Skoorsteenberg Formation, Permian, South Africa: "deep-sea turbidite"
reservoir analog reinterpreted as lake-shelf hyperpycnites
Roger Higgs, Geoclastica Ltd, 5 Breakwater Road, Bude, Cornwall EX23 8LQ, UK
Manuscript submitted 2009. Rejected.
PREAMBLE, 25/4/14
The sedimentologically renowned Skoorsteenberg outcrops are remote and largely on private land.
Consequently, very few sedimentologists are privileged to evaluate, in the field, the popular (but never
convincingly demonstrated) deep-sea interpretation of these flat-lying, foreland-basin strata.
Nevertheless, numerous oil companies have become persuaded that the Skoorsteenberg is an excellent
“outcrop analog” for exploring and developing deep-sea-turbidite reservoirs in ultra-high-cost operating
areas (e.g. deep water offshore Africa, Brazil, Gulf of Mexico), despite the vastly different tectonic setting
(supra-continental foreland basin versus supra-oceanic passive margin).
This unpublished contribution is based on: (1) the author’s careful observations during a 3-day AAPG field
trip in 2008; (2) a thorough review of the Skoorsteenberg literature; (3) 25 years (so far) studying
turbidites and tempestites worldwide for a living and for pleasure; and (4) intimate familiarity with the
Skoorsteenberg-lookalike Bude Formation (Carboniferous, UK), exposed in sea cliffs in exquisite wavepolished detail (unlike the Skoorsteenberg). The manuscript, emanating from a 2008 lecture at a
specialist AAPG conference in Argentina on hyperpycnites, was offered in 2009 for publication in the
conference book (released 2012). Unconventionally the senior editor chose, as main reviewer of my
manuscript, the hardly impartial co-originator of the Skoorsteenberg deep-sea-fan interpretation, who
strongly urged rejection, with which the editor emphatically agreed.
The author believes that the sedimentological community should be aware of this alternative, shallowwater (lake shelf) view of the Skoorsteenberg “deep-sea” turbidites, as should oil companies who (A)
funded most of the Skoorsteenberg research, including expensive behind-the-outcrop boreholes, (B)
finance the visits of hundreds of their staff geologists and geophysicists, and dozens of students, and (C)
run the billion-dollar risks of using improper outcrop analogs (e.g. misplaced boreholes; unwarranted
project development or abandonment).
To the list of other Skoorsteenberg lookalikes in the text can be added the Jackfork Formation
(Carboniferous, USA), likewise visited by hundreds (thousands?) of petroleum geologists who are taught
that this is an outcrop analog for passive-margin, deep-sea-turbidite reservoirs, notwithstanding an early
shallow-water interpretation, reported presence of possible HCS, and precious few fossils (again
suggesting a lake). Prominent among Jackfork deep-sea advocates is the aforementioned editor, who out
of courtesy I prefer not to name here.
For more on reinterpreting the “deep-sea” Skoorsteenberg, Jackfork, etc, please see references at
http://www.geoclastica.com/RogerHiggsCV.htm (Higgs 2008, 2009b,d, 2010b,c, 2014a,b).
Roger Higgs, Bude, April 2014
2
ABSTRACT
Many oil companies use the Permian Skoorsteenberg Formation (Ecca Group) of the
Karoo Basin, South Africa, as an "outcrop analog" of deep-sea-turbidite petroleum
reservoirs in costly-development deep-water regions like the Brazil, West Africa and
Gulf of Mexico passive margins. However, a literature review and the author's field
observations suggest that the Skoorsteenberg is instead of shallow lacustrine orogin.
Wave-influenced sedimentary structures, indicating deposition above storm wave base,
are illustrated here from the Skoorsteenberg, as are ichnofossils which, together with
ichnogenera reported by previous authors, suggest hyposalinity, supporting the large
fresh-to-brackish lake proposed for the Ecca Group by previous workers. This sea-level
lake, here named "Lake Karoo", was susceptible to ocean-water wedge intrusion up the
outflow channel (i.e. over the sill; Bosporus outflow of modern Black Sea), resulting in
variable lake brackishness and water level tied to glacioeustatic fluctuations. Consistent
with low-salinity lake water, favoring river-fed underflows (hyperpycnal turbidity currents)
lasting for weeks during summer melting of alpine glaciers in the adjacent Cape orogen,
Skoorsteenberg sand beds are mainly thin (individually < 40 cm), non-laminated,
ungraded, and fine- or very-fine grained. These features suggest deposition as
hyperpycnites from sustained, depletive flows fast enough to carry pre-suspended
(settling) fine sand but too slow to move it tractionally. Beds with climbing ripple
lamination are interpreted as proximal equivalents. Beds with any of the structures listed
above (HCS, etc.) are wave-modified hyperpycnites. The envisaged environment was a
lake shelf on which the coeval, lookalike Laingsburg Formation was also deposited. The
shelf was constricted northward (i.e. strait of gulf) and was flanked to the south by an
inferred flysch trough (later upthrusted and eroded by northward advance of Swartberg
branch of the Cape orogen). Instead of basin-floor fans fed by slope channels, the
Skoorsteenberg is reinterpreted as shelf tongues fed by shallow (< 10 m) non-leveed
channels crossing a muddy inner shelf. Underflows from the west (Cederberg ranges)
underwent Coriolis veering leftward (southern hemisphere), to flow axially along the gulf
or strait. A laminated siltstone facies is interpreted as lofting rhythmites, more likely in
brackish than fresh water. The evidence for deposition in shallow, hyposaline water
makes the Skoorsteenberg an improper analog for deep marine turbidites whose
dissimilar processes (more surge-type turbidity currents; no waves; less lofting) would
produce fan lobes differing from their lake "counterparts" in shape, area, grain size,
Coriolis curvature, architecture and heterogeneity, and channels that are deeper, leveed
and more sinuous. The economic implications are profound. The same shallow
lacustrine interpretation applies to lookalike strata of the same age (PermoCarboniferous) in other Pangean foreland basins on three continents, also widely used
as supposed deep-sea-turbidite analogues: the Laingsburg, Ross, Bude and Brushy
Canyon formations. For sedimentologists and petroleum geologists, shelf hyperpycnites
and wave-modified hyperpycnites, whether lacustrine, as exemplified by these five
"Bude-type turbidite" formations, or marine (newly recognised in Cretaceous western
interior seaway, USA), represent a fourth great class of gravity-flow subaqueous event
beds, after fluvio-deltaic crevasse beds, conventional shelf tempestites and deep-sea
turbidites.
