and the shelf environment had been recently flooded. This traps sediment in estuaries and back barrier settings leaving the shelf area lacking in a land-based sediment source.
Erosive forces related to tidal currents and wave action result in the cannibalization of the substrate producing a new sediment supply to the shelf setting.
The Cocoa sand ridge deposits described here constitute the expression of the transgressive systems tract in a shallow marine environment. The transgressive system tract is defined as sediments immediately overlying the transgressive surface with a diagnostic feature of retrogradational stacking pattern in bedding (Coe et al. 2003). When the shoreline is migrating landward, erosion of the North Twistwood Creek member starts supplying sand for the Cocoa Sand Member as the shelf environment is first significantly flooded. During this time, the newly flooded area is subjected to a variety of processes, such as wave action, tidal currents, and other shelf currents, that can significantly erode previously deposited sediments. The erosion of the substrate results in a temporary addition to the overall sediment supply in the depositional environment. The sediments that are being eroded from the substrate can locally be relatively coarse grained (Posamentier 2001). The Cocoa Sand ridge deposits show a retrogradational stacking pattern seen by the two small scale coarsening-upward sequences observed from the core sample, well log analysis, and the grain size analysis of the #1 Young core (Figure 17&18). The retrogradational stacking pattern specifically appears in the #1 Young core and its associated well log expression of the Cocoa Sand Member and exhibits landward

stepping in the cross sections (Figure 27). In this case, because the accommodation space is
63 greater than the rate of sediment supply, as the paleo-shoreline moved in a shoreward direction, the stacking-pattern seen in the Cocoa Sand Member is inferred to have been retrograded or backstepped. It is important to note that the transgressive systems tract can consist of a succession of small scale regressive events arranged in a progressively backstepping patterns since successive periods or events of relative sea-level changes result in the deposition of detached deposits (Posamantier et al.1999). The Cocoa Sand ridges are an example of detached deposits formed during a transgression. Sufficient wave energy significantly altered the original sea floor morphology to form the Cocoa Sand ridges.
These Linear sand ridges, up to
65 feet (21.67 m) thick, extending to the southeast in a retrogradational stepping back pattern supports this transgressive landward stepping in the shelf depositional environment perpendicular to the paleo-shoreline.
Three stages are suggested to describe the depositional model of the Cocoa Sand
Member (Figure 30). Stage one, during sea-level fall, the sediments of the North Twistwood
Creek Member were subaerially exposed forming soil horizons up to one foot thick as reported by Tew et al. (1992). Stage two, during transgression, the North Twistwood Creek Member was eroded by current related processes. With rising sea level, the sandy upper interval of North
Twistwood Creek Member was reworked and eroded by tidal currents and wave action. This resulted in the deposition of rip-up clasts as the base of the Cocoa Sand Member. Continued erosion and reworking of the sediments resulted in the winnowed, sandy sediments gradually migrating toward the new shoreline and starting to pile up in the formation of sand ridges.
During landward migration of the Cocoa Sand sediments, the rate of increase in

64 accommodation space is greater than the rate of sediments supply resulting in a retrogradational stacking pattern as the thickness changes in the cross section D-D’,E-E’,F-F’
(Figure 25, 26 &27). In this case, the paleo-shoreline has moved in a landward direction as Tew et al. (1995) suggested, and there is a retrogradational stacking package occurring at the same time. In Stage three, the Cocoa sand bodies continue to grow and migrate shoreward combining with some individual sand bodies to form larger sand bodies. This results in mud filled swales, deposited in areas of quiet water, being buried by the migrating sand. This is seen in the core, log, and grain size analysis, and illustrated in Figure 30.

