1Bureau of Economic Geology, University of Texas at Austin, Austin, TX ([email protected]) 2Department of Geology, University of Kansas, Lawrence, KS 3Department of Geosciences, University of Wisconsin, Milwaukee, WI
Abstract
Recent investigations into the facies and architecture of the Mackellar Formation (MF) in outcrops at Turnabout Ridge in the Beardmore
Glacier Region of the Central Transantarctic Mountains indicate that sediments were likely deposited by deltas that prograded into a marine
environment. The MF, deposited at 80-85° S paleolatitude, has previously been interpreted as deposits of a freshwater-dominated inland sea or
glaciolacustrine deposits. Our collaborative study combining sedimentology with detailed ichnology instead reveals facies, architectures, and
trace fossils indicative of marine submarine fan-channel complexes (lower MF) and prodelta and/or delta-front deposits (upper MF).
The MF overlies glacigenic deposits of the Lower Permian Pagoda Formation and is overlain by fluvial-deltaic environments of the Lower
Permian Fairchild Formation. Facies of the MF at Turnabout Ridge include shale, wave-to current-ripple cross-laminated and planar-laminated
siltstone to fine-grained sandstone, and trough cross-laminated fine-to medium-grained sandstone. Convolute bedding is pervasive. Siltstone
and sandstone commonly exhibit one or more of the following traces: Arenicolites, Bergaueria, Circulichnus, Cochlichnus, Conichnus,
during the late Paleozoic ice age; evaluating the equilibrium line altitude as a control on glaciation: Gondwana Research, v. 22/1, p. 1-19.
Miller, M.F., and R.S. Frisch, 1987, Early Permian paleogeography and tectonics of the central Transantarctic Mountains; inferences from the
Mackellar Formation: Antarctic Journal of the United States, v. 22/5, p. 24-25.
Miller, M.F., and J.L. Isbell, 2010, Reconstruction of a high-latitude, postglacial lake; Mackellar Formation (Permian), Transantarctic
Mountains, in O.R. Lopez-Gamundi, and L.A. Buatois, eds., Late Paleozoic glacial events and postglacial transgressions in Gondwana: GSA
Special Paper, v. 468, p. 193-207.
Veevers, J.J., and C.McA., Powell, 1994, Permian-Triassic Pangean basins and foldbelts along the Panthalassan margin of Gondwanaland:
GSA Memoir v. 184, 368 p.
Lower Permian Antarctic marine deltas of the Mackellar Formation: Turnabout Ridge, Beardmore Glacier
Region, Central Transantarctic Mountains, Antarctica
Peter Flaig1, Stephen Hasiotis2, Adam Jackson2, and John Isbell3
1Bureau of Economic Geology, University of Texas at Austin- Austin, Texas. 2Department of Geology, University of Kansas- Lawrence, Kansas
3Department of Geosciences, University of Wisconsin- Milwaukee, Wisconsin
• Sometimes in Antarctica you do science
just because you are dropped off by a helicopter for the entire day…
• Initially we thought this was going to be a Devonian outcrop (shoreface?), but were confused by the regional geologic map and landed on younger stratigraphy.
• So we asked ourselves “what is this succession?”
• When we realized it was the Permian Mackellar/Fairchild fms. we asked “what type of delta is this?”
• Did this delta prograde into a marine or lacustrine environment? (combine sedimentology and ichnology).
• How does our interpretation fit-in with other outcrops and interpretations of the Mackellar and Fairchild fms. in Antarctica (extensive work by Miller, Isbell, Collinson, Barrett, and others).
Reason for this study:
Study Area
Early Permian (Sakmarian)
PLATES/UTIG
Study area at 75° S to 85° S in early Permian
Study Area
Long et al., 2008
Location and Geologic Map
• Devonian to Jurassic succession • Icehouse to greenhouse transition • Mackellar is postglacial Permian
Icehouse Antarctica, ~290 MA
Isbell et al., 2008
Isbell et al., 2012
Regional Paleogeography
Veevers & Powell, 1994
Miller & Frisch, 1987
E
W
E W
1) High-latitude, postglacial lake similar in size and depth to glacial lake Agassiz, 350,000 km2, 200 m deep (Miller and Isbell 2010)?
2) Vast inland sea with salinities reduced by steady influx of freshwater (melting glaciers)?
Interbedded Basinal Shales, Fan-Sands, & Submarine Channels and Levees Mount Bowers (50 km away)
Submarine Channels/Levees
Submarine Channels and Levees - Compensationally stacked
• Climbing ripples- high sedimentation rate (levees) • Convolute bedding- sediment loading onto wet substrate, high sed. rate
Submarine Channels and Levees
Submarine Channels and Levees
Prodelta to Distal Delta Front
Prodelta to Distal Delta Front
- Erosionally based traction deposits - Wave reworked
Proximal Delta Front
Proximal Delta Front
Proximal Delta Front
Proximal Delta Front
- Channels - Climbing ripples - Wave modified current ripples
Proximal Delta Front - Subaqueous terminal distributaries
Floodback (transgression) or delta lobe switching?
Proximal Delta Front (again)
- Climbing ripples - Wave modified current ripples
Proximal Delta Front
Proximal Delta Front
- Hummocky cross-stratification?
Lower Delta Plain - Braidplain
Lower Delta Plain - Braidplain
Miller and Isbell, 2010
Lacustrine Trace Fossils – we expect…
• Relatively low ichnodiversity and high abundance • Dominantly horizontal feeding and grazing traces • Dominantly epifaunal (less burrowers) • Typically fine-grained, well oxygenated, lower energy systems so no
need for deep burrowing for food... also lower oxygen in bottom sediments
• Low diversity of morphotypes (gradational morphologies between ichnogenera)
• Normal sizes
Ichnology
In the Mackellar Formation
• Overall high ichnodiversity, low bed diversity • Marine ichnofaunal assemblages - impoverished Cruziana and
and Teichichnus. Planolites and Palaeophycus are facies breakers and occur in any environment so they are not diagnostic. Lockeia are found in both marine and freshwater settings, but these forms are typical of marine. Margaritichnus is also marine.
• Teichichnus is a key marine indicator
Trace fossil suite = stressed marine • Pronounced size reduction of traces compared to those expected for
fully marine
• Traces are marine-type (Arenicolites, Teichichnus and others), and facies breakers (e.g., Planolites, Palaeophycus)
• High overall ichnodiversity
• Low individual bed ichnodiversity, with high mono specific abundances
• Adam Jackson et al. poster, Monday afternoon booth 1D