INTRODUCTION
3
Subaqueously deposited yet sparsely fossiliferous formations of the Ecca Group
(Permian), including the Skoorsteenberg, deposited in the Karoo foreland basin
(Johnson et al., 2006), are of historically controversial water depth (10s vs 100s m) and
salinity (fresh or brackish lake vs marine; Table 1). Nevertheless, oil companies
consider the Skoorsteenberg an "outcrop analog" for turbidite channel and fan
reservoirs deposited in the deep sea (100s-1000s m), as in oil- and gasfields offshore
Brazil, West Africa, the Gulf of Mexico and the North Sea (Sullivan et al., 2000, 2004;
Wickens & Bouma, 2000a; Hodgson et al., 2007a; Hodgetts et al. 2004; Bouma et al.,
2007a,c,d). The same applies to three equally controversial, lookalike formations of
identical facies association, coeval (Permo-Late Carboniferous) age and Pangean
foreland-basin setting: the Laingsburg, Ross and Brushy Canyon (South Africa, Ireland,
USA; Chapin et al., 1994; Sullivan et al., 2000, 2004; Martinsen et al., 2000; Lien et al.,
2003; Larue, 2004; Fugelli & Olsen 2005; Shew, 2007a; Pyles, 2007a,b, 2008)). These
and the Skoorsteenberg comprise no fewer than 28 of the 154 chapters in the recent
AAPG Atlas of Deep-Water Outcrops (Nilsen et al., 2007). The possible economic
consequences of inappropriate analogs are profound (see below).
Another Late Carboniferous foreland-basin lookalike, the Bude Formation of SW
England, is equally controversial (Burne, 1998; Higgs, 1998), with almost fifty years of
published sedimentological study: "The precise depositional environment of the Bude
Sandstones is not easy to establish" (Reading, 1963). Despite its easy accessibility and
superb cliff exposure, the Bude is the only one of the five formations, here termed
"Bude-type turbidites" given Bude's long history of sedimentological study, not widely
invoked as a deep-sea fan outcrop analog. The Bude has been interpreted as lacustrine
(Higgs, 1991), and also the Ross (Higgs, 2004); their likeness was highlighted by Higgs
(2004). Similarity has been pointed out between the Skoorsteenberg and the
Laingsburg, Ross and Brushy Canyon (Sullivan et al., 2000, 2004; Wickens & Bouma,
2000; Wild et al., 2005), and is borne out by the inclusion of articles on the
Skoorsteenberg, Laingsburg and Brushy in a compendium on "fine-grained turbidite
sytstems" (Bouma and Stone, 2000).
On a reconnaissance three-day field trip to the Skoorsteenberg Formation in
2008, the author examined three of the five sandy "submarine fans" recognized in this
formation (Bouma and Wickens, 1991; Wickens and Bouma, 1991) and found diverse
wave-influenced sedimentary structures in all three, as described below. The reader is
referred to Johnson et al. (2001) and Hodgson et al. (2006, 2008) for Skoorsteenberg
location information, "fan" stratigraphy, graphic logs and facies details. These
structures, including symmetrical ripples, negate the deep-water interpretation.
Symmetrical ripples were reported previously (Wach et al., 2000; Wild et al. 2005;
Hodgson et al. 2008), in the topmost sandy unit of the Skoorsteenberg, called "Fan 5
slope fan" by Johnson et al. (2001) but renamed "Unit 5" by Wild et al. (2005) after the
discovery of these ripples. Similarly, Laingsburg "Fans B to F" (Grecula et al., 2003)
were renamed "Units B-F" (Flint et al., 2007a,b). The ripples are problematical for the
deep-water fan model as they occur in strata less than 100 m above "Fan 4" (e.g.,
Hodgson et al., 2006, fig. 3), a difficulty noted by Wach et al. (2000, p. 173): "Evidence
for the transition from submarine fans to deltaic deposition has been enigmatic, with
limited evidence of sediments representative of ... (the intervening) ... slope". The
ripples were interpreted unclearly as storm-wave generated, in a "deep-water" lower
4
slope setting by Wild et al. (2005), contradicting the deltaic interpretation of Wach et al.
(2000).
The marine (salinity) interpretation of the Skoorsteenberg is in doubt too, based
on ichnofossils, also illustrated here for the first time, which suggest fresh or brackish
water, as proposed by numerous early workers from other evidence (Table 1).
GEOLOGICAL BACKGROUND
Age
A paucity of fossils renders uncertain the precise age of the Skoorsteenberg Formation,
near the middle of the Ecca Group. The Ecca is no younger than Middle Permian,
based on vertebrates in the overlying Beaufort Group (Rubidge et al. 2006). An
anomalous, Late Permian age was proposed by Fildani et al. (2007), based on
interpreted zircon U-Pb ages of 253-257 Ma from Skoorsteenberg ash beds. Four lines
of evidence disprove these U-Pb dates. (1) The lookalike Bude, Ross and Brushy
Canyon formations, of early Middle Permian (Roadian) and Pennsylvanian age (Higgs,
2004; Lambert et al., 2007), correspond to times of high-frequency glacioeustatic sealevel oscillations (cf. Haq & Schutter, 2008, fig. 3), so the Skoorsteenberg probably does
too (Goldhammer et al. 2000; Johnson et al. 2001; Hodgson et al. 2006), but such
oscillations are unknown in Late Permian time (Haq & Schutter, 2008, fig. 3; Rygel et al.
2008). (2) A 255 Ma age for the top of the Ecca was misattributed (Johnson et al. 2001;
van de Werff & Johnson 2003a; Sixsmith et al. 2004; Wild et al. 2005; Hodgson et al.
2006, 2007b; Fildani et al., 2007; Flint et al. 2007b) to Rubidge (1991), who in fact
assigned lowermost Beaufort fossils to the Ufimian or succeeding Kazanian/Roadian
stage. The Kazanian upper limit, indeed previously taken as 255 Ma (Haq et al. 1988),
is now 268.4 Ma (Gradstein et al. 2004). (3) Nonmarine bivalves in the uppermost Ecca
match those of the Estrada Nova Formation of Brazil (Cooper and Kensley, 1984),
dated as Roadian or younger (Iannuzzi et al., 2004). The combined fossil evidence
suggests a probable Roadian age for the top of the Ecca. This and a radiometric date
from the base of 269-271 Ma (Turner, 1999) suggest that the entire Ecca Group is
Roadian (base 270.6 Ma). The Skoorsteenberg is therefore almost certainly Roadian,
like the Brushy Canyon (Lambert et al. 2007). (4) An interval of 700 m and supposedly
15 m.y. separate this 269-271 Ma sample from the interpreted 253-257 Ma samples of
Fildani et al. (2007, fig. 2a). This implies subsidence of only 50 m/m.y., slow for a
foreland basin (typically 100-1,000 m/m.y.; Jordan, 1995), as opposed to 800 m/m.y. if
the entire Ecca Group in the Skoorsteenberg area (c. 1700 m; Fildani et al., 2007, fig.
2a) represents the whole Roadian (2.2 m.y.). Thus, the Fildani et al. (2007) ages are
rejected; the few grains suggesting these ages (in all samples most are older) may
reflect Pb contamination.
Tectonic setting
Most authors consider the Karoo a retroarc foreland basin, following Johnson (1991).
However, in view of evidence for outboard terrane accretion (López-Gamundí 1997), a
peripheral foreland basin is more likely.
