New Mexico Geological Society Annual Spring Meeting — Abstracts

The late Paleozoic (Carboniferous-Permian) ice ages took place when New Mexico was located in the equatorial tropics of western Pangea. Two potential primary drivers of Pennsylvanian-Permian sedimentation in New Mexico have been identified: (1) global, glacio-eustatically driven sea-level cycles, a farfield effect of the ice sheets that waxed and waned in southern Gondwana; and (2) the regional, ancestral Rocky Mountain orogeny (ARM). Most sedimentologists advocate glacio-eustasy as the primary driver of late Paleozoic sedimentation in New Mexico, but new analyses of stratigraphic architecture, facies and biostratigraphy do not support that conclusion. The classic template for understanding late Paleozoic glacio-eustatically-driven sedimentation is in the Midcontinent, where cyclothems accumulated in a quiescent tectonic setting. They are inferred to record glacio-eustatic cycles that were orbitally forced by 100 and 400 kyr eccentricity cycles. Counting Midcontinent cycles indicates that the Desmoinesian, Missourian and Virgilian comprise 44, 23 and 50 cycles, each with an inferred duration of 100 kyr, which suggests durations of 4.4, 2.3 and 5.0 million years for each of these stages. Radioisotopic chronology agrees reasonably well with these astrochronological estimates. The Wolfcampian is more difficult to calibrate astrochronologically, but it represents at least 20 cycles inferred to represent 400-ka-eccentricity, so it has a minimum duration of about 8 million years. In the New Mexico sections, strata with good biostratigraphic age control contain far less cycles than do correlative strata in the Midcontinent. Examples include well studied Pennsylvanian sections in Sierra, Socorro, Valencia and Bernalillo counties, where cyclostratigraphy identifies between 25-50% the number of cycles. Particularly significant are recent precise biostratigraphic data from the Bursum Formation at Carrizo Arroyo in the Lucero uplift of Valencia County. Here, the Bursum Formation has too few cycles to be matched to the succession of Midcontinent cyclothems, and this is prima facie evidence that regional tectonic events of the ARM exerted a greater control over the creation and preservation of Bursum depositional sequences than did eustatic events. The large Gondwana ice sheets collapsed during the Early Permian (Sakmarian = middle Wolfcampian), and subsequent Permian ice fields were too small to have driven global eustasy. Yet, recent work continues to advocate a glacio-eustatic driver of sedimentation in some younger Permian rocks in New Mexico, such as late Sakmarian and Artinskian strata in the Robledo Mountains of Doña Ana County. The cycles these studies identify are better interpreted as autocyclic shifts along the topographically complex Early Permian seacoast. Eustatic farfield effects are present in some parts of the New Mexico upper Paleozoic section, as shallowing upward cycles correlated to actual cyclothemic events thought to reflect the drawdown of global sea level during ice-sheet expansion. However, these cycles are the minority of cycles seen in most sections. Instead, the majority of the late Paleozoic section has a disorganized stratigraphic architecture and a lack of completeness (hiatuses) indicative of prolonged, repeated and/or complex tectonic events. Thus, ARM tectonism, not glacio-eustasy, was the primary driver of late Paleozoic sedimentation in New Mexico.