Prof. Robert H. Blodgett
Austin Community College
GEOL 1404
Laboratory Exercise
Well Core Project
Safety
1. Dilute hydrochloric acid (HCl)
 Only use one drop for each test
 Rinse specimen after testing and wash rinse water into a beaker or sink
 Dry specimen with a paper towel
 Close dropper bottle lid when not in use
 Wash hands or skin if you get acid on it
 Tell the professor or lab technician if an acid bottle is leaking, has droplets on it, or if the
label has been defaced or removed
2. Binocular microscope
 Do not touch light source after it has been turned on
 Disconnect the light source by pulling on the plug not the electrical cord
Goals
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Learn to use a logging form to describe and interpret subsurface core
Correlate a geophysical log with subsurface rock types
Synthesize observations of rock type, textures and structures, fossils, and mineralogy
Formulate hypotheses about the sedimentary depositional environment
Work with a team to collect and interpret scientific data, and communicate the results of a
scientific study to other researchers in a group presentation
Write a concise and scholarly individual scientific report
Tasks
1. Core and Geophysical Log Analysis – describe and interpret part of a rock core and
geophysical log taken from the Edwards aquifer monitoring well on the Austin Community
College (ACC) Northridge Campus. Use a hand lens, binocular microscope, dilute HCl, a
water spray bottle, ruler, and meter stick to make observations of your interval of the well
core. Compare the rock types that you observe in the core with same interval on the
geophysical log.
2. Graphic Log – use the logging form provided by your professor to complete a graphic log of
your assigned interval of rock core. Each group will turn in one graphic log for their interval
of core. Below is an example of a graphic log of the Glen Rose Formation on Mt. Bonnell
(Figure 1).
3. Group Presentation – as a team, give a 10-minute presentation of the results of your study
to the entire class. The presentation will take place at your laboratory table where the group
can point out features in the core that support their hypotheses.
4. Individual Report – summarize the observations and write your personal interpretation of
the depositional environment for the assigned part of the core. This report must have a title
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page, be double spaced on 8.5 X 11 inch paper, use 12-point font, and have at least 1-inch
margins. Additional information may be included as tables and captioned figures following
the format that is used in your textbook. Any ideas, conclusions, illustrations, or text taken
from published or electronic reports must be credited to the original author(s) following the
format used in your textbook. Cited references should be listed on a References Cited page
following the format of your textbook. Read the ACC guidelines for documentation and
plagiarism (http://library.austincc.edu/help/documen/) to ensure that you are giving proper
credit.
Core
Core for this exercise was taken with a mud rotary drilling rig operated by MHC Xploration, Inc
in November 1998. Alcoa’s Sandow Mine in Rockdale and MHC Xploration in Tyler donated
this well to ACC and the Texas Bureau of Economic Geology (BEG) donated preparation of the
core. In drilling the well, 120 feet of 3"-diameter core was taken from the Upper Cretaceous
Atco, Eagle Ford, and Buda formations. The cylindrical core was then cut in half with a
diamond saw at the BEG’s Austin Core Research Center on the J.J. Pickle Research Campus of
the University of Texas at Austin. Half of the core is stored at the BEG for scientific study. You
will examine and describe part of the other half of this core.
The well core is stored in waxed cardboard boxes that are labeled with depths below the land
surface. Each box contains three, 3-foot sections of core with the total interval for the box
indicated on a white label at one end of the box. The interval for each section of core is also
written on the top and bottom of the inside of each box. Also, the top of each piece of core is
numbered to indicate the 3-foot interval to which it belongs. It is important to keep the pieces of
the core in their proper order and orientation within the box.
Preliminary study indicates that the following stratigraphic formations are present in the core:
12 to 79 ft. - Atco Formation of the Austin Group
79 to 125 ft. - Eagle Ford Formation of the Woodbine Group
125 to 154 ft. - Buda Formation of the Washita Group
Attached is a summary of the stratigraphy of these formations from a report on the
environmental geology of the Austin area (Garner and Young, 1976).
In studying the core you should focus on changes in rock type (a.k.a. lithologic changes), color,
sedimentary structures, types of body and trace fossils, mineralogy, and the degree of
bioturbation (i.e. amount of burrowing indicated by a lightning bolt symbol).
Observation and Testing of the Core
Pieces of the core can be removed from the boxes for viewing under a binocular microscope. If
you do this be sure to return the core to its original position in the box and protect the glass stage
of the microscope from scratches.
Water should only be sprayed on chalk or limestone in the core. Do not spray or otherwise place
water on mudrock. Many of the mudrock layers contain expandable clay that will swell when it
gets wet. Applying water to the clay-rich core will cause it to fall apart.
