Sedimentology and process stratigraphy of the upper Pennsylvanian, Pedregosa (Arizona) and Orogrande (New Mexico) basins.

Abstract

The primary factors that influence stratigraphy are tectonic subsidence, eustasy, and sediment supply. Change in any of these factors potentially produces a similar response in the form of a change in accommodation space. Accordingly, distinguishing the origin of a stratigraphic response is difficult, but theoretically possible by analysis of temporal and spatial extents of the accommodation signal. Correlation is critical for distinguishing eustasy from tectonism. Upper Pennsylvanian strata of the Pedregosa and Orogrande basins (southern Ancestral Rocky Mountains) were deposited during a time of continental collision and extensive continental glaciation, and contain a composite record of changing tectonism, eustasy, climate, and sediment supply. High-frequency stratigraphic cyclicity expressed as repetitive stacks of lithofacies at the scale of 10¹ m pervades all sections and displays features that collectively imply a primary glacioeustatic origin, notably: (1) abrupt juxtaposition of dissimilar lithofacies, signaling a rapid rate of baselevel change, (2) apparent intrabasinal, interbasinal and, provisionally, interregional correlation of high-frequency cycles across and between contrasting tectonic environments, and (3) cycle frequencies that approach the 413 ka periodicity of orbital eccentricity, the probable forcing mechanism for Pennsylvanian glaciations. Glacial-interglacial climate change expressed as precipitation and circulation fluctuations in the equatorial Pedregosa and Orogrande basins accompanied Pennsylvanian glacioeustasy. Intensified aridity and wind strength during peak glacials led to decreased fluvio-deltaic sedimentation and increased eolian activity where siliciclastics were available. Conversely, increased precipitation during interglacials reactivated and/or intensified fluvio-deltaic sediment yield. Eustasy dictated fluvial aggradation versus degradation and coastal sediment trapping versus bypassing. Coupled glacioeustatic-glacioclimatic change was sufficiently severe to reconfigure environments between climatic extremes, which implies that Pennsylvanian stratigraphic cycles should be viewed in at least partially non-Waltherian terms. Each cycle potentially recorded contrasting facies mosaics that were to some degree temporally exclusive. Multiple-cycle trends in facies and/or thickness also occur to define low-frequency stratigraphic patterns at the scale of 10² m. Qualitative analysis of these trends implicates distinct eustatic and tectonic processes as contributing influences. The eustatic component may derive from low-frequency glacioeustasy as well as tectonoeustasy related to evolving continental paleogeography. The tectonic component probably reflects late Paleozoic Marathon-Ouachita collisional orogenesis.