Previous interpretations of salinity and water depth
5
Conflicting with the recently popularized deep-water (100s m) marine fan model for the
Skoorsteenberg Formation (Wickens & Bouma, 2000; Johnson et al., 2001; Hodgson et
al., 2006), most previous authors agree that the water body in which the Ecca Group
(including Skoorsteenberg Formation) accumulated was fresh or brackish for most of
the time (Table 1). With regard to water depth, there is a consensus in favour of depths
not exceeding 500 m (Table 1).
NEW EVIDENCE FOR SHALLOW WATER:
WAVE-INFLUENCED STRUCTURES
Various wave-influenced structures are described and illustrated below, indicating
deposition of the Skoorsteenberg Formation above storm wavebase (< 150 m, see
below), at least in part. Many other photographs are available on request. Deposition of
the Skoorsteenberg, and the entire Ecca Group, in shelf depths is consistent with the
stratigraphic context, with glaciomarine shelf facies below (Dwyka Formation; Visser
1997) and fluvial above (Beaufort Group; Johnson et al., 1996), and solves the enigma
(Wach et al., 2000) of minimal vertical space in which to "fit" a slope succession
between the Skoorsteenberg "fans" and overlying strata interpreted as shelfal and
deltaic (Kookfontein, Waterford formations; Hodgson et al., 2007b, 2008).
Hummocky cross stratification
The Skoorsteenberg Formation has many beds (cm-dm) with interpreted hummocky
cross stratification (HCS; Figs 1-3), at least in Fans 2 and 3, interspersed with nonlaminated, ungraded beds typical of Bude-type turbidites. Skoorsteenberg HCS may
have been misidentified previously due to wave- or seismically-induced liquefaction and
oversteepening; or missed because of faintness ("blurring") caused by rapid fallout from
suspension, possibly reflecting the particular, hyperpycnal supply mechanism (see
below); or overlooked by workers with deep-water preconceptions, or focused on largerscale, potentially seismically resolvable features (channel, lobe geometry). Structures
described as "Rare trough cross-bedding and scour-and-fill" and "local scour and
migrating bedform" (Johnson et al., 2001, table 1 and fig. 3F) may in fact be HCS, or
asymmetrical HCS (Nøttvedt and Kreisa, 1987).
HCS is also present in the lookalike Bude, Ross and Brushy Canyon formations
(Table 2; Higgs, 1991, 2004 and unpublished observations). Structures previously
identified in the Brushy and Ross as "Helmholtz waves", "trough", "scour-and-fill", "cutand-fill", "aggradational 'plow-and-fill'", "local scour and migrating bedform", "low-angle",
and "cross" stratification (Newell et al., 1953; Chapin et al., 1994; Zelt and Rossen,
1995; Beauboeuf et al., 1999; Carr and Gardner, 2000; Gardner and Borer, 2000;
Gardner et al., 2003) may instead be HCS, variably masked. Sketches and photographs
from the Brushy show upward-convex lamination truncating low-angle laminae below
(Zelt and Rossen, 1995, figs 25.14a, 25.21a; Beauboeuf et al., 1999, figs 3.3c, 4.5e;
Carr and Gardner, 2000, figs 7, 9, 10), suggestive of HCS.
Symmetrical and near-symmetrical ripples
Symmetrical ripples indicative of wave action are well known in "Unit 5" of the
Skoorsteenberg Formation (Wach et al., 2000; Wild et al., 2005, fig. 4e; Hodgson et al.,
2008, p. 101). Near-symmetrical ripples, typical of combined-flow ripples in shelf storm
6
beds (Brenchley, 1985; Myrow et al. 1982; Pattison et al., 2007), occur lower in the
formation (Figs 4, 5).
Near-symmetrical ripples are also common in the Bude, Ross and Brushy
Canyon formations (Higgs, 1991, 2004, and in preparation). In the Brushy, symmetrical
ripples were reported by King (1948) and Newell et al. (1953), and slightly asymmetrical
"wave-dominated combined-flow ripples" by Harms (1969), whose figure 17 shows
ripple crests bifurcating, diagnostic of wave action (Reineck and Singh, 1980).
Multidirectional tool marks
Multidirectional bounce- and prod marks in the Skoorsteenberg Formation (Fig. 6)
indicate combined flows with a storm-wave component (Gray and Benton, 1982).
Non-linear grooves
Kinked and hooked grooves (Fig. 7), and low-angle-intersecting grooves, occur on bed
bases in the Skoorsteenberg Formation. These structures again indicate combined
flows involving storm waves (Martel and Gibling, 1994; Beukes, 1996).
Discordant, mud-draped bed tops
Mud-draped scours (Fig. 8) of concave-up to undulating geometry are common in the
Skoorsteenberg Formation , incising shale and/or sand beds and commonly deep
enough (dm-m) to truncate beds laterally. Such features, attributable to storm-wave
action (Walker et al., 1983; Brenchley, 1985), are also common in the Bude, Ross and
Brushy Canyon formations (Higgs 1991, 2004 and unpublished observations). A very
thin (mm) or absent sand or silt drape suggests that any unidirectional current
accompanying the waves imported little or no sediment. Megaflutes, well known in the
Ross Formation (Elliott, 2000) and reported in the Skoorsteenberg (Wild et al., 2005),
are a strongly asymmetrical variant, suggesting waves accompanied by a relatively
strong unidirectional flow, possibly storm-wind-driven lake circulation (see below).
ICHNOFOSSIL EVIDENCE FOR NON-MARINE SALINITY
Scott et al. (2000) were incorrectly cited as having proposed lacustrine deposition for
the Skoorsteenberg, based on the absence of fossils (Johnson et al., 2001; van de
Werff & Johnson, 2003a; see also Pazos, 2002). On the contrary, Scott et al. (2000)
invoked a submarine fan, without mentioning fossils. Normal marine salinity was
proposed by Johnson et al. (2001), based on the following reported (but not illustrated)
trace-fossil association: Chondrites, Cosmorhaphe, Gordia, Granulana, Gyrochorte,
Helminthoida, Helminthoides, Helminthopsis, Lophoctenium, Lorenzinia and
Paleodictyon (Johnson et al., 2001). Raising the possibility that this association reflects
instead a brackish water body, consistent with most previous interpretations of the Ecca
Group in southwestern Karoo Basin (Table 1), Paleodictyon is known in nonmarine
Carboniferous strata (Pickerill, 1990), and Chondrites may in fact be a
missinterpretation of gregarious, overlapping Planolites. The author has seen the
ichnogenera ?Arenicolites, ?Diplocraterion, Helminthoidichnites, ?Helminthorhaphe,
Planolites, Treptichnus and Undichna (Figs 9-14). The total Skoorsteenberg
assemblage thus bears a closer resemblance to the Mermia fresh-lake ichnofacies
(Buatois and Mángano, 1995) than to the conventional marine shelf (Cruziana), slope
7
(Zoophycos) or basin plain (Nereites) ichnofacies. Skoorsteenberg ichnogenera
consistent with the Mermia ichnofacies are Gordia, Helminthopsis, Helminthoidichnites,
Planolites, Treptichnus and Undichna (Buatois and Mángano, 1995). Lack of other
Mermia-ichnofacies genera like Mermia, Cochlichnus and Lockeia may simply reflect
inadequate study, or brackish rather than fresh water. While the ichnology of "brackish
seas" is becoming better uderstood (Pazos 2002), no formal ichnofacies model yet
exists, unlike brackish marine-marginal environments like estuaries (Wightman et al.
