A PLEISTOCENE PRO-GLACIAL LACUSTRINE DELTA DEPOSIT IN MICHIGAN, USA – A TERRESTRIAL ANALOG OF THE ANCIENT DELTA-LAKE SYSTEM AND FLOOD DEPOSITS AT JEZERO CRATER, MARS

Sedimentary facies exposed as scarps along the eroded edge of a large fan-shaped body at the western margin of Jezero crater have recently been interpreted as deposits of a typical prograding Gilbert-type delta. Two separate coarsening-upward units consist of bottomset finer-grained material overlain by a ~8-10 m thick foreset units. Each consists of a single tabular tangential cross-bedded predominantly sandstone bed with scattered cobbles, and is capped by poorly sorted, commonly clast-supported, locally lenticular cobble conglomerate with boulders to > 1 m in size.

Here we describe highwall exposures 10-15 m high in a gravel pit in Pleistocene glacial deposits in southern Michigan. Three episodes of gravel deposition and delta progradation are indicated. The Lower Gravel consists of ~10 m of ~0.5-2 m cross-bedded sandy conglomerate that was formed as interbedded tabular tangential and either trough cross-bedded bedload-dominated topset channel-fills or meter-scale tangential delta foreset beds. The ~10 m thick Lower Gravel terminates laterally at a foreset the height of the entire Lower Gravel unit’s thickness. Two overlying sand-gravel couplets are much thinner, coarsening-upward topset-only facies representing delta progradation beyond the study area. Bottomset beds were not exposed at this location.

The meter-scale thickness of individual Lower Gravel beds (interbedded gravel bar and bedform topset and cross-bedded gravel foreset) suggests meter-scale rise in lake level between deposition of successive topset beds. The sand foreset-beds downflow of the prograding Lower Gravel are similar in thickness to the gravelly sand foreset beds at Kodiak. This suggests similar lake depths during deltaic progradation into their respective lakes.

Compared with the deltaic deposits of Jezero crater, the terrestrial deposit exhibits many similarities in the gravel and conglomerate facies, important differences in the sandy facies, and evidence of paleo-lake level rise and re-advance of the Gilbert-type delta. Both the similarities and differences support the main interpretations of episodes delta progradation during episodes of gradual lake-level rise to depths of a few tens of meters during a longer-term trend of lake-level regression in Jezero crater, as proposed by the Mars2020 science team.