IMMEDIATE SCIENCE REPORT
K040 : The Sedimentology and Trace Fossils of Devonian Strata at Table Mountain
Antarctica New Zealand December 1993 - January 1994
IMMEDIATE SCIENTIFIC REPORT TO THE ROSS DEPENDENCY RESEARCH COMMITTEE
THE SEDIMENTOLOGY AND TRACE FOSSILS OF DEVONIAN STRATA AT TABLE MOUNTAIN (EVENT K040)
Popular Summary of Scientific Work Achieved
The origin of quartzose sandstones in the lower part (Devonian) of the Beacon Supergroup, southern Victoria Land, Antarctica has generated considerable debate. The controversy centres on the presence of varied and abundant trace fossils, frequently used by some geologists as indicators of marine paleo-environments. However, these same trace fossils have been found by others in nonmarine paleoenvironments. Most previous studies of these sandstones have been conducted on a regional scale and insufficient data has been collected to establish sedimentary processes and depositional environment with confidence.
This study was designed to examine the lowermost Beacon Supergroup in detail. The New Mountain Sandstone at Table Mountain has excellent 3-dimensional exposures, and contains a variety of trace fossils and sedimentary structures. The exposures are ideal for lateral profiling techniques, which take into account lateral as well as vertical fades changes in order to provide a refined depositional model. Improved understanding of the depositional system will contribute not only to resolving the trace fossil problem, but also to southern Victoria Land paleogeographic and tectonic reconstructions.
Detailed examination of the New Mountain Sandstone revealed no evidence of marine deposition. Most of the sandstone was found to be eolian (wind blown), with evidence of water-lain deposition in the upper part. Features which support an eolian environment include climbing translatent strata, pinstripe laminations, adhesion ripples and ripple forms with coarse-grained tops. Thin intervals of trough cross-bedded sandstone represent periodic flooding by braided rivers. Paleowind directions are primarily from the northwest and the rivers flowed towards the northwest. Deposition in the uppermost New Mountain Sandstone was likely in a complex intermixture of eolian and fluvial environments. Features indicative of a water-lain environment include massive sandstones, slumped cross beds, mudcracks, and a change in paleocurrent direction to the northwest.
The origin of the 1000 m thick sequence of mostly quartzose sandstones of the lower Beacon Supergroup (Devonian), southern Victoria Land, Antarctica has generated considerable debate. The controversy centres on the presence of varied and abundant trace fossil assemblages, especially sediments containing Skolithos, frequently used by geologists to interpret marine paleoenvironments. However, the trace fossils, in the lower Beacon Supergroup (Taylor Group) are considered by some (Woolfe 1990) to be consistent with nonmarine deposition. Most previous studies have been conducted on a regional scale and insufficient data has been collected to establish sedimentary processes and depositional environment with confidence.
Improved understanding of the depositional system will contribute not only to resolving the trace fossils problem, but also to southern Victoria Land paleogeographic and tectonic reconstructions.
- To reconstruct the depositional environment of the lower Taylor Group
- To determine the habitats in which the trace fossils were made.
- Further investigate the enigmatic Pivot Member (Arena Sandstone).
Scientific Endeavours and Achievements
This study was designed to examine the lower Taylor Group in detail, collecting data from a relatively limited area and stratigraphic interval. Excellent 3-dimensional exposures at Table Mountain are ideal for lateral profiling techniques, which take into account lateral as well as vertical fades changes in order to provide a refined depositional model. Data was collected by measuring (primarily describing facies and recording paleocurrent directions) closely spaced vertical stratigraphic sections and laterally tracing individual bedding units.
Detailed examination of the New Mountain Sandstone and casual observations of the Windy Gully Sandstone, Terra Cotta Siltstone, Altar Mountain Formation and Arena Sandstone revealed no conclusive evidence of deposition in a marine environment as previously suggested by other workers (e.g., Bradshaw, 1981; Gevers and Twomey, 1982). The majority of the 250 m thick New Mountain sandstone was deposited in an eolian environment, with increasing evidence of water-lain deposition in the upper part of the formation. Deposition in the uppermost 50 m of the New Mountain Sandstone, was likely in a complex intermixture of eolian, fluvial and possibly marginal marine environments.
