Facies character and stratal responses to accommodation in Pennsylvanian bioherms, western Orogrande Basin, New Mexico

Abstract

ABSTRACT Upper Paleozoic shallow-water (algal) bioherms were sensitive to changes in accommodation, and thus preserve potentially high-resolution records of the (glacio)eustatic and tectonic perturbations that prevailed during late Paleozoic time. Detailed field and petrographic study of a high-relief (> 100 m), well-exposed mound complex of the western Orogrande basin (New Mexico) indicates that it consists of a series of stacked high-frequency sequences bounded by surfaces of paleo-subaerial exposure. Facies within and proximal to this complex include (1) boundstones (cement-, algal-, peloidal-, and foraminiferal-rich variants) within mound-core regions, (2) packstones (skeletal-, foraminiferal-, algal-, and peloidal-rich debris) within mound-flank regions, and (3) auxiliary facies (oncoidal wackestone, algal bindstone, carbonate mudstone) formed in low-relief off-mound regions. Sequence stacking in this system was the result of high-frequency, high-amplitude glacioeustasy that prevailed during late Paleozoic time. Paleorelief on subaerial exposure surfaces records glacioeustatic amplitudes in excess of 80-100 m; preserved paleoslopes locally exceeding 40° (compactionally corrected) indicate that the mounds were cemented geologically instantaneously and not easily eroded, even at lowstands. Timing of stratal accretion within a given sequence varied significantly as a function of both spatial and temporal position within the biohermal complex. Stratal accretion in mound-nucleation stages produced late-falling-stage (catch-down) sequences both on- and off-mound. During the acme phase of mound growth in this complex, accretion occurred during sea-level rise and maximum inundation phases, nearly pacing production of short-term (glacioeustatic) accommodation and producing an anomalously thick, near keep-up sequence in the mound-core region. Subsequent sequences of the mound core nucleated atop significant paleobathymetric relief, accreted during maximum inundation to incipient fall, and therefore display thicknesses that likely reflect long-term accommodation potential. In a shallow-water biohermal system capable of accretion rates commensurate with production of short-term accommodation space, very thick near keep-up sequences are laterally offset from one another in progradational, retrogradational, or random shifting patterns owing to the limits of short-term accommodation production in any given locality. Sequence thicknesses in shallow-water biohermal systems vary substantially laterally as a result of variable sedimentation (accretion) rates as well as other environmental factors. Accordingly, sequence thicknesses should never be used as faithful and consistent proxies for calculations of accommodation space or eustatic magnitudes.