Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Lateral Facies Architecture and the Role of Antecedent Topography on Carbonate Facies Geometries in Incipient Icehouse Climatic Conditions Carboniferous/Lower Pennsylvanian (Baskirian/Morrowan): Hueco Mountains Texas
(1) Bureau of Economic Geology, Austin, TX.
Carbonate successions representing the initial stages of Pennsylvanian (Morrowan) ice house climatic conditions are rare within the greater Permian Basin, the Hueco Mountains (HM) escarpment being an exception. Phylloid algal bioherms and mounds are distinguishing facies and dominant reservoir hosts within mid-late Carboniferous carbonates worldwide.
Previously proposed frequency and magnitudes of early Pennsylvanian eustatic variations are low (~1/Ma, < 40 m) yielding architectures and facies patterns that are relatively laterally continuous, flat, and generally devoid of abrupt facies transitions (i.e. layer cake). The HM outcrop succession does not conform entirely to a laterally continuous facies architecture model.
Two ~12 m-(40 ft) thick cycles within the HM succession are characterized by lateral facies changes over 10’s of m’s. The transgressive component of the lower cycle is a phylloid/rugose coral boundstone biostrome correlative at the km scale. The regressive superjacent facies laterally grade from stromatolitic/Chaetetes facies, to peloid packstone and bioclastic- to phylloid grain-dominated facies.
The upper cycle initiates with coarse (≤ cm) crinoid grainstones (encrinites). These transgressive shoals have lateral thickness and facies changes over meters to 10’s of meters, pinch out entirely, and shingle. Within the transgressive tract, locally, these shoals are overlain by a phylloid bioherm that thickens in their absence, but also laterally grades from phylloid boundstone into a series of cross-bedded encrinite shoals, 1-1.5 m thick. The facies spectrum of the regressive component in the upper and lower cycle is similar.
We propose that the upper cycle phylloid biohermal community colonized the topographically higher substrate provided by subtidal encrinite shoals. Encrinites in this depositional setting represent a distinct transgressive facies and lateral equivalent to phylloid bioherms, contrasting with depositional models (Miss. and late Penn.) where encrinites are debris beds flanking phylloid mounds.
The architecture of the HM succession may support recent analyses proposing Morrowan eustatic fluctuations of up to 100 m. Alternatively, the HM succession represents tectonically driven local relative base level change. The lateral facies changes identified in an apparent stratiform system reveals that reservoir continuity in this setting is as complex as those in the Virgilian and Permian of the Permian Basin.