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The core can also be tested with dilute HCl on the outer, curved surface. If you do perform a
HCl test be sure to use only one drop of dilute acid and clean up the spot with water after you
have observed the results. Do not drop acid on the smooth face of the core.
Finally, a great deal of information can come from observations of the “butt” ends of each piece
of core because they are freshly broken during the coring process. Note that some sweeping
marks on the butt end and flat surface of the core have come from the diamond saw.
Geophysical Log
Geo Cam, Inc. of San Antonio donated a geophysical and video log of the well to the College.
Instrumented probes (a.k.a. logging tools) were lowered into the well to produce the geophysical
log. The logging tools that GeoCam used measured:
1. Temperature of water (Temp) measured in degrees Celsius
2. Width of the borehole (Caliper) measured in inches
3. Natural gamma radiation (Gamma) from the rock measured in counts per second (cps); Th
gamma log may be the most useful log for your core interpretation.
4. Resistance of the fluid (Fres) in the well to an electrical current measured in ohm-m
5. Four measurements (R8, R16, R32, and R64) of the resistance of the formation (resistivity)
to an electrical current are given in ohm-m. The spacing of electrodes on the logging tool
controls how far into the rock that the resistance is measured (8", 16", 32", and 64") for the four
logs.
You will use the Gamma log for this exercise. In general, radioactivity increases in sedimentary
deposits with an increase in the content of organic matter or an increase in the detrital content
from igneous sources such as volcanic ash or weathered intrusive or extrusive rock.
Other Resources
Your professor has numerous articles on the geology of these formations and on more general
aspects of Cretaceous fossils, trace fossils, paleoecology, and volcaniclastic sedimentation.
These articles are included in the reference list at the end of this exercise. You may want to
borrow several of these to take home overnight to read. Also attached is a chart summarizing the
Cretaceous chronostratigraphy of the Austin area. Other books on Austin-area geology are on
reserve in the Learning Resource Center.
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Figure 1. Graphic log of the Glen Rose Formation, Mt. Bonnell, Austin, Texas.
Measurements are in feet and the degree of bioturbation is shown by the number of lightning bolt
symbols. Modified from Loucks and others (1978).
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Stratigraphic Units in the Edwards Aquifer Monitoring Well Core
From Garner and Young (1976):
BUDA FORMATION
The Buda Formation consists of an upper hard, resistant, fine-grained, burrowed, glauconitic,
shell-fragment limestone and a lower marly, nodular, and less resistant limestone. Total
thickness of the Buda in the Austin area is about 35 feet, but the unit thins northward. Freshly
broken surfaces of the Buda are characteristically colored shades of tan to orange-brown that
resemble discolorations caused by heating. Many early descriptions of this unit termed it the
"burnt" limestone (Hill and Vaughan, 1902). Scattered outcrops of the Buda Formation
occur from the area near Mount Bonnell southward to the vicinity of the town of Buda.
EAGLE FORD FORMATION
In the Austin area the Eagle Ford Formation comprises four members: from bottom to top,
the Pepper shale, the Cloice shale, the Bouldin flags, and the South Bosque shale. These
members are not mapped separately.
The South Bosque member is a calcareous, marly claystone which is primarily
montmorillonitic clay. This member ranges from 21 to 25 feet thick in the Austin area.
The Bouldin flags member constitutes about 11 feet of interbedded montmorillonitic shale
and limestone flags. The flag beds range from 4 inches to 1 foot thick, and some of them are
discontinuous. Several small bentonite beds (from 0.5 to 3 inches thick) are present.
Northwards, the Bouldin flags thickens to about 15 feet.
The Cloice shale is a fissile, gray, montmorillonitic, silty formation. The shale is about 11
feet thick in the Austin area and thickens to the north.
The Pepper shale is a montmorillonitic, non-calcareous, unctuous, black claystone, which on
the weathered surfaces contains much selenite and jarosite. Since the Pepper has very little
material admixed with the clay, it is a structurally unstable unit. The Pepper ranges from
about 1 foot thick at the Travis-Hays county line to around 5 feet thick near Round Rock.
AUSTIN GROUP
The Austin Group consists of about 350 feet of light gray chalk, limestone, marly limestone,
and marl generally referred to as the Austin chalk. Formations recognized within this
interval are, from base to top, Atco, Vinson, Jonah, Dessau, Burditt, Pflugerville, and
McKown. These units are not mapped separately.
The Atco Formation is mostly a chalky limestone alternating with thin beds of clayey
limestone and marl. Thickness is about 125 feet.
