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

Upper Paleozoic shallow-water (algal) bioherms were sensitive to changes in accommodation, and thus preserve potentially high-resolution records of th e (glacio)eustatic and tectonic perturbations that prevailed during late Pa leozoic 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 proxima l to this complex include (1) boundstones (cement, algal-, peloidal-, and f araminiferal-rich variants) within mound-core regions, (2) packstones (skel etal-, foraminiferal-, algal-, and peloidal-rich debris) within mound-flank regions, and (3) auxiliary facies (oncoidal wackestone, algal bindstone, c arbonate mudstone) formed in low-relief off-mound regions. Sequence stackin g in this system was the result of high frequency, high-amplitude glacioeus tasy that prevailed during late Paleozoic time. Paleorelief on subaerial ex posure surfaces records glacioeustatic amplitudes in excess of 80-100 m; pr eserved paleoslopes locally exceeding 40 degrees (compactionally corrected) indicate that the mounds were cemented geologically instantaneously and no t 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 comp lex. Stratal accretion in mound nucleation stages produced late-falling-sta ge ("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 (glac ioeustatic) accommodation and producing an anomalously thick, near keep-up sequence in the mound-core region. Subsequent sequences of the mound core n ucleated 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 sy stem 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 p atterns owing to the limits of short-term accommodation production in any g iven locality. Sequence thicknesses in shallow-water biohermal systems vary substantially laterally as a result of variable sedimentation (accretion) rates as well a s other environmental factors. Accordingly, sequence thicknesses should nev er be used as faithful and consistent proxies for calculations of accommoda tion space or eustatic magnitudes.