1987).
DISCUSSION
Other probable wave-influenced structures
Sinusoidal ripple lamination
This structure comprises rounded, near-symmetrical, sub-vertically climbing ripples.
Sinusoidal ripple lamination (SRL) was described in the Skoorsteenberg Formation by
Basu and Bouma (2000a) and, as sigmoidal ripple lamination, by Johnson et al. (2001).
SRL was originally defined from Pleistocene glaciolacustrine deposits of, significantly,
very shallow-water origin (< 10 m; Jopling and Walker, 1968), consistent with wave
influence. The structure is wave-related and associated with HCS according to
Campbell (1966; his "small-scale truncated wave-ripple laminae"), Kreisa (1981; his
climbing wave-ripple lamination) and Chaudhuri (2005). The steep climb is interpreted
here to indicate: (A) nearly symmetrical flow, comprising storm waves combined with a
comparatively weak unidirectional current supplying sand/silt (hyperpycnal flow; see
below), hence the ripple symmetry; and (B) very high suspended sediment
concentrations, compatible with “wave support” (Pattison et al., 2007). SRL is
interpreted here as produced by combined flows comprising (1) a unidirectional current
supplying suspended sand/silt, of low velocity, hence the symmetry and steep climb,
and (2) uniform (monochromatic) waves, hence the regularity of the SRL, implying fairweather waves rather than (polychromatic) storm waves which produce ripple
lamination with irregular, erosional, "scooping" set boundaries (de Raaf et al., 1977).
Intervals with SRL were thus deposited above fair-weather wave base, possibly over
periods of many weeks (see below).
Conventional climbing ripple lamination, also present in the Skoorsteenberg
Formation (Basu and Bouma 2000a, b; Johnson et al. 2001) and the Bude (Jopling and
Walker, 1968), is by association with SRL also probably a wave-influenced structure, as
proposed by Chaudhuri (2005).
Scooping cross lamination
Ripple cross lamination with scooping set boundaries, diagnostic of wave influence (de
Raaf et al., 1977), has been photographed in the Skoorsteenberg Formation (Bouma et
al., 2007b, fig. 5).
Laingsburg Formation: wave-formed structures?
The Laingsburg Formation, of the same age and facies association as the
Skoorsteenberg and occurring in an adjacent sub-basin (Wickens & Bouma, 2000), is
reported to have climbing ripple lamination (Sixsmith et al., 2004) and mud-draped
scours (Flint et al. 2007a,b), both of which may be wave related, as inferred above.
8
Reported "wavy-bedded sandstone" (Grecula et al., 2003) and "undulating lamination"
(Sixsmith et al., 2004) are possibly HCS. Near-orthogonal "interference ripples"
interpreted by Grecula et al. (2003, fig. 12B) as indicating unidirectional flow reflection
off bottom topography, appear symmetrical and may instead represent two sets of storm
waves, one after the other. The combined evidence suggests deposition of at least part
of the Laingsburg above storm wavebase.
Lainsgburg Formation paleosalinity
The only ichnofossils reported from the Laingsburg Formation are arthropod tracks and
Undichna (Sixsmith et al., 2004), consistent with deposition in fresh or brackish water
(Goldring, 1978; Higgs, 1988; Buatois and Mángano, 1995).
Depositional environment: Skoorsteenberg and lookalike formations
Some remarks follow on the interpreted depositional environment of all five formations
comprising "Bude-type turbidites": the Bude, Ross, Brushy Canyon, Laingsburg and
Skoorsteenberg. The evidence presented above suggests that the Skoorsteenberg was
deposited in fresh or brackish water, as invoked by many previous authors (Table 1), in
a large lake (Veevers et al., 1994), here named "Lake Karoo". The Laingsburg
Formation accumulated in the same lake. A similar "Lake Bude" has been proposed for
the Bude and Ross formations (Higgs, 1991, 2004), and a "Lake Brushy" is suggested
for the Brushy Canyon Formation (Higgs, in preparation). In all cases, the foreland basin
setting was amenable to thrust-front salients cutting off the sea, forming "sea-level
lakes" (Goldring, 1978), of variable brackishness, like the modern Black Sea and Lake
Maracaibo. Such lakes are susceptible to ocean-water wedge intrusion up the outflow
channel (i.e. over the sill; Bosporus outflow of modern Black Sea; Higgs, 1991),
resulting in variable lake brackishness and water level tied to glacioeustatic fluctuations.
Various workers have invoked the Black Sea analog for the Karoo Basin during Ecca
deposition (Table 1). The lowered salinity in Lake Karoo favored river-fed underflows
lasting weeks during summer melting of alpine glaciers that can be presumed, given
that the Dwyka continental glaciation (Visser, 1997) was not long over, to have existed
in the adjacent orogen. These hyperpycnal flows deposited turbidites of the
hyperpycnite variety (Mulder et al., 2002). (The term "underflowite" (Higgs, 1987) has
priority and is more self-explanatory.) Tentative claims to be "the first systematic outcrop
documentation of hyperpycnal flow beds from ancient turbidite systems" (PlinkBjörklund and Steel, 2004), and to describe "for the first time ancient lacustrine
hyperpycnites from an outcrop perspective" (Zavala et al., 2006b), are incorrect (Higgs,
1991, 2004).
Skoorsteenberg individual (non-amalgamated) turbidites are, like those of the
lookalike formations, mainly fine- or very-fine grained, thin (<40 cm), non-laminated, and
ungraded except in the top 1-3 cm. These characteristics suggest deposition from
sustained (weeks-months), uniform, depletive flows (Kneller, 1995), just fast enough to
carry fine sand in suspension but too slow to move it tractionally (< 30 cm/sec?; cf.
Sundborg, 1967). Sand "pre-suspended" in the source river settled continuously from
the underflow, the rate of settling retarded by turbulence (Cuthbertson and Ervine,
2007). Beds with wave-related structures (HCS, SRL, etc.) are wave-influenced
hyperpycnites (cf. Pattison et al., 2007). These possibly pass distally into non-laminated
9
hyperpycnites deposited below wavebase (fairweather or storm, as appropriate). Such a
transition from proximal laminated beds (including climbing ripples) to distal
structureless beds was demonstrated in interpreted marine hyperpycnites (PlinkBjörklund and Steel, 2004), but was conventionally attributed to distally increasing
fallout suppressing traction, which is counter-intuitive.