The lower 50 m of the New Mountain Sandstone contains alternating tabular units of small-scale (0.5 m thick by 1-2 m wide) trough cross-bedded sandstones with pebble lags and units of nested low-angle large-scale trough to tangential cross beds (1-2 m thick by several 10's of m wide) with very thin ('flaggy') page 3 foresets. The tabular sandstone units, up to 2 m thick, extend laterally from several hundred metres to several kilometers. Paleocurrents recorded from trough cross beds indicate sediment transport towards the southeast. The low-angle cross beds, on the other hand, have paleocurrent directions towards the west. The bottom sets of the trough-tangential cross beds as well as the lower 0.5 m of some foresets commonly contain abundant Heimdallia burrows. The remainder of the lower part of the New Mountain Sandstone, contains a complex arrangement of the nested low-angle large-scale trough to tangential cross beds, with rare interludes of the tabular units.
The flaggy foreset beds are overwhelmingly the most common type of cross stratification. The beds consist of well-sorted upper medium- to lower coarsegrained sandstone laminae, generally less than 5 mm thick, with rare cross laminae and preserved ripple forms (with coarse tops and finer bases). Exhumed foreset beds reveal the laminae are oriented at a very low angle across the foreset dip direction. These laminae are climbing translatent strata (Hunter, 1977), the product of wind generated ripples migrating across the slipface of eolian dunes. The flaggy character of the foresets is a result of pin stripe laminations (thin silt-rich laminae), also distinctive of eolian cross beds (Fryberger and Schenk, 1988). Other types of cross beds recognised are grain fall and grain flow laminae. The latter type result from avalanching of non-cohesive sand on dune slipfaces. Grain fall laminae result from settling of suspended particles on the dune slipface.
Both small-scale features and large-scale bedding geometries (cf. Trewin, 1993a and 1993b) suggest deposition in a eolian environment with periodic flooding by braided fluvial systems (tabular units of trough cross bedding).
The middle to upper part (except for the upper 50 m) of the New Mountain Sandstone consists of very large-scale (up to 8 m thick) cross beds. The scoop to gently curving geometry of the cross bed sets suggest some curvature to the crest of the bedform that deposited them, but often the sets can be traced laterally (both along and perpendicular to foreset dip) for at least two hundred metres. Internally the cross beds are also composed of flaggy foresets, with grain-flow and grain-fall beds less common. Slumped foresets and breccias of foreset beds are relatively common on the upper parts of the cross beds. Although generally on the order of metres in scale, one slumped cross bed, up to 5 m thick, could be traced laterally for over 1 km. Another cross-bed type is featureless or massive sandstones, up to 0.5 m thick. Lenticular to tabular shaped, these beds are devoid of sedimentary structures but their external geometries (including scour bases) and faint internal grading indicate they are depositional features and not the product of post-depositional alteration. Laterally these features can be traced into bottomset beds and are often interbedded with the features outlined below, but in some areas intervals of mostly massive beds, up to 10 m thick, are found.
Other bottomset beds consist of thin laminated ripple strata, disrupted and wavy thin laminae, adhesion ripples, wave ripples, mudstone beds (less than 20 cm thick) and desiccation cracks. Together with the massive beds these features suggest some deposition in shallow water as well as damp and dry substrates.
Similar interbeds of eolian strata, with common slumping, and massive sandstones have been reported by Gradzinski and Jerzykiewicz (1974) and page 4 Eschner and Kocurek (1986). The latter study describes coastal dune deposits flooded by trangressive seas. However, the New Mountain Sandstone contains no body fossils, and in the upper interval bioturbation is generally rare, except for trackways, which are generally found on (eolian) foreset beds. The formation described by Gradzinski and Jerzykiewicz (1974), interpreted as eolian deposits interbedded with sediments of intermittent rivers and lakes, is more akin to the New Mountain Sandstone beds. Thus it is inferred that the upper New Mountain Sandstone was deposited in a eolian setting with occasional inundation by a river system, resulting in flooding of the interdune areas and slumping of sand off the fronts of the eolian dunes into the ponded water.
The upper 50 m of the New Mountain Sandstone contains primarily low angle coarse beds, 1-2 m thick tangential cross bed sets and subhorizontal bedding. Skolithos bioturbation, commonly obliterating most of the sedimentary structures, is widespread in this interval. A regional and dramatic change in paleocurrent direction, back to an orientation similar to the lower tabular (fluvial) units, suggests a continued, but overwhelming, influence by the river system.
