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
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.