Larger (mm) particles in some Brushy Canyon sandstones are marine fossils and
fragments, whose characteristic abrasion (King, 1948; Fischer & Sarnthein, 1988)
suggests transportation. The fusulinid species (King, 1942, 1947, 1948; Hayes, 1964)
are consistent with reworking from underlying limestones of the lower San Andres
Formation (see below). The fusulinids are interpreted to have been rolled into position
by the underflow.
The envisaged Skoorsteenberg environment was a lake shelf, of continental shelf
dimensions (cf. modern Black Sea), on which the Laingsburg Formation was also
deposited, as was the coeval, lookalike Ripon Formation farther east, interpreted by
Kingsley (1981) as slump-generated turbidites forming base-of-slope fans. Similar,
coeval facies in the Malvinas/Falkland islands, deposited alongstrike in the same
foreland basin prior to Gondwana breakup, and interpreted by Trewin et al. (2002) as
lacustrine "basin-floor turbidites", though with wave ripples, were deposited on the same
shelf. The Lake Karoo shelf was flanked to the south by an inferred flysch trough
(underfilled foreland basin; Covey, 1986), no longer identifiable, probably upthrusted
and eroded by northward advance of the southern, Swartberg branch of the Cape
orogen (e.g., Turner, 1999; Catuneanu et al., 2005). The trough accommodated excess
sediment shaved off the shelf by combined storm waves and wind-drift currents,
maintaining an equilibrium shelf profile (Seilacher, 1982; Higgs, 2004), preventing
sediment from aggrading to sea level. Overlying the Skoorsteenberg, the Kookfontein
Formation comprises shallow-marine (though unfossiliferous) parasequences according
to Wild et al. (2007). The lowermost parasequence is interpreted to downlap onto the
Skoorsteenberg (Wild et al., 2007). This would suggest drowning of the Lake Karoo
shelf, followed by construction of a new, prograding, shelf-and-slope equlibrium profile,
perched on the drowned shelf. Drowning may reflect long-term eustatic rise over the sill,
probably increasing the lake salinity (still no marine fossils).
Lake Karoo was probably brackish whenever glacioeustatic highs overtopped the
sill enough for an oceanic wedge to intrude, possibly culminating in undiscovered
marine bands (cm-dm) in the shaly "inter-fan" intervals, like the rare marine bands of the
Bude and Ross formations (Higgs, 2004). During glacioeustatic lows, the lake water
level fell toward the spillpoint (lowstand perched level), eventually cutting off the
intruding ocean-water wedge, so that the lake freshened by river and rain inflow, and
may have turned completely fresh (Higgs, 1991). The sill thus regulated the minimum
lowstand lake level, confining eustatically forced emergence to the innermost shelf
(Higgs, 1991). Emergence by delta progradation was also confined to the innermost
shelf because, in fresh- or brackish lakes, direct offshore sediment supply by
hyperpycnal flows enhances vertical aggradation, at the expense of delta progradation
(Higgs, 2004). The water depth on the Karoo shelf, by comparison with modern
continental shelves facing oceans (large fetch), was probably less than 150 m.
The Skoorsteenberg shelf is inferred to have been enclosed (gulf) or constricted
(strait) toward the north, between an eastward-thrusting mountain belt oriented north-
10
south (Cederberg branch of Cape orogen) and, in the east, a probable corresponding
forebulge (cf. Catuneanu, 2004). Skoorsteenberg deposition occurred on the west flank
of the gulf or strait; the depositional surface was a thus and east-dipping "ramp" in eastwest profile (Van Wagoner et al., 1988). Instead of base-of-slope fans fed by slope
channels or canyons (Beauboeuf et al., 1999; Gardner et al., 2003; Grecula et al., 2003;
Lien et al. 2003; Sixsmith et al., 2004; Wild et al., 2005; Hodgson et al., 2006; Pyles,
2008), the Skoorsteenberg and lookalikes are reinterpreted here as mid-shelf sandy
tongues (Higgs 1991, 2004) fed by shallow (< 10 m) channels crossing the muddy inner
shelf, as in newly recognised (marine) examples in the Cretaceous seaway (ramp) of
North America (Pattison et al., 2007). Muddy intervals previously interpreted as slope
deposits containing channel sands, like Skoorsteenberg "Unit 5", Laingsburg "Units B to
F", the proximal mud (silt) belt of the Brushy Canyon, and the Gull Island Formation
overlying the Ross (Beauboeuf et al., 1999; Wignall and Best, 2000; Martinsen et al.,
2000; Wild et al., 2005; Flint et al., 2007a,b), are reinterpreted here as inner-shelf mud
belts, consistent with "slumps" (Wild et al., 2005) showing gradational bases and nearupright folds or load balls/pillows (Fig. 15; Wild et al., 2005, figs 3a, 4d; Hodgson et al.,
2008, photos on p. 97, 101, 102), suggesting in situ deformation (Hodgson et al., 2008;
Oliveira et al., in review), implying minimal gradient; the deformation trigger may have
been earthquakes (seismites) or wave loading. Similarly, chaotic units up to 40 m thick
underlying the Laingsburg, interpreted as "mass-transport complexes" marking the
ititiation of "deep-water" sedimentation (Flint et al., 2007a,b), instead have no such
bathymetric significance if they formed in situ, either seismically or by (tsunami-?) waveloading. Syneresis cracks confirm seismic shocks (Pratt, 1998) during Skoorsteenberg
deposition. The examples seen were reticulate (Fig. 16), relatively unusual (Pratt, 1988)
and superficially resembling desiccation cracks, possibly reflecting seismic waves
propagating from orthogonal directions (Cederberg, Swartburg thrust belts). Sandstone
dikes in the Laingsburg Formation (Grecula et al., 2003) also reflect seismicity.
Instead of river supply for the Brushy Canyon Formation, Fischer and Sarnthein
(1988) suggested eolian dunes feeding slump-generated turbidity currents. However,
Brushy sand is characterisitically angular (Newell et al., 1953; photomicrographs in
Montgomery et al., 1999, Justman and Broadhead, 2000; Shew 2007a,b), contradicting
the eolian model. Also implying aridity, Harms (1974) proposed shelf-derived saline
density currents, at odds with both coeval basin-margin karstification (San Andres
Formation; Stoudt and Raines, 2004) and Brushy-equivalent strata elsewhere in the
basin (Word Formation; Lambert et al., 2007) containing plant remains (Hill, 1999). The
lack of reported macroscopic plant remains in the Brushy suggests that the sedimentsupplying rivers occupied, in their final reaches, caves that (A) restricted entry of local
plant material and (B) trapped far-traveled leaves, branches and logs in narrowings
(logjams), where it bio-fragmented to sand and mud size, finally reaching Lake Brushy
at drowned gorges (rias) or cave mouths.
The author's observations indicate that channels in the Skoorsteenberg, Bude,
Ross and Brushy Canyon formations are simply incised, lacking levees, in agreement
with some authors (Harms, 1974; Johnson et al., 2001; Lien et al., 2003) but not all
(Melvin, 1986; Zelt and Rossen, 1995; Basu & Bouma, 2000a,b; Bouma et al., 2007b;
Flint et al., 2007a). The Laingsburg Formation includes levee facies according to
Grecula et al. (2003) and Sixsmith et al. (2004).
