Stratigraphic responses to geologic processes: Late Pennsylvanian eustasy and tectonics in the Pedregosa and Orogrande basins, Ancestral Ricky Mountains

摘要

The primary factors that influence stratigraphy are tectonics, eustasy, and sediment supply. Because a change in any of these factors produces a concomitant change in relative base level or accommodation space, the link between a given process and its stratigraphic response is nonunique. Despite this ambiguity, distinguishing the origins of stratigraphic signals is theoretically possible by examining the temporal and especially the spatial scales of the signal. Correlation is a particularly critical tool for distinguishing global eustasy from regional tectonism. Upper Pennsylvanian strata of the Pedregosa and Orogrande basins (southern Ancestral Rocky Mountains) were deposited during a time of major continental collision coincident with extensive continental glaciation, and thus they contain a composite record of significant changes in tectonism, eustasy, and sediment supply. High-frequency stratigraphic cyclicity expressed as repetitive successions of lithofacies at the 10 m scale is prevalent in all sections and displays several features that collectively imply a primarily glacioeustatic origin: (1) abrupt juxtaposition of dissimilar lithofacies, signaling rates of base-level change best attributed to glacioeustasy; (2) apparent intrabasinal, interbasinal, and (provisional) interregional correlation of high-frequency cycles across and between contrasting tectonic environments; and (3) apparent cycle frequencies that bracket the 413 k.y. periodicity typical of orbital eccentricity, the probable forcing mechanism for Pennsylvanian glaciations. Aside from the prevalent high-frequency "cyclostratigraphy," multiple-cycle trends in facies and/or thicknesses define low-frequency stratigraphic patterns at the 102 m scale. Isolating and identifying the controlling factors for these patterns is not straightforward; nevertheless, qualitative analysis of multicyclic trends implicates distinct eustatic and tectonic processes as contributing influences. The eustatic component may derive partially from low-frequency glacioeustasy. The tectonic component almost certainly reflects Marathon-Ouachita collisional orogenesis.