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Cretaceous Chronostratigraphy of Austin, Texas
Ma
________
Austin Group
Pflugerville Formation
Burditt Marl
Dessau Chalk
Jonah Formation
Vinson Chalk
Atco Formation
______________
Campanian Stage
?--- 83.5 +0.7 -------------------________ 85.8 +0.7 Santonian Stage
Coniacian Stage
________ 89.3 +1.0 ______________
Woodbine Group
Eagle Ford Formation
South Bosque Member
Bouldin Member
Cloice Shale Member
Pepper Shale Member
Turonian Stage
?--- 93.5 +0.8______________
Cenomanian Stage
________ 95.1
Washita Group
Buda Formation
Del Rio Formation
Upper Cretaceous Series
______________99.6 +0.9______________
Lower Cretaceous Series
Georgetown Formation
Fredericksburg Group
Edwards Formation
Comanche Peak Formation
Albian Stage
Walnut Formation
Keys Valley Marl Member / Whitestone Limestone Member
Cedar Park Limestone Member
Bee Cave Marl Member
Bull Creek Limestone Member
Trinity Group
Glen Rose Formation
------- 112.0 +1.0----------------------Aptian Stage
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References
Bralower, Timothy J. 2008. Volcanic cause of catastrophe: Nature, v. 454, p. 285-287.
Byerly, Gary R. 1991. Igneous activity, in Salvador, A., ed., The Gulf of Mexico basin: Boulder,
Colorado, Geological Society of America, The Geology of North America, v. J,
p. 91-108.
Charvat, William A., and Grayson, Robert C. 1981. Anoxic sedimentation in the Eagle Ford
Group (Upper Cretaceous) of central Texas: Abstract, Transactions, Gulf Coast
Association of Geological Sciences, v. 31, p. 256.
Dawson, William C. and Donald F. Reaser, 1985. Ichnology and paleoenvironments of the
middle and upper Austin Chalk (Upper Cretaceous), northeastern Texas, in McNulty,
Charles L. and John G. McPherson, eds., Transactions, Southwest Section – A.A.P.G.,
1985 Convention, p. 47-67.
Dravis, Jeffery James, 1979. Sedimentology and diagenesis of the Upper Cretaceous Austin
Chalk Formation, South Texas and northern Mexico: Houston, Texas, Rice University,
Ph.D. dissertation, 513 p.
Ekdale, A.A. and Mason, T. R. 1988. Characteristic trace-fossil associations in oxygen-poor
sedimentary environments: Geology, v. 16, p. 720-723.
Ewing, Thomas E. and S. Christopher Caran. 1982. Late Cretaceous volcanism in south and
central Texas – stratigraphic, structural, and seismic models: Transactions, Gulf Coast
Association of Geological Sciences, v. 32, p. 137-145.
Fisher, R.V. and Schmicke, H.-U. 1984. Pyroclastic rocks: New York, Springer-Verlag, 472p.
Garner, L.E., and Young, K.P. 1976. Environmental geology of the Austin area: An aid to urban
planning: Austin, University of Texas Bureau of Economic Geology Report of
Investigations No. 86, 39 p.
Hill, R.T., and Vaughan, T.W. 1902. Description of the Austin quadrangle: U.S. Geological
Survey Atlas, Austin Folio, No. 76, 8 p.
Liro, Louis M., Dawson, William C., Katz, Barry J., and Robison, Vaughn D. 1994. Sequence
stratigraphic elements and geochemical variability within a “condensed section”: Eagle
Ford Group, east-central Texas: Transactions, Gulf Coast Association of Geological
Sciences, v. 44, p. 393-402.
Loucks, R.G., Scott, A.J., Bebout, D.G., and Mench, P.A. 1978. Lower Cretaceous carbonate
tidal facies of central Texas: Austin, University of Texas Bureau of Economic Geology
Research Note 10, 45 p.
Moore, Carl A. 1963. Handbook of subsurface geology: New York, Harper & Row Publishers,
235 p.
Phipps, Stephen Paul. 1988. Deep rifts as sources for alkaline intraplate magmatism in eastern
North America: Nature, v. 334, p. 27-31.
Simonson, Bruce M. 2003. Petrographic criteria for recognizing certain types of impact
spherules in well-preserve Precambrian successions: Astrobiology, v. 3, p. 49-65.
Simonson, Bruce M. and Glass, Billy P. 2004. Spherule layers–records of ancient impacts:
Annual Review of Earth and Planetary Sciences, v. 32, p. 329-361.
Turgeon, Steven C. and Creaser, Robert A. 2008. Cretaceous oceanic anoxic event 2 triggered by
a massive magmatic episode: Nature, v. 454, p. 323-326.
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