65
Figure 30 Depositional model of the Cocoa Sand Member

66
CONCLUSIONS
1.
The Cocoa Sand Member, in general, is a coarsening –upward sequence.
In parts of the study area, it consists of two small parasequences of about 14 feet thick each. The stacking pattern of this parasequence is retrogradational caused by increasing accommodation space that was greater than the sediment supply during transgression.
2.
The thickness trend of the Cocoa Sand Member is variable based on the isopach map and cross sections. The Northwest-southeast thickness trend shown on the isopach map indicates the Cocoa Sand Member piles up towards the southeast direction and is thickest near the state boundary line of Mississippi and Alabama. Cross Sections
A-A’, B-B’ and C-C’ support this thickening trend. Variations in thickness of the Cocoa
Sand distribution is best illustrated in the 3D isopach map (Figure 20). The thickest part is around 65 feet (21.67 m) in the state boundary line of Mississippi and Alabama.
3.
Two linear, elongated in a northwest –southeast direction, sand ridges are present in the isopach map, which are parallel to the paleo-shoreline (Figure 19).
Limited subsurface data from Alabama prevents further estimates of the sand ridges thickness into Choctaw and Washington Counties, but the Cocoa Sand ridges could continue southeast across the state line into Alabama. Associated with six cross sections, there is a landward migrating backstepping stacking pattern of the Cocoa Sand Member during relative sea-level rise.
4.
Three stages are suggested to describe a depositional model for the
Cocoa Sand Member of the Yazoo Formation. After the reworking and erosion of the
North Twistwood Creek Member during sea-level rise, the Cocoa Sand Member is

67 formed on the top of underlying mudstone above the transgressive surface of the North
Twistwood-Cocoa Sand contact. Mud filled swales formed in areas of quiet water associated with the sand ridges and were later buried as migrating ridges coalesced into larger sand bodies.

68
REFERENCES
Baum, G. R., and Vail, P. R., 1986, Sequence stratigraphy of the Eocene carbonates of the
Carolinas, in Harris, W.B., Zullo, V.A., and Otte, L.J. eds., Eocene Carbonate Facies of the North
Carolina Coastal Plain: SEPM Field Guidebook, Southeastern United States, p. 264-269.
Baum, G. R., and Vail, P.R., 1987, Sequence stratigraphy, allostratigraphy, isotope stratigraphy and biostratigraphy, putting it all together in the Atlantic and Gulf Paleogene, in C. L. McNulty, and Waterman, A.S., eds., Innovative biostratigraphic approaches to sequence analysis; new exploration opportunities: Eighth annual research conference, Gulf Coast Section, SEPM
Foundation, Houston, Texas, p.15-23.
Baum, G. R., and Vail, P.R., 1988, Sequence stratigraphic concepts applied to Paleogene outcrops, Gulf and Atlantic basins, in C. K. Wilgus, Hastings, B.S., Ross, C.A., Posamentier, H.,
Van Wagoner, J., and Kendall, eds., C.G.S.C.: Sea-level changes, SEPM Special Publication, v.42, p. 309-327.
Berne, S., 1996, Donghai Cruise, preliminary report, Brest, France: French Institute of Research and Exploitation of the Sea, p.74.
Boggs, S. J., 1995, Principles of sedimentology and stratigraphy: Prentice Hall, 585p.
Brissette, N. O., 2004, The Cocoa Sand Member of the Yazoo Formation (Eocene), Mississippi: A petrologic and depositional model study, Ball State University, 93p.
Bybell, L. M., 1982, Late Eocene to early Oligocene calcareous nannofossils in Alabama and
Mississippi: Gulf Coast Association of Geological Societies Transactions, 32. p. 295-302.
Coe, A. L., Bosence, D. W.J, Church, K. D., Flint, S. S, Howell, J. A. and Wilson, R. L., 2003, The sedimentary record of sea-level changes: Cambridge University Press, 288p.
Cooke, C. W., 1918, Correlation of the deposits of Jackson and Vicksburg ages in Mississippi and
Alabama: Journal of the Washington Academy of Sciences, v. 8, p.186-198.
Cooke, C. W., 1933, Definition of Cocoa Sand Member of Jackson Formation: Bulletin of the
American Association of Petroleum Geologists, v.17, Issue 11, p.1387-1393.
Deboo, P. B., 1965, Biostratigraphic correlation of the type Shubuta Member of the Yazoo Clay and Red Bluff Clay with their equivalents in south-western Alabama: Geological Survey of
Alabama Bulletin 80, p.84.
Diaz, J. I., and Maidonado, A., 1990, Transgressive sand bodies on the Maresme continental shelf, western Mediterranean Sea: Marine Geology, v. 91, p.53-72.
Dockery III, D. T. I., and Siesser, William G., 1984, Age of the Upper Yazoo Formation in Central
Mississippi: Mississippi Geology, v.5, Issue 1, p. 1-10.

