GEOL 2920C The Sedimentary Rock Cycle of Mars & Earth Eberswalde fan deposits: deltaic or alluvial? Deltas at Aeolis Dorsa(?

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Mark Wood
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1 GEOL 2920C The Sedimentary Rck Cycle f Mars & Earth Week 12 Basin Scale Prcesses n Mars I (April 16, 2018) Discussin Pints, Key Equatins, Key Figures (prepared by Jesse Tarnas) Eberswalde fan depsits: deltaic r alluvial? Existence f palechannels (Jerlmack et al. 2004, Lewis & Aharnsn 2006). What is the emplacement/cementatin mechanism fr the carser-grained r mre highly cemented palechannel depsits? Are these different mechanisms than thse that perate n Earth t cause landscape inversin? Is steepness f fan frnt an aelian ersinal feature r is it indicative f a delta? (Jerlmack et al. 2004, Lewis & Aharnsn 2006). Is aelian ersin definitely capable f prducing these steep slpes? Distance between Lbe 1 and Lbe 2 indicates that aelian ersin has erded < 100s f meters f the fan frnt (Lewis & Aharnsn 2006, Fig. 1). Existence f channels eastward f the fan indicates that nly minr aelian ersin ccurred (Lewis & Aharnsn 2006). Bedding rientatin f steeply dipping fresets indicates sediment was depsited in aggradatinal delta (Lewis & Aharnsn 2006). Fig. 2 shws histgram f dips. What is the minimum time f depsitin f this fan? What is the sediment and fluid discharge fr frmatin f this fan? Jerlmack et al. 2004: several decades t centuries depending n assumed grain size. Lewis & Aharnsn 2006: several centuries. Requires basin fills mre than 8 times t 1400 m cntur (where the highestelevatin fluvial features are), meaning it must empty 7 times via evapratin and infiltratin. Minimum times f depsitin assumes n time gaps during depsitin. Is this realistic even if precipitatin was nt the primary driver f fluvial activity? What is the dependency f time scale f frmatin, t eq, n length f the fan? Hw can cmpactin skew calculatin f t eq? (see slides) What is the dependency f vlume, V, n length, L? What is the dependency f sediment discharge, Q s, n length? Hw much cmpactin will burial and exhumatin frm ice/dirt/dust cver change vlume, V, and prsity, λ? Deltas at Aelis Drsa(?): Depsitin interpreted t be ceval with fan depsitin in Terby crater, interpreted t pre-date depsitin f Ebserwalde, Jezer, and Melas fans (DiBiase et al. 2013). DiBiase et al use dip angle f fresets t blster their argument fr depsitin as a prgradatinal delta, but shw n histgram f their measured dip angles (suspicius?). Minimum frmatin timescale f 400 years fr sand-bed and 7300 years fr gravel-bed (DiBiase et al. 2013, Table 1). N expsed bttmsets fr this fan. Hw des this affect ur ability t discern the actual length f the delta? Requires standing bdy f water (n turbidity currents) (DiBiase et al. 2013). Hw is this evidence fr a Nrthern cean if the preferred depsitin envirnment was a standing bdy f water (lacustrine style) rather than submarine (cean style)? Ice cvered cean? General: Hw can in situ bservatins imprve ur characterizatin f alluvial fan versus deltaic depsits n Mars?

2 Jerlmack et al. 2004: Lewis & Aharnsn 2006:

3 DiBiase et al. 2013

5 Parker et al (equatins in Jerlmack et al. 2004):

6 Jesse Tarnas GEOL2920C Week 12 Discussin Summary As is a cmmn theme in martian sedimentatin studies, the bradest cnclusin drawn frm ur discussin f Jerlmack et al. 2004, Lewis & Aharnsn 2006, and DiBiase et al is that researchers must be careful when interpreting depsitinal envirnments f sedimentary depsits n Mars. Jerlmack et al cmpute a minimum timescale f frmatin fr the fan depsit in Eberswalde crater, which they interpret t be an alluvial fan (fr the sake f cmputing the actual minimum timescale f frmatin, nt because they interpret the depsit t be an alluvial fan based n bservatins). This analysis is helpful fr cnstraining the lwest pssible vlume f water input int Eberswalde, but the authrs shuld have mre explicitly demnstrated the dependence f their calculated parameters (sediment discharge and fluvial discharge) n the length f the fan, which they assume t be 45 km based n extraplatin f the mdern fan slpe, which may r may nt resemble the palefan slpe. Mre recent publicatins, such as Lewis & Aharnsn 2006, argue that the fan depsit in Eberswalde is likely an aggradatinal delta that experienced limited aelian ersin (100s f meters laterally). This interpretatin f limited vertical ersin is based n bservatins f existent palechannels, meaning vertical ersin is less than ne palechannel depth. The interpretatin f limited lateral ersin is based n the distance between Lbes 1 & 2 f the fan. The interpretatin that the Eberswalde fan is an aggradatinal delta is based primarily n measured freset dip angles. The authrs argue that frmatin f the Eberswalde aggradatinal delta requires several centuries f fluvial activity that must have filled the basin 8 times t its 1400 m cntur, where the highestelevatin fluvial features are, meaning the basin must empty 7 times via evapratin and grundwater infiltratin. Bth papers cvering the Eberswalde fan estimate minimum timescales f frmatin based n sediment transprt mdels. A key nuance t these calculatins is that fluvial activity can be episdic, meaning the minimum timescale can be drawn ut ver timespans that are multiple rders f magnitude lnger than the actual minimum timescale. Episdic sediment depsitin n Earth is ften driven by precipitatin, which may r may nt have been active n Mars, s translating this tendency frm Earth t Mars is ptentially errneus t an uncharacterizable extent. DiBiase et al argue that a prgradatinal delta exists in Aelis Drsa, which wuld have fed int a standing bdy f water in the nrthern plains f Mars. Depsitin f this sediment is interpreted t be ceval with fan depsitin in Terby crater, which is interpreted t pre-date depsitin f fans in Eberswalde, Jezer, and Melas. These authrs argue that the flw directin was the ppsite f that implied by mdern tpgraphy (the water flwed in the directin that is, tday, uphill) based n the dip angles f the interpreted deltaic fresets. These authrs measure many dip angles fr this interpreted prgadatinal delta, but d nt shw the histgram f these dip angles. Much f ur discussin centered n the degree t which dip angle f fresets can be used t differentiate between alluvial fan and deltaic depsitin envirnments. Additinally, much discussin revlved arund the extent f ersin experienced by these systems pst-depsitin, and hw this can make it difficult t differentiate between alluvial fan and deltaic depsits, since a highly-aelianerded alluvial fan lks very similar t a delta, except fr the dip angle f its fresets. There are situatins n the martian surface where ne can interpret a deltaic depsit with very high cnfidence, such as Jezer crater, which has multiple input channels, as well as an utflw channel, as well as a fan depsit at the muth f an input channel that has dip angles cnsistent with a delta. These multiple lines f evidence strngly supprt the interpretatin f a deltaic depsit in Jezer crater. When all f these cmpnents are nt currently bservable n the martian surface, as is the case in Aelis Drsa and Terby crater, it is much mre difficult t cnfidently interpret a fan as deltatic versus alluvial. In-situ bservatins will help cnstrain these interpretatins mre accurately than we currently can frm rbit.

UNIT IX: ROCKS NOTES PACKET

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