11
Interbedding of channels and sheet sands (cf. channel-lobe transition zone;
Gardner et al. 2003; Hodgson et al., 2006) can be attributed to lake-level fluctuations
due to glacioeustatisy and possibly tectonism (forebulge minor retreat or advance?).
Lake-level fluctuations made channel-tongue pairs retro- and prograde. In
retrogradation, the outer channel backfilled and was blanketed by tongue deposits. In
progradation, the channel incised the backfill and, if the relative fall was sufficient, the
underlying tongue interval and preceding channel fill. Thicker (10s m), composite
channel fills were thus formed, with narrower paleocurrent dispersion than the
enveloping tongue facies (not levee). This model has some similarities with the Brushy
Canyon "build-cut-fill-spill" model of Gardner and Borer (2000), except those authors
invoked autocyclicity and avulsion as the controlling mechanism. Other authors
suggested Skoorsteenberg glacioeustatic cyclicity, without specifying the exact
mechanism of channel and sheet alternation (Goldhammer et al. 2000; Johnson et al.
2001; Hodgson et al. 2006).
East-flowing underflows emerging from east-west channels underwent Coriolis
veering leftward (southern hemisphere), enhanced due to flow slowness and longevity
(Hill, 1984; Higgs, 2004), causing flows from the west (Cederberg ranges) to swing
north (i.e. leftward, southern hemisphere) along the gulf axis, consistent with
Skoorsteenberg paleocurrents (Johnson et al., 2001; Hodgson et al., 2006), in particular
the evidence for leftward veering (Luthi et al., 2006, fig. 11). The Karoo hyperpycnites
accumulated at shelf depths in two orthogonal sub-basins, the Tanqua (N-S, overfilled)
and Laingsburg (E-W, underfilled). In contrast, the Laingsburg "open shelf" sector was
probably bordered by an east-west forebulge in the north, and by the "missing" southern
deep-water flysch trough, which trapped northward flows from the Cape orogen
Swartberg branch. Laingsburg paleocurrents overall are eastward (Sixsmith et al.,
2004), interpretable as hyperpycnal flows derived from the far south of the Cederberg
range, traveling east along the Laingsburg shelf sector, the predicted leftward Coriolis
force balanced by the southward shelf gradient. Laingsburg isopach variations and local
paleocurrent anomalies, attributed to basin-floor topography (Grecula et al., 2003;
Sixsmith et al., 2004), may instead reflect (1) growth faulting (differential subsidence)
without topographic expression, and (2) erratic combined-flow paleocurrents. In both
formations, published sand petrology indicates a metamorphic and plutonic provenance
(Johnson, 1991; Scott et al., 2000), interpreted here as a now-eroded nappe atop the
CFB (weakly metamorphosing it), rather than a source situated anomalously far away
(200-500 km, i.e. wider than most foreland basins) in South America (Scott et al., 2000).
A distinctive parallel-laminated siltstone facies in the Skoorsteenberg (e.g. Fig. 5,
upper one-third), Laingsburg, Bude and Brushy Canyon formations (Higgs, 1991;
Harms, 1974; Wickens and Bouma, 2000; Grecula et al., 2003) is interpretable as lofting
rhythmites (Zavala et al., 2006a), abbreviated here to "loftites". Such lofting requires a
strong salinity contrast, therefore loftites are considered a marine indicator (Zavala et
al., 2006a), but brackish lakes stages (as opposed to fresh) are also considered here
suitable for lofting. Eolian fallout has previously been proposed for this facies (Fischer
and Sarnthein, 1988).
CONCLUSION: ECONOMIC AND ACADEMIC IMPLICATIONS
12
The Skoorsteenberg and lookalike formations are shallow lacustrine, improper analogs
for deep marine turbidites whose dissimilar processes (more surge-type turbidity
currents; no waves; less lofting) would produce fan lobes differing from their lake
"counterparts" in shape, area, grain size, Coriolis curvature, architecture and internal
heterogeneity (e.g., no wave-scour mud baffles or barriers), and channels that are
deeper, leveed and more sinuous. The economic consequences of inappropriate
analogs can amount to billions of dollars, including non-optimum placement of wells and
perforations, unreliable prediction of flow rates and reserves, and incorrect judgment of
field commerciality (Higgs, 2004, 2009).
Besides the salinity and water-depth problems raised above is the issue of
tectonic setting. The questionability of using foreland-basin outcrops as analogs for
passive-margin reservoirs is underscored by the observation that passive-margin-slope
or -rise strata can only ever achieve outcrop in a metamorphosed and/or intensely
deformed state (orogenic collision belt), yet the supposed outcrop analogues are largely
sub-horizontal, except where overrun by the foreland-basin deformation front (e.g.,
Bude, Laingsburg).
Nevertheless, the five Bude-type formations are magnificent sedimentological
workshops, and invaluable for oil companies wishing to learn what deep-sea turbidites
do not look like, or seeking outcrop analogs to optimize development of oil-producing
lacustrine hyperpycnal reservoirs like the Brushy Canyon itself (Montgomery et al.,
1999). Brushy exploration will also benefit from the new model, e.g., predicting Coriolisdeviated sand-tongue reservoirs, ideal stratigraphic traps due to nearly four-way
shaleout. For the wider sedimentological community, shelf hyperpycnites and wavemodified hyperpycnites, whether lacustrine (Bude-type, exemplified also by the Brushy,
Ross, Laingsburg and Skoorsteenburg formations) or marine (Pattison et al., 2007),
represent a fourth great class of gravity-flow subaqueous event beds, after fluvio-deltaic
crevasse beds, "conventional" shelf tempestites and deep-sea turbidites.
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Figures 1-16 (next 16 pages), followed by Tables 1-2 ....
24
Figure 1. Sandstone bed at center c. 20 cm thick (15 cm scale, center right) has flat
base, undulatory top (hummocky in 3D?), and faint internal lamination, possibly HCS.
Skoorsteenberg Formation, "Fan 3", Rondawel.
25
Figure 2. Decimetric bed of fine- or very-fine sandstone, with flat base and convex-up
top, capped by shale (receding, forming ledge). Faint horizontal lamination in the lower
portion (e.g., below scale) is overlain (truncated?) by a set of convex-up laminae. This
structure is reasonably interpreted as HCS, with poorly defined ("blurred") lamination
due to rapid fallout from suspension. Skoorsteenberg Formation, "Fan 3", Los Kop
South.
26
Figure 3. Brick-sized/shaped sample (float) of an entire thin bed (c. 8 cm) of very fine
sandstone or siltstone, viewed on-end. The bed has a sharp flat base, hummocky or
rippled top, and interpreted HCS (unprovable due to sample's small dimensions). The
back and sides of the sample show similar lamination, without angle-of-repose dips or
obvious preferred directionality. Coin 2.6 cm diameter. Skoorsteenberg Formation, "Fan
3", Kanaalkop.
