Environmental drivers of cyclicity recorded in lower Permian eolian strata, Manitou Springs, Colorado, western United States

Abstract

Ancient eolian deposits are effective records of environmental conditions and commonly exhibit cyclicity. Accordingly, these deposits can reveal information that allows assessment of the earth system drivers controlling that cyclicity. Here, we assess those drivers in the lower Permian Ingleside Formation exposed near Manitou Springs, Colorado. The unit is characterized by two major eolian depositional intervals, exposed as topographic ridges, and punctuated by alluvial facies, exposed as a valley between the two ridges. Eolian facies are moderately sorted, sub-rounded, fine-grained, sub-arkosic sandstone. Sets of cross strata are up to 10.5 meters thick, and foresets exhibit internal laminae. Eolian cosets are regularly truncated by parallel to sub-parallel bounding zones composed of laterally continuous massive to planar laminated muddy sandstone. Rhizoliths are locally present in these zones. Thin section petrography demonstrates that clay and carbonate cement are abundant in these zones when compared to eolian cosets. Bounding zone thicknesses range from surfaces (with no thickness) to 1.7 m. The bounding zones cut across all other surfaces within the eolian stratigraphy, and are inferred to be super bounding zones (SBZ). Based on similar relationships to interior erg deposits of the coeval Cedar Mesa Sandstone and in the context of late Paleozoic glacioeustasy, we propose the following model for creation of the SBZ. Dry eolian dunes mobilized and climbed during regression when abundant sand was available. During transgression, the encroaching shoreline trapped sediment resulting in deflation due to the loss of sediment flux. Relative sea level rise also raised the water table and when deflation reached a depth near the capillary fringe of the water table, the deflationary surface stabilized resulting in the SBZ.Three levels of cyclicity are observed in the Ingleside Formation that are inferred to reflect changes in local and global paleoenvironmental conditions. The first is the long-term gradation from the predominantly fluvial upper part of the Fountain Formation to the predominantly eolian Ingleside Formation. This trend is inferred to be the result of long-term aridification of the Pangaean interior. The second scale of cyclicity is denoted by punctuation of the two eolian ridges by alluvial facies comprising the intervening valley. We propose that this dry—wet—dry environmental change is driven by relative global ice volume during the early Permian. Higher volumes of global ice, occurring as Gondwanan ice sheets, appear to correlate to the eolian intervals, whereas, the alluvial interval may be coeval with feckless or complete loss of Gondwanan ice sheets. The third scale of cyclicity is indicated by the SBZ-coset couplets and is inferred to be the result of glacioeustatic variations in sea level resulting from the waxing and waning of relatively large Gondwanan ice sheets.