Holden Stratal Geometries & Depositional Hypotheses Dawn Sumner, Gilles Dromart, Ralph Milliken, Ken Edgett, Mike Malin (by way of Ken) with input from entire LSWG Depositional Hypotheses Depend on Stratal Geometries Bedforms Help Uzboi Breach Provides Context 1

Grant et al’s Strat Column Contact exposed – onlap of light toned sediments onto mound Lower Member Poorly Exposed; Probably not Lowest Sed Unit Upper Member: More Information * Middle Member: Few Observations Upper Unit: Best Depositional Model * * Units Discussed Here 2

Depositional Models for Upper Member of Lower Unit Lacustrine Deposition (favored) Distal Fluvial Deposition (possible) Aeolian Deposition (possible) Gilles’ Likeliness Chart: 3

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? channel ? downlap Strata down dip north or northwestwards 5

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Possible Stratal Geometries (but need to map on DEMs) 7

Upper Unit Lower Unit Upper Member Lower Unit – Middle Member South 30 m Same area 8

Stratal Geometry Questions for Upper Member of Lower Unit Are the Downlap (or Onlap) relationships real? How far can individual beds be traced vs how far are bedding patterns similar? Do bedding characteristics change with distance from the crater wall, e.g. sediment source? Are these changes more or less consistent with a specific depositional environment? 9

Preserved dunes in upper member of lower unit (next slide) 10

10 m Fairly straight-crested, no apparent ‘edge’ effects, minimal bifurcation. Resolution is such that aeolian or wave ripple origin can’t be determined. 11

Depositional Models for Upper Unit Uzboi Breach Erosion & Deposition – Evidence for erosion – 100 m blocks – Undermining & collapse of upper member of lower unit (with little transport) – Erosion and transport via debris (or viscous) flows – Very large sand waves (turbulent flow required) – M-scale light toned boulders in sand waves All characteristic of rapid, large volume flow slowing down in response to broadening of flow and decrease in slope 12

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N 200 m 14

Large, Angular, Layered, Light-toned Lithic Clasts Edgett, p. 15 From Figure 5c of Grant et al Geology paper (I think N is down) Clasts were broken from the underlying bedrock when a catastrophic flood came through Uzboi Vallis. Did they “fall” here or were they transported?

Large, Angular, Layered, Light-toned Lithic Clasts Edgett, p. 16 From Mike Malin. Large basalt lithic clasts transported in Iceland catastrophic floods. These have not been moved far—generally they are just a few kilometers from their source. Top: Flood-transported clasts ~30 km from Dyngjufjökul Glacier. Bottom: Flood-transported clasts ~43 km from the same, but these are not the largest clasts in the area. 1 Ravine = the height of Mike Ravine (the person in each picture).

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Large, Angular, Layered, Light-toned Lithic Clasts Edgett, p. 18 Sub-frame of HiRISE PSP_001468_1535 This is a landslide! Mike Malin pointed out this observation.

subaqueous dunes (next slide) Also present here 19

Fluvial features high-energy flooding event associated with Uzboi breaching the crater rim (duneforms, rib & furrow, large clasts). Paleocurrent is aligned with Uzboi breach. 20

Summary Upper Unit: Uzboi Breach Erosion & Deposition – High Confidence in General Model – Many Details to pursue Erosion & transport of lower unit clasts Fully fluvial, flow into ponded water, or debris flows? Lower Unit: Lacustrine, Fluvial or Aeolian Deposits – Medium Confidence in Lacustrine Interpretation; Other options viable – Need regional bedding geometries 21

Habitability & Preservation Depends on Depositional Environment – Lakes excellent, although transient lakes less so – Fluvial or Aeolian not as good Clays Generally Good – Multiple sources (crater rim, Uzboi Vallis) More Extensive Discussion with Eberswalde – to allow more discussion of geological relationships today 22