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A Seal Breach on Mars – How We Closed in on a Mudstone and Teased Information from Images and Comparison with Experimental and Earth Analogs


The Sheepbed mudstone is the first mudstone described on another planet. From a distance, initial indications that we had found a soft and fine grained rock were the stark erosional contrast to the overlying sandstones. Closer in, its eolian abrasion features compared closely to those observed on experimentally abraded mudstones from Earth. Grooving of the rock with the “Brush Tool” showed close correspondence to clay-rich mudstones from Earth. Imaging with the MAHLI camera established that the grain size of the rock was definitely below the 62 micron limit for mudstones, and probably substantially finer. XRD analysis with Chemin established that the rock contained abundant clay minerals. These mudstones show discrete soft-eroding layers that appear otherwise featureless, range in thickness from a few cm's to 10 cm and are separated by more resistant (and likely coarser) thin layers (mm-cm thickness) that occur singly or as sets of multiple layers. In places these resistant horizons show features suggestive of ripples. Depositional cycles of several dm's thickness are seen in the distance, apparently sandstone based and softer towards the top, likely an expression of upwards increase of clay content and fining upwards. Such cycles might reflect deposition from ashfalls, but may also reflect climate fluctuations, tectonic pulses, base level fluctuations, or also pulses of fluvial sediment input into a lake. Intercalated resistant beds may reflect winnowing by wave action, as well as current action and smaller scale sediment pulses. These rocks also contain small early diagenetic nodules, cemented erosion resistant vein fills, and later diagenetic calcium-sulfate filled fractures. The latter closely resemble fracture networks generated by hydraulic fracturing of overpressured mudstones. Sub-horizontal and horizontal fractures suggest that the fluids from which Ca-sulfates precipitated exceeded lithostatic pressure. The rocks may have been buried to substantial depths (in excess of 1 km) by the time they were hydraulically fractured. To generate the substantial volumes of mineralizing fluids that must have passed through these rocks required a thick section of underlying compacting sediments. The Sheepbed mudstones appear to have acted as a seal for fluid movement from underlying strata, and the breach of this seal attests to substantial burial and vertical movement of evolved brines from underlying or laterally adjacent strata.