27
Figure 4. Float sample of a thin bed of very fine sandstone or siltstone, viewed on-end,
showing a near-symmetrical ripple, overlying a single set of leftward dipping ripple
foresets. Skoorsteenberg Formation, "Fan 3", Gemsbok valley.
28
Figure 5. Centimetric bed (center) of siltstone or very fine sandstone, encased in
siltstone with millimetric lamination. The sandstone bed shows irregular, nearsymmetrical ripples on top. Internally, possible HCS passes up into right-dipping ripple
lamination. Skoorsteenberg Formation, "Fan 3", Ongeluks River.
29
Figure 6. Base of a thin (cm) siltstone or very fine sandstone bed (float), showing multidirectional bounce-, prod- and groove marks. Note numerous inclined, triangular
projections, interpreted here as pointed-leaf prod marks, not previously described in the
sedimentological literature to the author's knowledge (see also Fig. 13; cf. Glossopteris
leaf photograph in Johnson et al. 2006, fig. 23a). Skoorsteenberg Formation, "Fan 3",
Grootfontein.
30
Figure 7. Base of a thin (cm) siltstone or very fine sandstone bed (float), showing three
features indicative of a wave-component in the depositing combined flow (see text): (1)
two sets of grooves intersecting at a low angle; (2) sinuous grooves (right and left); and
(3) two hooked grooves (adjacent, at upper center, one possibly widened by scour). The
hooked grooves are short (terminate), suggesting "touch down" and "lift-off" (Beukes,
1996). Skoorsteenberg Formation, "Fan 3", Rondawel.
31
Figure 8. Series of sandstone units, each up to 2 m thick, comprising amalgamated
thinner (< 40 cm) beds of mostly fine sandstone. The top of the penultimate (thickest)
unit dips to the right, and is overlain by recessive shale. This surface truncates internal
sub-horizontal partings (interpreted amalgamation planes), suggesting a "mud-draped
scour" (see text) rather than constructional bedform topography. Note lack of obvious
vertical bed-thickness trends, a result of premature amalgamation (see text).
Skoorsteenberg Formation, "Fan 3", Rondawel.
32
Figure 9. Pervasively burrowed concretion weathered out of shale. The 3D burrow
morphology, inferred from 2D intersections with the concretion surface, is tongueshaped, oblique- or vertical to bedding, possibly Diplocraterion. Skoorsteenberg
Formation, shale interval between "Fan 2" and "Fan 3", Gemsbok valley.
33
Figure 10. Plan view of upper surface of a thin (cm) bed of siltstone or very fine
sandstone bed. A low, symmetrical ripple trending left-right occupies the upper half of
the photo. The adjacent ripple trough shows Helminthoidichnites (dark, narrow, slightly
sinuous trace, left of coin). Coin 2.6 mm diameter. Skoorsteenberg Formation, "Unit 5",
Droog Kloof ridge.
34
Figure 11. Base of a sandstone bed (float), showing flow scours (longitudinal furrows)
locally overprinted by a horizontal, meandering(?) burrow system, possibly Agrichnium
or Helminthorhaphe. Coin 2.6 mm diameter. Skoorsteenberg Formation, "Fan 3",
Rondawel.
35
Figure 12. Plan view of gradational, non-laminated, silty top of a thin (cm) bed of very
fine sandstone or siltstone, encased in shale. The siltstone shows abundant crosscutting (not branching) burrows interpreted as Planolites, possibly misinterpreted in the
past as Chondrites. Coin 2.6 mm diameter. Skoorsteenberg Formation, shale interval
between "Fan 2" and "Fan 3", Los Kop South.
36
Figure 13. Base of a sandstone bed (float), showing Treptichnus burrows. Note inclined,
triangular, possible leaf-prod mark left of coin (see also Fig. 6). Coin 2.6 mm diameter.
Skoorsteenberg Formation, "Fan 3", Rondawel.
37
Figure 14. Base of a sandstone bed (float), showing a composite fish trail (running leftright, 5 cm from top of view). This is probably Undichna insolentia, whose type locality is
in the Dwyka Formation in the Karoo Basin (Anderson, 1976). Note the prod mark
(lower left), sub-parallel to the fish trail. The numerous millimetric projecting "dimples"
are of uncertain origin. Skoorsteenberg Formation, "Fan 3", Rondawel.
38
Figure 15. Disturbed interval comprising sandstone balls and/or pillows separated by
siltstone flames. The deformed interval is under- and overlain by undisturbed strata,
indicating that deformation was syn-sedimentary. The base of the siltstone is
undisturbed, and the sandstone balls/pillows have near-vertical axes, both observations
indicating that deformation occurred in situ, probably by seismic- or wave-loading, not
by slumping. Note 15 cm scale, upper right. Skoorsteenberg Formation, "Unit 5", Droog
Kloof Ridge.
39
Figure 16. Plan view of reticulate syneresis cracks in a thin (mm) mudstone layer
separating centimetric siltstone or very fine sandstone beds. The base of a sandstone
slab upended by the author (upper right) shows load casts associated with the
syneresis cracks. Skoorsteenberg Formation, "Fan 3", Gemsbok valley.
40
Table 1 (next 2 pages). Previous interpretations of depositional water depth and salinity
of Ecca Group formations deposited subaqueously in the "inland sea" of the
southwestern Karoo Basin. NS = not stated by original author(s).The Ecca Group
comprises (Tanqua sub-basin): Prince Albert, Whitehill, Collingham, Tierberg,
Skoorsteenberg, Kookfontein and Waterford fms, in conformable succession.
Author(s)
Formation(s)
Interpreted
salinity
Interpreted water depth
Remarks
du Toit 1954
Whitehill; Laingsburg
(coeval, eastern lookalike
of Skoorsteenberg Fm)
"estuary", implying "shallow water", implying
brackish
10s m max (Laingsburg)
(Whitehill)
Hart 1964
"Ecca shales"
fresh or brackish,
with "marine
intercalations"
Ryan 1968
entire Ecca Gp
NS
"relatively deep water";
"deep water" (min 100s m
implied?)
"continental sea" and "inland sea" imply less
than marine salinity?
Stratten 1968
Prince Albert & Whitehill
NS
NS
"nearly isolated sea" and "a marine
incursion" imply mainly brackish?
McLachlan 1973
Ecca Gp
fresh to brackish
NS
p. 9 refers to entire basin (p. 10 discusses
coal-bearing NE Karoo only)
McLachlan &
Anderson 1973
Ecca Gp
"sea";
"nonmarine"
NS
brief "marine invasion" near the base; "by
White Band times conditions were probably
already nonmarine"
Marchant 1978
"Ecca formation" (Group)
fresh
NS
"the preponderance of available geological
and geochemical data for the Ecca
formation suggests ... lacustrine origin"
(cites Danchin 1970 Ph.D. thesis)
Visser & Loock
1978
Ripon (E equivalent,
same strat. level & facies
as Skoorsteenberg &
Laingsburg fms)
NS
max. about 500 m
500 m est. based on report of Nereites in
Kingsley 1977 PhD thesis, not confirmed by
Kingsley 1981; "extreme shallow water" at
top of Dwyka tillite, c. 150m below base of
Ripon
Stanistreet et al.