Dockery, D. T. I., 1990, The Eocene-Oligocene boundary in the northern Gulf-A sequence boundary, in Armentrout, J.M., Coleman, J.M., Galloway, W.E., and Vail, P.R., eds., Sequence
Stratigraphy as an Exploration Tool-Concepts and Practices in the Gulf Coast: Eleventh Annual
Research Conference, Gulf Coast Section, Society of Economic Paleontologists and
Mineralogists Foundation, Programs and Extended Abstracts, p.141-150.
69
Dockery, D. T. I., Thompson, David E., and Ingram, Stephen L., 1994, The Mobil-Mississippi
Office of Geology Core-Hole Project: Mississippi Geology, v.15, Issue 1, p. 8-15.
Echols, R. J., Armentrout, J. M., Root, S. A., Fearn, L. B., Cooke, J. C., Rodgers, B. K. and
Thompson, P. R., 2003, Sequence Stratigraphy of the Eocene/Oligocene Boundary Interval: southeastern Mississippi: in D. R. Prothero, Ivany, L. C., Nesbitt, E. A., eds.
, From greenhouse to icehouse : the marine Eocene-Oligocene transition : Columbia University Press, p.189-222.
Fluegeman, R. H., 1996, Preliminary Paleontological report of the Foraminifera of the Mossy
Grove Core, Hinds County, Mississippi: Mississippi Geology, v. 17, Issue. 1, p. 9-15.
Fluegeman, R. H., 2003, Late Eocene-Early Oligocene Benthic Foraminifera in the Gulf Coastal
Plain: regional vs. Global Influences: in D. R. Prothero, Ivany, L. C., Nesbitt, E. A., eds., From greenhouse to icehouse : the marine Eocene-Oligocene transition: Columbia University Press, p.283-293.
Fluegeman, R. H., Balakrishnan, A., Banser, C. J., McGuire, A. K., Tiedemann, N. S., Wolfe, D. M.,
Crume, M. A., Thurber, D. C. and Funk, B. A., 2008, Comparison of Two Planktonic Benthonic
(P:B) Curves from the Yazoo Clay (Eocene; Priabonian) of Mississippi: Gulf Coast Association of
Geological Societies Transactions 58, p.287-296.
Fluegeman, R. H., and Grigsby, J. D., 2009, Eocene-Oligocene greenhouse to icehouse transition on a subtropical clastic shelf: The Jackson-Vicksburg Groups of the eastern Gulf Coastal Plain of the United States: The Geological Society of America Special Paper 452, p.261-277.
Fluegeman, R. H., 2009, Late Eocene and early Oligocene benthic foraminiferal paleoecology and sequence stratigraphy in the eastern Gulf Coastal Plain, U.S.A.: SEPM Special Publication, v.93, p. 293-307.
Gilliland, W. A., and Harrelson, D. W., 1980, Clarke County Geology: Mississippi Department of
Natural Resources, Bureau of Geology, 306 p.
Hurley, J. V., and Fluegeman R.H., 2003, Late Middle Eocene Glacioeustacy: Stable Isotopes and
Foraminifera from the Gulf Coastal Plain.
in D. R. Prothero, Ivany, L. C., Nesbitt, E. A., eds., From greenhouse to icehouse : the marine Eocene-Oligocene transition: Columbia University Press, p.223-231.
Huthnance, J. M., 1982, On one mechanism forming linear sand banks: Estuarine, Coastal and shelf Science, v.14, p.79-99.