Inspection of the lower Taylor Group resulted in several important observations that complement previous work of others, primarily the extensive study of Bradshaw (1981). Previously reported Skolithos, vertical to steeply inclined linear cylindrical burrows about 3-5 mm diameter, were discovered to be commonly (20-40%) curved. Strongly curved burrows may deviate from the vertical by as much as 45° for lengths up to 5 cm long. Because of the bending and grouping of burrows a braided pattern is commonly observed. However, whether the burrows are actually branched or whether they appear to branch because of the intersection of two or more burrows, could not be determined. Research into previously described Skolithos is currently being undertaken to determine if the curved tubes can be considered a feature of this type of trace fossils, or whether the burrows are of another variety. Furthermore, in several instances Skolithos was found in the same bedding units as Diplichnites, an occurrence not previously reported.
Diplichnites trackways occur throughout the New Mountain Sandstone, primarily on foresets (cross beds), some with up to 30° dip. In several examples the trails are very well preserved, with well-defined imprints (common), pushed piles of sediment at edges of individual prints (common), slide marks of the imprints (abundant), and tail drag (rare) indicating the trackways are primary (non-reworked) features. However, in many trackways, recent weathering of the outcrops has removed much detail. These conclusions differ from Bradshaw (1981), who suggested current reworking of trails. Furthermore, we believe that the preservation and clarity of many trackways is a result of eolian depositional processes (i.e., firm substrates a result of wind hardening) and not binding of subaqueous marine sands by algae (??Bradshaw, 1981; Gevers and Twomey, 1982). The trackways are nearly always preserved on foresets of ripple translatent strata, which apparently consists of very well packed sand. In a few instances, trackways were found on grain-flow cross beds, and the relatively loose-packing of the bed is manifested by the relatively deep imprint and the page 5 diffuse edges of the trail.
A new type of trackway has been discovered which consists of prints previously described by Bradshaw (1981) as 'large isolated prints'. The trails, about 5 cm wide, consist of tear- to horseshoe-shaped prints up to 3 cm wide and 1 cm deep, that are tapered towards the inside of the trackway. Whether this type of trail has been previously described elsewhere is currently being investigated.
Another previously unrecognised trace fossil, 'pit and pile' structures, are common in bottomset beds of the very large-scale foresets in the New Mountain Sandstone. These features consist of a elongate pit (about 2-3 cm deep, 4-5 cm wide and up to 20 cm long) with a small (less than 2 cm tall by about 5 cm diameter) adjoining pile of sediment. Most occurrences were isolated, but on one bedding plane exposure about 20 of the features were found. The form of these clearly biogenic structures suggest they are excavation features. No trackways or other trace fossils appear to be associated with the pit and pile features.
Heimdallia burrows usually occur in the interdune areas, and also along reactivation surfaces on the foreset beds (extending upwards to less than 20 cm). The intensity of bioturbation attests to the slow rates of deposition of the New Mountain Sandstone. Heimdallia is associated with Agrichnium, epichnial grooves (Bradshaw, 1981), 'hairpin' beds (type H trace fossils of Plume, 1976), Diplichnites, cylindricum, and Skolithos (rare). Although frequently homogenised by bioturbation, the interdune deposits in the Heimdallia-bearing interval are generally devoid of mudstone beds and thick (<5 cm) massive or featureless sandstones. This may indicate that the organism(s) responsible for creating the Heimdallia burrows was not tolerant of subaqueous environments.
The Pivot Member contains fine-grain sedimentary rocks that in places have been extensively altered to Fe-Ti minerals by hydrothermal fluids related to the emplacement of dikes and sills of the Ferrar Dolerite (Woolfe et al., submitted). Preliminary studies (Woolfe et al., submitted) of the mineralisation suggest that the mechanisms in this type of alteration may be relevant to mineralisation styles in economic deposits worldwide. Two important discoveries related to the Fe-Ti alteration were made on the expedition.
Reconnaissance of the Platform Spur area revealed that fine-grained deposits of the Terra Cotta Siltstone appeared to be altered by Fe-Ti mineralisation. Thus, the mineralisation may be more widespread stratigraphically that previously thought. The site was sampled and the rocks will be analysed by Woolfe.
Woolfe et al. (1989) speculated from observations at a distance, that altered Pivot Member sediments may crop out along the southern end of Sickle Ridge. Close inspection by helicopter and from ground observations on the lower part of the fine-grained sediments at Sickle Ridge revealed that they are not extensively altered by the intrusive Ferrar Dolerite. Thus, all occurrences of the Pivot Member are not significantly altered by Fe-Ti mineralisation.
Contributions by Party Members
Dr. Woolfe, visiting from James Cook University in Townsville, Australia, had the unfortunate experience of having to be evacuated from the field for medical reasons. However, until the time that his condition precluded field work, he performed with great enthusiasm and efficiency. The knowledge, from several seasons of Antarctic fieldwork with NZAP, he was able to pass to the other, less experienced, members of the party was invaluable for the expedition's success.
