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Sedimentologic and Diagenetic Analysis of Microbialite-Bearing Lacustrine Rift Sequence in Lower Cretaceous Yucca Formation, Indio Mountains, West Texas – An Outcrop Analog to Pre-Salt Microbialite Reservoirs Along the South Atlantic Margins


Microbial carbonate reservoir facies in Cretaceous rift basins along the South Atlantic margins are poorly understood regarding their spatial and temporal distribution and modes of diagenetic porosity enhancement and occlusion. In order to better understand controls on reservoir and trap architecture in these complex systems, we present an outcrop analog study of the depositional/diagenetic features and stratigraphic framework of the Lower Cretaceous Yucca Formation lacustrine microbialite facies (MF) deposited within a syn-rift basin on the eastern margin of the Chihuahua trough. The syn-rift succession was inverted during Laramide-age shortening and is exceptionally well-exposed within 3 thrust panels in the Indio Mountains of West Texas. Microbialite-bearing cycles were also evaluated as to proximity to faults (syndepositional?). Three distinctive types of MF are recognized: (1) gray to greenish microbial carbonate concretions within reddish, light purple or greenish siltstone to mudstone. Concretion interiors contain septarian-style fractures mainly filled by coarse-crystalline calcite (CCC), which is commonly partially silicified following local brecciation. Additionally, saddle dolomite, carbonate fans, spherulites and shrubby textured precipitates (microbialite?) are locally present within the concretions. Porosity networks are formed by dissolution or fracture. The surrounding mudstones show multiple phases of replacement including detrital quartz replaced by euhedral dolomite and dolomite replaced by authigenic quartz; (2) low-porosity stromatolitic bindstone with dark irregular chert nodules. Phreatic dolomite, silica and CCC cements progressively filled in the primary porosity of microbial framework; (3) greenish, slightly dolomitized thrombolite of low porosity in light purple siltstone to mudstone. The three facies are organized into 3 to 5m thick lacustrine cycles separated by fluvial facies. Each cycle progresses upward from concretions, to stromatolites, to thrombolites. The lacustrine cycles are commonly eroded laterally by the overlying fluvial channels that contain basal gravels including MF clasts. The MF are depositionally and diagenetically highly variable over short distances, possibly related to proximity to faults. Microbial reservoir quality is mainly improved by diagenetic dissolution and fracturing during burial diagenesis.