GEOL 2920C – The Sedimentary Rock Cycle of Mars & Earth Week 12 – Basin Scale Processes on Mars I (April 16, 2018) Discussion Points, Key Equations, Key Figures (prepared by Jesse Tarnas) Eberswalde fan deposits: deltaic or alluvial?

• Existence of paleochannels (Jerolmack et al. 2004, Lewis & Aharonson 2006). o What is the emplacement/cementation mechanism for the coarser-grained or more highly

cemented paleochannel deposits? Are these different mechanisms than those that operate on Earth to cause landscape inversion?

• Is steepness of fan front an aeolian erosional feature or is it indicative of a delta? (Jerolmack et al. 2004, Lewis & Aharonson 2006).

o Is aeolian erosion definitely capable of producing these steep slopes? o Distance between Lobe 1 and Lobe 2 indicates that aeolian erosion has eroded < 100s of

meters of the fan front (Lewis & Aharonson 2006, Fig. 1). o Existence of channels eastward of the fan indicates that only minor aeolian erosion

occurred (Lewis & Aharonson 2006). • Bedding orientation of steeply dipping foresets indicates sediment was deposited in aggradational

delta (Lewis & Aharonson 2006). Fig. 2 shows histogram of dips. • What is the minimum time of deposition of this fan? What is the sediment and fluid discharge for

formation of this fan? o Jerolmack et al. 2004: several decades to centuries depending on assumed grain size. o Lewis & Aharonson 2006: several centuries.

§ Requires basin fills more than 8 times to 1400 m contour (where the highest-elevation fluvial features are), meaning it must empty 7 times via evaporation and infiltration.

o Minimum times of deposition assumes no time gaps during deposition. Is this realistic even if precipitation was not the primary driver of fluvial activity?

o What is the dependency of time scale of formation, teq, on length of the fan? How can compaction skew calculation of teq? (see slides)

§ What is the dependency of volume, V, on length, L? § What is the dependency of sediment discharge, Qs, on length? § How much compaction will burial and exhumation from ice/dirt/dust cover

change volume, V, and porosity, λ? Deltas at Aeolis Dorsa(?):

• Deposition interpreted to be coeval with fan deposition in Terby crater, interpreted to pre-date deposition of Ebserwalde, Jezero, and Melas fans (DiBiase et al. 2013).

• DiBiase et al. 2013 use dip angle of foresets to bolster their argument for deposition as a progradational delta, but show no histogram of their measured dip angles (suspicious?).

• Minimum formation timescale of 400 years for sand-bed and 7300 years for gravel-bed (DiBiase et al. 2013, Table 1).

o No exposed bottomsets for this fan. How does this affect our ability to discern the actual length of the delta?

• Requires standing body of water (no turbidity currents) (DiBiase et al. 2013). o How is this evidence for a Northern ocean if the preferred deposition environment was a

standing body of water (lacustrine style) rather than submarine (ocean style)? § Ice covered ocean?

General: • How can in situ observations improve our characterization of alluvial fan versus deltaic deposits

on Mars?

Jerolmack et al. 2004:

Lewis & Aharonson 2006:

DiBiase et al. 2013

Parker et al. 1998 (equations in Jerolmack et al. 2004):

Jesse Tarnas GEOL2920C Week 12 Discussion Summary

As is a common theme in martian sedimentation studies, the broadest conclusion drawn from our discussion of Jerolmack et al. 2004, Lewis & Aharonson 2006, and DiBiase et al. 2013 is that researchers must be careful when interpreting depositional environments of sedimentary deposits on Mars. Jerolmack et al. 2004 compute a minimum timescale of formation for the fan deposit in Eberswalde crater, which they interpret to be an alluvial fan (for the sake of computing the actual minimum timescale of formation, not because they interpret the deposit to be an alluvial fan based on observations). This analysis is helpful for constraining the lowest possible volume of water input into Eberswalde, but the authors should have more explicitly demonstrated the dependence of their calculated parameters (sediment discharge and fluvial discharge) on the length of the fan, which they assume to be 45 km based on extrapolation of the modern fan slope, which may or may not resemble the paleofan slope.

More recent publications, such as Lewis & Aharonson 2006, argue that the fan deposit in Eberswalde is likely an aggradational delta that experienced limited aeolian erosion (100s of meters laterally). This interpretation of limited vertical erosion is based on observations of existent paleochannels, meaning vertical erosion is less than one paleochannel depth. The interpretation of limited lateral erosion is based on the distance between Lobes 1 & 2 of the fan. The interpretation that the Eberswalde fan is an aggradational delta is based primarily on measured foreset dip angles. The authors argue that formation of the Eberswalde aggradational delta requires several centuries of fluvial activity that must have filled the basin 8 times to its 1400 m contour, where the highest-elevation fluvial features are, meaning the basin must empty 7 times via evaporation and groundwater infiltration.

Both papers covering the Eberswalde fan estimate minimum timescales of formation based on sediment transport models. A key nuance to these calculations is that fluvial activity can be episodic, meaning the minimum timescale can be drawn out over timespans that are multiple orders of magnitude longer than the actual minimum timescale. Episodic sediment deposition on Earth is often driven by precipitation, which may or may not have been active on Mars, so translating this tendency from Earth to Mars is potentially erroneous to an uncharacterizable extent.

DiBiase et al. 2013 argue that a progradational delta exists in Aeolis Dorsa, which would have fed into a standing body of water in the northern plains of Mars. Deposition of this sediment is interpreted to be coeval with fan deposition in Terby crater, which is interpreted to pre-date deposition of fans in Eberswalde, Jezero, and Melas. These authors argue that the flow direction was the opposite of that implied by modern topography (the water flowed in the direction that is, today, uphill) based on the dip angles of the interpreted deltaic foresets. These authors measure many dip angles for this interpreted progadational delta, but do not show the histogram of these dip angles.

Much of our discussion centered on the degree to which dip angle of foresets can be used to differentiate between alluvial fan and deltaic deposition environments. Additionally, much discussion revolved around the extent of erosion experienced by these systems post-deposition, and how this can make it difficult to differentiate between alluvial fan and deltaic deposits, since a highly-aeolian-eroded alluvial fan looks very similar to a delta, except for the dip angle of its foresets. There are situations on the martian surface where one can interpret a deltaic deposit with very high confidence, such as Jezero crater, which has multiple input channels, as well as an outflow channel, as well as a fan deposit at the mouth of an input channel that has dip angles consistent with a delta. These multiple lines of evidence strongly support the interpretation of a deltaic deposit in Jezero crater. When all of these components are not currently observable on the martian surface, as is the case in Aeolis Dorsa and Terby crater, it is much more difficult to confidently interpret a fan as deltatic versus alluvial. In-situ observations will help constrain these interpretations more accurately than we currently can from orbit.