Mr. Thornley, student at VUW, also performed admirably. Under the difficult circumstances of losing a third of the party, he was able to rise to the occasion and work at a sustained level of enthusiasm and excellence that assured sufficient data could be collected in the field to attain the event objectives.
Two or possibly three abstracts will be presented at international meetings (Geological Society of America, 1994; VII International Symposium on Antarctic Earth Sciences, 1995, possibly 2 papers) as well two papers at the New Zealand Geological Society Conference (1994).
Dr. Wizevich will prepare for publication the principle findings on the depositional environments in an international journal of sedimentolgy (Journal of Sedimentary Petrology or Sedimentology) and also the results of the petrographic analyses (Antarctic Science, New Zealand Journal of Geology and Geophysics, or New Zealand Antarctic Record). Mr. Thornley, with assistance from Dr. Woolfe will be responsible for writing a paper on the trace fossils (New Zealand Journal of Geology and Geophysics). Dr. Woolfe intends to publish the results of the analyses of altered sediments, possibly in Economic Geology.
We would like to thank the RDRC for approving the project, Victoria University (Internal Grants Committee) for funding the expedition (MW and ST), and NZAP for overseeing the logistics. Additional funding was provided by James Cook University (KW).
Special thanks to Peter Barrett (VUW) for his support throughout the project, to Alex Pyne (VUW) for his assistance and advice, to Gillian Wratt and Neville Jones (NZAP) for making additional helicopter time available and thus allowing the completion of our final objective, to Grant Avery and Bruce Jakes? for assistance in acquiring samples at Platform Spur, and to all the staff at Scott Base, whose thoroughness and professionalism enabled our expedition to achieve success.
BRADSHAW, M.A., 1981, Paleoenvironmental interpretations and systematics of Devonian trace fossils from the Taylor Group (lower Beacon Supergroup), Antarctica: New Zealand Jour. Geol. and Geophys., v. 24, p. 615-652.
ESCHNER, T.B., AND KOCUREK, G., 1985, Marine destruction of eolian sand seas: origin of mass flows: Jour. Sed. Petrology, v. 56, p. 401-411.
FRYBERGER, S.G. AND SCHENK, C.J., 1988, Pin stripe lamination: a distinctive feature of modern and ancient eolian sediments: Sed. Geology, v. 55, p. 1-15.
GEVERS, T.W., AND TWOMEY, A., 1982, Trace fossils and their environment in Devonian (Silurian?) Lower Beacon Strata in the Asgard Range, Victoria Land Antarctica: in CRADDOCK, C., ed., Antarctic Geoscience, Madison, Wisconsin, Wisconsin Press, p. 639-647.
GRADZINSKI, R, AND JERZYKIEWICZ, T., 1974, Dinosaur- and mammal-bearing aeolian and associated deposits of the Upper Cretaceous in the Gobi Desert (Mongolia): Sed. Geology, v. 12, p. 249-278.
HUNTER, R.E., 1977, Basic types of stratification in small eolian dunes: Sedimentology, v. 24, p. 361-387.
PLUME, R.W., 1976, Stratigraphy, sedimentology and paleocurrent analysis of the basal part of the Beacon Supergroup (Devonian and older (?) to Triassic), southern Victoria Land, Antarctica: unpublished M.S. thesis, Victoria University of Wellington, New Zealand, p. 205.
TREWIN, N.H., 1993a, Mixed aeolian sandsheet and fluvial deposits in the Tumblagooda Sandstone, Western Australia: in North, C.P., and Prosser, D.J., eds., Characterization of Fluvial and Aeolian Reservoirs, Geological Soc. Spec. Publ. No. 73, p. 219-230.
TREWIN, N.H., 1993b, Controls on fluvial deposition in mixed fluvial and aeolian facies within the Tumblagooda Sandstone (Late Silurian) of Western Australia: Sed. Geology, v. 85, p. 387-400.
WOOLFE, K.J., 1990, Trace fossils as paleoenvironmental indicators in the Taylor Group (Devonian) of Antarctica: Palaeogeography, Palaeoclim., Palaeoecol., v. 80,301-310.
WOOLFE K.J., ARNOT, M.J., AND BRADLEY, G.M., (submitted), The formation of ilmenite-bearing stratiform ironstones at Pivot Peak, Antarctica by low-temperature replacement of phylosilicates: Submitted to Economic Geology.