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Keywords:

  • Bioturbation;
  • geomodel;
  • Kansas;
  • loess;
  • Lower Permian;
  • Naktodemasis;
  • pedogenesis

Abstract

Siliciclastic intervals in Lower Permian carbonate–siliciclastic cyclothems in western Kansas record climate control on facies progression, deposition and preservation. The 26 000 km2 study area comprises seven marine-continental (carbonate–siliciclastic) cyclothems caused by glacioeustasy. Core data and a three-dimensional geological model provide a detailed view of the sub-surface on a gently sloping ramp. Siliciclastic intervals in the cyclothems are fine-grained red beds with extensive pedogenic features, indicating a continental origin. Bed geometry (sheet-like deposits that thin to the east), lateral grading, grain size (very fine-grained sand to silt) and grain angularity (sub-angular to angular) suggest that the sediment is loess sourced from the west, probably the Ancestral Rocky Mountains. There is a repeated record of glacial-cycle-scale, climate-controlled cyclicity within siliciclastic intervals that has not been recognized previously. Aeolian silt grain size coarsens upward towards the middle, then fines upward in each siliciclastic interval. When sea-level was high (interglacial) and carbonate production flourished, aeolian sedimentation nearly ceased, suggesting increased vegetation and rainfall at the source. As sea-level fell, fine-grained siliciclastic sediments were deposited under relatively dry, but seasonally wet conditions on an exposed ramp. Laterally graded coarser grained siliciclastic sediments with diagnostic fabrics indicate drier conditions with seasonal rainfall during a continued relative fall in sea-level. The coarsest siliciclastic sediments were deposited during the lowest sea-level and driest conditions, but still with sufficient seasonal moisture to allow vegetative cover and bioturbation. Subsequent upward fining is correlated with sedimentological indications of wetter conditions during relative sea-level rise. Unlike common sequence stratigraphic models that relate siliciclastic sediment accumulation to base-level rise, continental deposits were preserved because plants and pedogenesis stabilized aeolian sediment. The aggradational landscape formed by this process had several metres of positive relief that reduced accommodation for overlying marine carbonate strata. Thus, this mechanism for continental siliciclastic aggradation has a significant effect on sequence stratigraphic architecture.