1980
Vryheid (NE equivalent of marine episodes
Prince Albert, Whitehill & implied
Tierberg fms; Faure &
Cole 1999)
NS
glauconite & supposed marine trace fossils
occur in "abandonment phase" between coal
seams
Anderson 1981
Ecca Gp
brackish implied
NS
"marginally marine...'sea'", following a basal
Ecca fossiliferous "marine horizon"
Kingsley 1981
Ripon (E equivalent,
same strat. level & facies
as Skoorsteenburg and
Laingsburg fms)
"lake", implying
fresh or brackish
max 500-700 m
"land-locked lake less than 1000 m deep";
"wave ripples" < 200 m above top of Ripon
Fm (p. 31 & Figs 8, 9); "Helminthopsis worm
trails attest to ... relatively deep water"
Tankard et al.
1982
Laingsburg/Ripon (E
equivalent, same strat.
level & facies as
Skoorsteenberg Fm)
less than marine
("rare marine
incursions")
100s m
"landlocked basin" with "rare marine
incursions"; present Black Sea (brackish)
analog
"Lower Karroo Sea"; cites "South African
Sea" (Ryan 1963)
42
Cooper &
Kensley 1984
Ecca Group (southern
"flyschoid rocks")
brackish
"shallow"
"shallow inland sea"
Smith 1990
Laingsburg/Ripon (same
age/facies as
Skoorsteenberg)
NS
NS
"flysch-type"; "landlocked sea"
Yemane & Kelts
1990
Whitehill
fresh to slightly
brackish
NS
Late Pleistocene Black Sea analogue
Bouma &
Wickens 1991
Skoorsteenberg
marine
("submarine fan")
NS
Wickens &
Bouma 1991
Skoorsteenberg
marine
("submarine fan")
NS
Visser 1992
Laingsburg
brackish
"basin floor turbidite fans"
imples min. 100s m
Marine Prince Albert/lr Whitehill fms;
brackish from up. Whitehill
Visser 1994
Prince Albert, Whitehill
marine then
(upper Whitehill)
brackish
"shallow to moderate (<400
m)"
"large sea"; thin (< 4 m) basal shale with
marine fossils; carbonate bands with
supposed marine microfossils (pers.
comm.); shale Rb/K ratios
Veevers et al.
1994
Skoorsteenberg/Laingsbu brackish to fresh
rg/Ripon (lateral
lake
equivalents; same strat.
level & facies)
<500 m (cites Visser &
Loock 1978); under- &
overlain by "shallow basin"
"lack of indubitably marine sediment"
anywhere above basal Ecca unit with marine
fossils
Johnson et al.
1996
Skoorsteenberg
marine
("submarine fan")
min. 100s m implied by
"submarine fan"
Faure & Cole
1999
Whitehill, upper Prince
Albert
fresh to brackish
NS
"most likely lacustrine with fresh to brackish
water"
Johnson et al.
2001
Skoorsteenberg
marine
min 100s m implied ("deepwater")
"the trace fossils identified in this study
suggest a basin with marine salinity, as
proposed by Wickens (1994)"
Braddy & Briggs
2002
Ecca Gp
fresh
shallow
"The succession is interpreted as freshwater
(shallow lacustrine)"
Pazos 2002
Laingsburg
brackish
NS
"brackish seas characterized by reduced
salinity as result of the input of glacial melt
water during deglaciation"; "marineconnected"
McCarthy &
Rubidge 2005
Ecca Group
NS
"deep"
"Deep water trough". "Karoo Sea" was
"Perhaps ... similar to the Black Sea, with
only a narrow connection to the ocean"
Hodgson et al.
2006
Skoorsteenberg
marine implied
("submarine fan")
min 100s m implied
("deepwater")
Herbert &
Compton 2007
Prince Albert
fresh
NS
"fresh water lake", based on nodule
geochemistry
43
Table 2 (next 2 pages). Facies and sedimentary structures shared by the five "Bude-type
turbidite" formations: Ross, Bude, Skoorsteenberg, Laingsburg and Brushy Canyon formations.
NR = not reported. Modified after Higgs 2004.
Feature
Bude Fm
Laingsburg Fm
Brushy Canyon
Fm
Three main facies: (1) shales 1-10 m, Melvin 1986,
Collinson et al. 1991,
(2) cm-dm sandstones and shales, (3) Higgs 1991, Burne Chapin et al. 1994,
amalgamated sandstones 1-10 m
1995
Elliott 2000a,b, Elliott
et al. 2000, Martinsen
et al. 2000, Lien et al.
2003, Higgs 2004
Johnson et al.
Grecula et al.
2001, Hodgson 2003, Sixsmith et
et al. 2001,
al. 2004
2008, Higgs
this paper
Zelt & Rossen
1995, Beaubouef
et al. 1999,
Gardner et al.
2003, Higgs, in
prep.
Sand beds are fine- and very-fine
As above
grained, whether amalgamated or not
As above
As above
As above
As above.
Amalgamated-sand component beds
are mostly thin (cm-dm)
As above
As above
As above
As above
As above
Most cm-dm sandstone beds are
As above
massive, or massive with minor (mmcm) parallel/cross-laminated top.
As above
As above
As above
As above
Channels up to 10 m thick (and
Burne 1995, Higgs As above
thicker composite channels) filled with 2004
amalgamated sand beds
As above
As above
As above
Other amalgamated sand bodies are Melvin 1986,
As above
sheet-like (observed or inferred lateral Higgs 1991, Burne
extent at least 100 m in all directions) 1995,
As above
As above
As above
Clear vertical bed-thickness
successions rare. Instead,
amalgamated sandstone "packets"
alternate abruptly with interbedded
sandstone and mudstones
As above
As above
As above
As above
Ross Fm
As above
Skoorsteenberg Fm
Remarks
Except, in Brushy,
abraded fusulinids and
fragments of other fossils
(King 1948)
45
Amalgamated units deamalgamate
Higgs 1991
laterally. Each component sand bed
occupies a flat-based channel < 0.4
m deep), stacked to form a composite
body here termed "pseudo-channel"
Chapin et al. 1994,
As above & van Grecula et al.
Elliott 2000a,b, Elliott der Werff &
2003
et al. 2000, Lien et al. Johnson 2003b
2003, Higgs 2004
Beauboeuf et al. Frayed ribbons (Higgs
1999, Higgs, in
1991). "Transitional archiprep.
tectural element"
(Johnson et al. 2001).
"Pinchout fingers" (van de
Werff & Johnson 2003b).
"HASTZ" (Hodgson et al.
2007a).
Rare beds with hummocky cross
stratification
Higgs 2004
Higgs, in prep.
Higgs 1991, 2004
Higgs this
paper
NR