Abstract

The 'White Rock', constituting a portion of what may be a lacustrine sedimentary sequence near the margin of a crater c. 90 km across in Sabaea Terra, Mars, measures 18 × 15 km × 180–540 m high. It is re-interpreted as a lens of magnesium carbonate precipitated where ground waters seeped into an ancient evaporating crater lake. Were life to have emerged on Mars, as seems feasible, then the 'White Rock' might be expected to comprise a complex of stromatolitic mounds. Salda Gölü (Lake) in Turkey, is taken as an analogue. This enclosed lake is nearly surrounded and underlain by partially serpentinized harzburgite. Hydromagnesite stromatolites (microbialites) up to 7 m high coalesce to form a group of small islands 200 m across. The microbialites are seen to be growing near the mouth of the usually dry Salda River in the southwestern sector. Smaller developments of hydromagnesite encircle the lake and image processing of satellite data reveals a second extensive zone beneath the lake surface over a delta in the southeast. Individual columns a few centimetres high constitute bulbous mounds which are about 2 m in diameter. These columns terminate in domes a centimetre or so across. The domes are often annulated and are covered with a green biofilm a few millimetres thick comprised of cyanobactrial filaments. The columns consist of alternating fine and coarse hydromagnesite layers differentiated on a millimetric scale. The coarser layers near the surface still contain traces of the biofilm.

Fossil microbialites were also discovered in the friable hydromagnesite cliffs shoreward of the main developments, though the structures of the individual microbes have not survived. Instead the vestiges of microbialites are easily recognized and delineated by their coarse grain size and high porosity. Annular structures on their upper surfaces can be seen in places. The intervening and overlying material, also comprised of hydromagnesite, is a semi-lithified mud.

Bulbous megascopic structures, separated by finer grained magnesium carbonate mudstone, within strata in the 'White Rock', would be strong evidence of a photosynthetic microbial genesis. Another deposit of white rock on the western margin of Juventae Chasma could have a similar origin.

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