70
Jonhson, H. D., and Baldwin C. T., 1986, Shallow siliciclastic seas, in Readings, H. G., eds.,
Sedimentary Environments and Facies, Blackwell Scientific Publications, Oxford, 229-282
Keller, G., 1985, Eocene and Oligocene stratigraphy and erosional unconformities in the Gulf of
Mexico and Gulf Coast: Journal of Paleontology, v. 59, Issue.4, p.882-903.
Kenyon, N. H., Belderson, R. H., Stride, A. H., and Johnson, M. A.,1981, Offshore tidal sandbanks as indicators of net sand transport and as potential deposits: in S. D. Nio, Schuttenheim,
R.T.E., and Van Weering, T.C.E., eds., Holocene Marine Sedimentation in the North Sea Basin,
Special Publication 5, p.257-268.
Krutak, P. R., 1961, Jackson Eocene ostracoda from the Cocoa Sand of Alabama: Journal of
Paleontology, v. 35, Issue 4, p.769-788.
Liu, Z., Xia, D., Berne, S., Wang, K., Marsset, T., Tang, Y., and Bourillet, J. F., 1998, Tidal depositional systems of China's continental shelf, with special reference to the eastern Bohai
Sea: Marine Geology, v.145, p.225-253.
Mancini, E. A., 1979, Eocene-Oligocene boundary in Southwest Alabama: Transactions-Gulf
Coast Association of Geological Societies 29, p.282-289.
Mancini, E. A., and Water, Laura A., 1986, Planktonic foraminiferal biostratigraphy of upper
Eocene and lower Oligocene strata in southern Mississippi and southwestern and south-central
Alabama: Journal of Foraminiferal Research, v. 16, Issue 1, p.24-33.
Mancini, E. A., Tew, Barry H., and Water, Laura A., 1987, Eocene-Oligocene boundary in southeastern Mississippi and southwestern Alabama--a stratigraphically condensed section of type 2 depositional sequence: Special Publications-Cushman Foundation for Foraminiferal
Research, v. 24, p.41-50.
Mancini, E. A., 2000, Sequence Stratigraphy and chronostratigraphy of upper Eocene and Lower
Oligocene strata in Eastern Gulf Coastal Plain: Transactions-Gulf Coast Association of Geological
Societies, v.50, p.379-388.
Mancini, E. A. and Copeland., C W., 1986, St. Stephens Quarry (Lone Star Cement Company
Quarry), St. Stephens, Washington County, Alabama, where is a near complete Oligocene section, including the Eocene-Oligocene boundary, is exposed: Centennial Field Guide, p.373-
378.
Mancini, E. A. and Tew, B. H., 1991, Relationship of Paleogene stage and planktonic foraminiferal zone boundaries to lithostratigraphic and allostratigraphic contacts in eastern Gulf
Coastal Plain: Journal of Foraminiferal Research, v.21, Issue.1, p.48-66.
Mancini, E. A. and Tew, B. H., 1994, Claiborne-Jackson Group contact (Eocene) in Alabama and
Mississippi: Transactions of the Gulf Coast Association of Geological Societies, v.44, p.431-439.
May, J. H., 1974, Wayne County Geology: in May, J.H., eds., Wayne County Geology and Mineral resources: Mississippi Geological, Economic and Topographic Survey, Bulletin 105, p.13-194.

71
Miller, K. G., Browning, J. V., Aubry, M., Wade, B. S., Katz, M. E., Kulpecz, A. A., and Wright J. D.,
2008, Eocene-Oligocene global climate and sea-level changes: St. Stephens Quarry, Alabama:
Geological Society of American Bulletin, v.120, Issue.1, p.34-53.
Moore, W. H., 1974, Wayne County: Geology and mineral resources, Jackson, Mississippi:
Mississippi Geological, Economic and Topographical Survey, 293p.
Murray, G. E., 1947, Cenozoic deposits of central Gulf Coastal Plain: American Association of
Petroleum Geologists Bulletin, v.31, p.1825-1850.
Obradovich, J. D., Dockery III, D. T., and Swosher III, C.C., 1993, 40 AR39 AR ages of bentonite beds in the upper part of the Yazoo Formation (Upper Eocene), west-central Mississippi:
Mississippi Geology, v.14, No.1, p.1-9.
Off, T., 1963, Rhythmic linear sand bodies caused by tidal currents: American Association of
Petroleum Geologists Bulletin, v.47, p.324-341.
Park, S., Han, H., and Yoo, D., 2003, Transgressive sand ridges on the mid-shelf of the southern sea of Korea (Korea Strait): formation and development in high energy environments: Marine
Geology, v.193, p.1-18.
Pasley, M. A., and Hazel, J. E., 1995, Revised sequence stratigraphic interpretation of the
Eocene-Oligocene boundary interval, Mississippi and Alabama, Gulf Coast Basin, U.S.A.: Journal of Sedimentary Research, Section of Stratigraphy and Global Studies, v.65, Issue.1, p.160-169.
Posamentier, H. W., 2002, Ancient shelf ridges-A potentially significant component of the transgressive systems tract: Case study from offshore northwest Java: American Association of
Petroleum Geologists Bulletin, v. 86, Issue.1, p.75-106.
Posamentier, H. W. a. A., G. P., 2000, Siliciclastic Sequence Stratigraphy: Concepts and
Applications, Society for Sedimentary Geology.204p.
Siesser, W. G., 1983, Paleogene calcareous nannoplankton biostratigraphy-Mississippi, Alabama, and Tennessee: Bureau of Geology Bulletin, Mississippi Department of Natural Resources, 61p.
Snedden, J. W., Tillman, R. W., Kreisa, R. D., Schweller, W. J., Culver, S. J., and Winn, R. D, 1994,
Stratigraphy and genesis of a modern shoreface-attached sand ridge, Peahala ridge, New Jersey:
Journal of Sedimentary, v.B64, p.560-581.
Snedden, J. W., and Dalrymple, Robert W., 1999, Modern shelf sand ridges: form historical perspective to a unifies hydrodynamic and evolutionary model: in K. M. Bergman, and Snedden,
J.W., eds., Isolated Shallow Marine Sand Bodies: Sequence Stratigraphic Analysis and
Sedimentologic Interpretation, SEPM Special Publication, p.191-204.
Stainforth, R. M., J. L. Lamb, H. Luterbacher, J. H. Beard, and R. M. Jeffords, 1975, Cenozoic planktonic foraminiferal zonation and characteristics of index forms: University of Kansas
Paleontological Contributions, Article 62, p.425.

