A TALE OF TWO DELTAS: COMPARATIVE ANALYSIS OF ANCIENT, “RIVER-DOMINATED” DELTAIC SYSTEMS FROM OUTCROPS OF THE PERMIAN IN ANTARCTICA AND THE CRETACEOUS OF COLORADO

We compare two “river-dominated” deltaic systems deposited under Permian icehouse-to-greenhouse (Mackellar Formation) and Cretaceous greenhouse conditions (Loyd Sandstone). Our purpose is to 1) discuss similarities-differences between the two systems, and 2) examine the validity of the river-dominated classifications.

The Early Permian Turnabout Ridge Delta, Mackellar Formation, Central Transantarctic Mountains, is dominated by facies-sedimentary structures indicating recurring turbulent jets and hyperpycnites. Trough cross-stratified to climbing-ripple filled channel-form sandbodies with soft sediment deformation encased in prodelta to delta front deposits are interpreted as subaqueous terminal distributary channels (TDC) and levees. TDCs are overlain by low-angle planar-laminated mouth bars, TDCs, and hyperpycnites along the delta front, and a sand-dominated braidplain. Ichnology indicates that 12 of 30 traces are marine, 18 traces are found across marine-brackish-freshwater systems, and none are known solely from freshwater systems, suggesting a river-dominated, marine delta stressed by freshwater-sediment input. Sediment-laden glacial meltwater freshets likely prepped a marine basin, producing recurring hyperpycnites and jets along a river-dominated delta along the shoreline of a Permian Antarctic epeiric seaway.

The Cretaceous Loyd Delta of Colorado contains low-angle planar-laminated foresets and Bouma-like sequences suggesting recurring hyperpycnal flows, indicating that the delta should be classified as river-dominated. In contrast, a high-abundance high-diversity trace fossil assemblage, deeply penetrating burrows, and flaser-wavy-lenticular bedding between foresets is evidence that the delta front experienced extended periods of slow sedimentation and oxygen influx during which marine fauna flourished and tidal and wave forces dominated.

Although sedimentary structures, stratal geometries, and ichnology vary in these two deltas, the predominance of traction dominated underflows (hyperpycnites) in each system indicates that fluvial-flood processes were the driving force behind the bulk of the preserved stratigraphy, although the ichnology of both deltas suggest that significant time is represented by a lack of freshwater or sediment input.