Sedimentology and climatic environment of alluvial fans in the martian Saheki crater and a comparison with terrestrial fans in the Atacama Desert

Highlights

• Wind erosion reveals Saheki crater fan stratigraphy.

• A distributary network of fluvial channels fed extensive mudflow overbank deposits.

• The fans are up to 850 m thick and contain 550 km³ of sediment.

• Fan-forming discharges derived from annual or episodic melting of crater rim snow.

• Thousands of years were required to deposit the fans.

Abstract

The deflated surfaces of the alluvial fans in Saheki crater reveal the most detailed record of fan stratigraphy and evolution found, to date, on Mars. During deposition of at least the uppermost 100 m of fan deposits, discharges from the source basin consisted of channelized flows transporting sediment (which we infer to be primarily sand- and gravel-sized) as bedload coupled with extensive overbank mud-rich flows depositing planar beds of sand-sized or finer sediment. Flow events are inferred to have been of modest magnitude (probably less than ∼60 m³/s), of short duration, and probably occupied only a few distributaries during any individual flow event. Occasional channel avulsions resulted in the distribution of sediment across the entire fan. A comparison with fine-grained alluvial fans in Chile’s Atacama Desert provides insights into the processes responsible for constructing the Saheki crater fans: sediment is deposited by channelized flows (transporting sand through boulder-sized material) and overbank mudflows (sand size and finer) and wind erosion leaves channels expressed in inverted topographic relief. The most likely source of water was snowmelt released after annual or epochal accumulation of snow in the headwater source basin on the interior crater rim during the Hesperian to Amazonian periods. We infer the Saheki fans to have been constructed by many hundreds of separate flow events, and accumulation of the necessary snow and release of meltwater may have required favorable orbital configurations or transient global warming.

Introduction

An alluvial fan is a semi-conical landform that develops where a channel exits a confined valley, and through avulsions and channel branching spreads sediment across the unconfined terrain (Blair and McPherson, 2009). The combination of slope reduction and lateral spreading reduces the carrying capacity and forces progressive sediment deposition. Martian alluvial fans have been identified ranging in scale from sub-kilometer (Williams and Malin, 2008) to a few kilometers (Burr et al., 2009) to tens of kilometers (Moore and Howard, 2005, Kraal et al., 2008, Anderson and Bell, 2010, Grant and Wilson, 2011, Grant and Wilson, 2012). The well-preserved, mid-latitude fans of the Hesperian (and perhaps even younger) (Grant and Wilson, 2011, Kraal et al., 2008, Moore and Howard, 2005, Morgan et al., 2012a, Morgan et al., 2012b) are of particular interest because they, along with deltas (e.g., Malin and Edgett, 2003, Moore et al., 2003, Lewis and Aharonson, 2006, Pondrelli et al., 2008, Pondrelli et al., 2011, Mangold et al., 2012b, Wilson et al., 2013) and small valleys in the mid-latitude regions (e.g., Hynek et al., 2010, Fassett et al., 2010, Howard and Moore, 2011, Mangold, 2012), may represent a widespread episode of large-scale fluvial landform construction and modification on Mars occurring well after the Late Noachian to Early Hesperian epoch of valley network incision (Grant and Wilson, 2011, Howard and Moore, 2011). This later period of fluvial activity occurred in an environment thought to be characterized by a relatively thin atmosphere and global cryosphere (Carr and Head, 2010, Fassett and Head, 2011, Lasue et al., 2013). Although difficult to decipher, the effects of water (both fluid and ice) on a paleo-landscape are the most unambiguous markers of past climatic environment and have significant potential to further our understanding of the climate evolution and potential late-stage habitability of Mars.

Almost all mapped martian alluvial fan systems have been found to be enclosed within basins (craters) and source from deeply incised crater rim alcove basins (Moore and Howard, 2005, Kraal et al., 2008, Wilson et al., 2013)., which strongly constrains both the hydrological and sedimentary environments. The association between the sediment and water source areas in the dissected crater wall and the fan system is short and direct. The presence of a large alluvial fan in an enclosed basin limits the possible range of the hydrologic regime; on the low end it is constrained by the necessity to erode and transport sediment from the headwater source and on the high end by the apparent absence of coincident deep lakes within the crater. Previous work on the larger equatorial fans has concluded that they formed during periods of enhanced precipitation (probably as snowfall) primarily through hundreds of flow events over tens of thousands (to perhaps millions) of years (Armitage et al., 2011, Grant and Wilson, 2011, Grant and Wilson, 2012, Moore and Howard, 2005).

This study focuses on Saheki crater (85 km-diameter, 21.7°S, 73.2°E), one of several fan-bearing craters along the northern rim of the Hellas basin (Fig. 1, Fig. 2). Fans in this crater are among the largest catalogued by Kraal et al. (2008) and Moore and Howard (2005) and contain the clearest exposed stratigraphy yet identified on Mars (Table 1). The studies of alluvial fans in southern Margaritifer Terra (Grant and Wilson, 2011, Grant and Wilson, 2012) revealed that the fans and fan-deltas of this region date to the Late Hesperian to Early Amazonian rather than being coeval with the extensive valley networks of Late Noachian to Early Hesperian, which had been suggested by Howard et al. (2005). The exquisite alluvial fan stratigraphy exposed in craters of the Hellas north rim (Fig. 1), and Saheki crater in particular (Fig. 2), permits a comprehensive assessment of several unresolved issues concerning martian alluvial fans, including the mode of sedimentation (e.g., fluvial versus debris flows), the magnitude, frequency and duration of formative flows, the age of the alluvial fans and the associated climatic environment. We address these issues through a detailed stratigraphic analysis, crater count-derived ages, quantitative interpretation of the flows (velocity and discharge) forming the fans, and a comparison with a terrestrial analog fan system in the Chilean Atacama Desert. This is followed by our synthesis of the fan sedimentology, geologic history of fan deposition, and the associated hydrologic and climatologic environment. We conclude that the Saheki crater fans were deposited by a combination of channelized fluvial and muddy overbank flows by many separate flow events numbering in the hundreds to thousands over an extended time period around the Hesperian–Amazonian boundary. Snowmelt sourced from upper crater walls is found to be the most tenable water source.