72
Swift, D. J.P, and Hudelson, P.M , 1987, Shelf construction in a foreland basin: storm beds, shelf sandbodies and shelf-slope depositional sequences in the upper Cretaceous Mesaverde Group,
Brook Cliff, Utah: Sedimentology, v.34., p.423-457.
Tew, B. H., and Mancini, E. A., 1992, An integrated lithostratigraphic, biostratgraphic, and sequence stratigraphic approach to paleogeographic reconstruction: examples from the upper
Eocene and lower Oligocene of Alabama and Mississippi: Gulf Coast Association of Geological
Societies Transactions, v.42, p.735-756.
Tew, B. H., and Mancini, E. A., 1995, An integrated stratigraphic method for paleogeographic reconstruction: examples from the Jackson and Vicksburg Groups of the Eastern Gulf Coastal
Plain: Palaios, v.10, p.133-153.
Tillman, R. W., 1985, A spectrum of shelf sands and sandstone: in R. W. Tillman, Swift, D. J.P., and Walker, R. G., eds., Shelf sands and sandstone reservoirs: SEPM Short Course, v.13, p.1-46.
Toulmin, L. D., 1977, Stratigraphic distribution of Paleocene and Eocene fossils in the eastern
Gulf Coast region: Monograph-Geological Survey of Alabama, v. 1, Issue.13, p. 601-602.
Turnbull, W. D.,1948, Cocoa Sand Type Locality: Journal of Paleontology, v.22, p.372.
Van der Zwaan, G. J., Jorissen, F. J. and de Stigter, H. C., 1990, The depth dependency of planktonic/benthic foraminiferal ratios: Marine Geology, v. 95, p.1-16.
Wagle, B. G., and Veerayya, M., 1996, Submerged sand ridges on the western continental shelf off Bombay, India: evidence for late Pleistocene-Holocene sea-level changes: Marine Geology, v.136, p.79-95.
Water, L. A., and Mancini, Ernest A., 1982, Lithostratigraphy and biostratigraphy of upper
Eocene and lower Oligocene strata in southwest Alabama and southeast Mississippi:
Transactions-Gulf Coast Association of Geological Societies, v.32, p.303-307.
Yang, C. S., Sun, J. S., 1988, Tidal sand ridges on the East China Sea shelf: in Reidel, Dordrecht, P.
L. De Boer, Van Gelder, A., Nio, S.D., eds., Tide-influenced Sedimentary Environments and
Facies, p.23-38.
Yang, C. S., 1989, Active, moribund and buried tidal sand ridges in the East China Sea and the southern Yellow Sea: Marine Geology, v. 88, p.97-116.