STRATIGRAPHIC AND DIAGENETIC ANALYSIS OF THE LOWER CRETACEOUS LACUSTRINE MICROBIALITE SUCCESSION OF THE YUCCA FORMATION ALONG THE EASTERN MARGIN OF THE CHIHUAHUA TROUGH, INDIO MOUNTAINS, WEST TEXAS

Successful hydrocarbon exploration in microbial carbonate reservoirs from Cretaceous rift basins along the South Atlantic margins has generated extensive interest in studies of lacustrine microbialite facies. In order to understand controls on reservoir and trap architecture in these complex systems, this study documents the petrographic attributes and stratigraphic architecture of lacustrine microbialite facies exposed in Lower Cretaceous strata in the Indio Mountains, West Texas. The Lower Cretaceous strata were deposited along the eastern margin of a NW-SE trending rift basin (Chihuahua Trough). The basin was inverted during the Laramide Orogeny exposing in cross section the former rift stratigraphy within three thrust panels. The Yucca Formation comprises the rift-fill sequence, which is transgressively overlain by shallow marine carbonate strata of the Bluff Mesa Formation. The lower member of the Yucca Fm. comprises mostly coarse-grained siliciclastic fluvial and alluvial facies. The upper member of the Yucca Fm. (focus of this study), contains interstratified fluvial siliciclastic and lacustrine facies.

Three distinctive types of microbialite facies are recognized in the upper member of the Yucca Fm.: (1) light gray to greenish microbial carbonate concretions set within reddish to light purple calcareous siltstone to mudstone matrix. Concretion interiors contain septarian-style fractures filled by coarse-crystalline calcite and are often partially silicified. The concretion matrix shows multiple phases of replacement – detrital quartz replaced by euhedral dolomite and dolomite replaced by authigenic quartz; (2) stromatolitic bindstone with spherulites and nodular black chert; (3) light green thrombolite (~1cm across), locally dolomitized, within light purple siltstone to mudstone matrix. The three microbial facies are organized into 3 to 5m thick lacustrine cycles progressing upward from concretions, to stromatolites, to thrombolites. Each lacustrine cycle is bounded by fluvial siliciclastic facies. The lacustrine cycles are commonly erosionally terminated laterally by fluvial channels containing various clast types including microbialite facies. The cycles represent an upward increase in lake water depth and circulation, capped by baselevel fall and fluvial incision.