Lexie Stodden - Geology Department, Western Washington University

WWU Graduate School Collection, 953, 2020.

Abstract

Preserved fluvial and deltaic sedimentary deposits found within martian crater-fill basins are important evidence documenting past warmer, wetter climatic periods on Mars. The morphologic and stratigraphic record variably complex transgression and regression histories of crater-lake levels, driven by fluctuations in the prevailing hydroclimatic conditions. Yet this tendency for direct inversion of sedimentary characteristics to formative boundary conditions largely neglects large-scale autogenic processes operating in crater-fill basins. The goal of this research is to illustrate an idiosyncratic feature of these basin types, wherein attributes of the sediment source play an outsized role in dictating conditions of the sink that ultimately stores the sediment. This linkage is rarely a concern in sedimentary basins on Earth but appears to strongly influence martian stratigraphy. Here, we examine sequence stratigraphic patterns produced in an experimental sedimentary basin wherein an initially empty basin passively receives a constant sediment flux and water discharge until reaching a spill-point elevation. This experimental setup captures the simplest of all feasible crater-fill basin evolution histories, and we present data from five experiments that vary the initial basin size and sediment flux parameters. We used Computed Tomography (CT) scans to analyze internal deltaic stratigraphy and found that five main sequence stratigraphic phases are produced. These phases are spontaneously generated under constant boundary conditions, indicating they are autogenic and directly attributable to the consequences of mass balance. As sediment and water input volumes "compete" for accommodation in the crater they cause five major depositional phases: 1) an early fluvial progradational phase; 2) a transitional phase from fluvial to deltaic progradational deposition; 3) a retreat phase of retrogradation/aggradational back-stepping deltaic deposition; 4) an over-topping phase of late progradational delta deposition; and 5) a forced progradational phase when steady state base level is reached. This experimental sequence stratigraphic pattern compares favorably with well-studied martian crater-fill sedimentary packages such as the Eberswalde Delta, the Southwest Eberswalde Deposit and the Jezero Western Delta. Thus, we suggest our experiments constitute a alternative, but useful "starting" sequence stratigraphic framework for approaching crater-fills rather than a direct application of marine sequence stratigraphic models, which assume an infinitely large ocean basin.

How Our Software Was Used

Dragonfly was used to analyze CT scan files.