Walther's law, climate change, and upper Paleozoic cyclostratigraphy in the ancestral Rocky Mountains

Geologists routinely apply Walther's Law in interpreting paleoenvironments and paleogeographic scenarios from stratigraphic successions. The application of Walther's Law, however, may be problematic in strata that have been influenced by significant and geologically rapid climate change. Such climatic fluctuation can force depositional environments to change fundamentally rather than migrate laterally, resulting in a stratigraphic succession wherein depositional facies vertically superposed rarely or never coexisted laterally. Upper Pennsylvanian carbonate-siliciclastic cycles deposited in basins of the southern Ancestral Rocky Mountains display stratigraphic relations that cannot be explained by autogenic and/or glacioeustatic mechanisms alone; rather, intracyclic (glacial-interglacial) climate change was a significant influence. Cycles of the northern Pedregosa basin, for example, comprise eolian-marine siltstone in sharp contact with subtidal-peritidal carbonate facies; absence of facies mixing suggests a climatic control on silt influx. Cycles of the western Orogrande basin comprise deltaic siliciclastic strata in sharp contact with subtidal-peritidal carbonate facies; absence of facies mixing, truncated siliciclastic progradation, and carbonate facies with emergence features similarly support a climatic influence on the siliciclastic component. In both systems, intracyclic (glacial-interglacial) climatic shift reconfigured depositional environments. In cases such as these, application of Walther's Law could produce erroneous conclusions. Severe glacial-interglacial climate change helps explain the pervasive association of mixed carbonate-siliciclastic cyclicity that typifies upper Paleozoic strata in many parts of the world.