ANACHRONISTIC PERIODS IN EARTH HISTORY AND THEIR ROLE IN THE SEARCH FOR EXTRATERRESTRIAL MICROBIALITES: AN EXAMPLE FROM THE EARLY TRIASSIC

Precambrian rocks provide an important analogy for extraterrestrial environments and possible biota as they offer a wealth of examples of microbial systems in unusual surroundings. Anachronistic periods, or those times in Phanerozoic history when environmental conditions appear more similar to those of the Precambrian, may also provide an important means of examining microbial systems under atypical circumstances. These periods of Phanerozoic history may have an advantage over Precambrian examples as geochemical signals are less likely to have been altered over time and the volume of rock available for study is typically greater. Lower Triassic rocks from the southwestern United States, deposited in the aftermath of the Permian-Triassic mass extinction under unusual paleoceanographic conditions, offer an excellent point of comparison with possible extraterrestrial systems, specifically one where alkaline waters are present or were present in the past. The shallow water Virgin Limestone (Moenkopi Formation) of southern Nevada and southeastern Utah and its deeper water equivalent, the Union Wash Formation of east-central California, were deposited in a carbonate ramp setting and contain a lateral sequence of abiotic and microbial carbonates that formed in environments ranging from basinal to intertidal. The sequence consists of: 1) Basin: laterally-extensive crusts of abiotic calcite in which the crystals grew with their c-axes perpendicular to the seafloor; 2) Outer Ramp to Slope: abiotic fans and hemispheres comprised of calcite crystals that occur as solitary bodies or as interlocking mosaics; 3) Middle Ramp: stromatolitic or thrombolitic microbial buildups; 4) Inner Ramp: stratiform stromatolitic or thrombolitic masses; and 5) Intertidal: ooids, oncoids, and coated grains. Documentation of these features provides an example of the types of microbial and abiotic features that might appear in extraterrestrial environments or rocks, while an understanding of the lateral differences in abiotic and microbial carbonates may offer information about the type of environment in which those features accumulated in the past. Abiotic carbonates may not preclude the presence of microbial carbonates elsewhere, but instead point towards regions or strata where microbial carbonates are likely to occur.