Recognition and significance of stratal geometries and unconformities in Martian Polar Layered Deposits

Pyroclastic surges are dilute flows of gas and rock fragments, typically generated by the interaction of magma and water. Due to their hazardous nature, very little is known about sediment transport during these eruptions. However, the cross-stratified deposits that they leave behind provide an important record of flow conditions, if properly interpreted. In the absence of geologic context (and volcanic indicators such as bombs and lapilli), it may be difficult to distinguish bedforms in pyroclastic surge deposits from those in eolian or fluvial deposits. There has been some debate about the identification of pyroclastic surge deposits on Mars, suggesting a need to establish better criteria for recognizing these deposits in remote sensing applications. The goals of this study are to use physical characteristics to better understand bedform kinematics and gain insight into the flow dynamics of pyroclastic surges, and to establish criteria to distinguish pyroclastic surges from other depositional environments on Mars.

 

Two examples of pyroclastic surge deposits are exposed in Hunt's Hole and Kilbourne Hole in southern New Mexico. These volcanic craters expose up to 13 m of stratigraphy, dominated by dm-to-m-scale bedforms. The geomorphic pattern around the rim of Hunt's Hole provides 3D exposures at the scale of the bedforms, which enables observations of bedform geometries. We identify several facies, and measure bedform characteristics in the cross-stratified facies.  We propose that bedforms in pyroclastic surges can be identified by a unique style of stoss-side accretionary cross-stratification. Previous studies, of other pyroclastic surge deposits, identified chute and pool structures and potential antidunes, indicating high Froude number flow conditions. However, all bedforms observed at Hunt's Hole are consistent with downstream transport under lower flow regime conditions.

 

This study brings a new approach to bedform reconstructions, through the use of Terrestrial Laser Scanning (TLS) to studycross-stratification. TLS is based on Light Detection and Ranging (LiDAR). Large grain size variations in surge deposits make them an ideal target for TLS, because the intensity of the returned laser varies with grain size and packing, making individual beds visible in LiDAR data. We then produce digital outcrop models with mm-to-cm-scale resolution. In addition, we apply consumer-level technology to create 3D models from digital photography. The combination of these methods allows for visualization and mapping of geological surfaces in 3D. This high-resolution dataset can be used for bedform reconstructions and to establish quantitative metrics for the identification of pyroclastic surge deposits on Mars.

 

 

 

En route to Endeavour crater, the Mars Exploration Rover Opportunity embarked on a short but significant campaign at Santa Maria crater during sols 2450-2551.  Santa Maria crater is a relatively young impact crater, approximately 100 m in diameter and 11-17 m deep.  Opportunity performed detailed analyses on several ejecta blocks and completed an extensive imaging campaign around the crater.  Many of the ejecta blocks are composed of sandstone with abundant pinstripe laminations, indicative of eolian deposition.  However, other ejecta blocks are massive, fine-grained, and exhibit a nodular texture.  These rocks are interpreted to be the first fine-grained rocks – possibly mudstones  – observed by the rover.  If the inferred mudstones were deposited in a lacustrine setting, then surface water may have created a distinctly different environment than previously regarded at Meridiani Planum.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

HiRISE images PSP_009337_2600 and PSP_009293_2645 showing multiple truncation surfaces in the North Polar Layered Deposits.

Above: Bedforms in pyroclastic surge deposits, Hunt’s Hole, NM. Right: Comparison of LiDAR data and visible imagery.  Cross-bedding is clearly seen in LiDAR  scan; LiDAR intensity is a function of grain size, sorting and composition.

A) Super-resolution mosaic of Cape Verde, acquired during sols 1342-1356.  White dashed line shows location of erosional surface. B) At the base of Cape Verde, angled beds are truncated by low-angle cross-stratification, indicating different migration directions.  C) Climbing ripples superimposed on the larger dune stratification.

 

The Mars Exploration Rover Opportunity has investigated bedrock outcrops exposed in several craters at Meridiani Planum, Mars, in an effort to better understand the role of surface processes in its geologic history.  Opportunity has recently completed its observations of Victoria crater, which is 750 m in diameter and exposes cliffs up to ~15 m high.  The plains surrounding Victoria crater are ~10 m higher in elevation than those surrounding previously-explored Endurance crater, suggesting that Victoria crater exposes a stratigraphically higher section than Endurance crater; however Victoria strata overlap in elevation with the rocks exposed at Erebus crater.  Victoria crater has a well-developed geomorphic pattern of promontories and embayments that define the crater wall, and reveal thick bedsets (3-7 m) of large-scale cross-bedding, interpreted as fossil eolian dunes.  Opportunity was able to drive into the crater at Duck Bay, located on the western margin of Victoria crater.  Data from the Microscopic Imager and Panoramic Camera reveal details about the structures, textures, and depositional and diagenetic events that influenced the Victoria bedrock.  A lithostratigraphic subdivision of bedrock units was enabled by the presence of a light-toned band that lines much of the upper rim of the crater.  In ascending order three stratigraphic units are named Lyell, Smith and Steno; Smith is the light-toned band.  In the Reference Section exposed along the ingress path at Duck Bay, Smith is interpreted to represent a zone of diagenetic recrystallization, however, its upper contact also coincides with a primary erosional surface.  Elsewhere in the crater the diagenetic band cross-cuts the physical stratigraphy.  Correlation with strata present at nearby promontory Cape Verde suggests that there is an erosional surface at the base of the cliff face that corresponds to the erosional contact below Steno.  The erosional contact at the base of Cape Verde lies at a lower elevation, but within the same plane as the contact below Steno, which suggests that the material above the erosional contact was built on significant depositional paleotopography.  The eolian dune forms exposed in Duck Bay and Cape Verde, combined with the geometry of the erosional surface, suggests that these outcrops may be part of a larger-scale draa architecture.  This insight is possible only due to the larger-scale exposures at Victoria crater, which significantly exceed the more limited exposures at Erebus, Endurance, and Eagle craters.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

False color Pancam image of target Ruiz Garcia acquired on sol 2521.  Ruiz Garcia is massive and has a nodular appearance, unlike the typical eolian sandstones found at Eagle, Endurance, Erebus and Victoria craters. White box shows approximate location of MI mosaic.