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Abstract

Upper Cambrian Sawatch and Peerless formations exposed in the vicinity of Manitou Springs, Colorado, U.S.A., are part of the Cambrian inner detrital belt of North America. Transgressive systems tract (TST) deposits composed of quartz-rich shoreline and nearshore sandstone (Sawatch Formation) are overlain by coarse-grained, glauconite-rich, tidal dune deposits at the base of the Peerless Formation. These compound cross-bedded sandstone units include spectacular complete formsets up to 3.5 m thick and 3- to 5-m-thick co-sets, the latter composed of two or three stacked-to-shingled, cross-stratified sets. The formsets have near-symmetrical cross-sectional shapes and low stoss and foreset dips, which, in conjunction with their internal structure, indicate deposition under strong but nearly symmetrical tidal currents. Slight asymmetry in current strength is, however, indicated by the dip directions of large-scale foresets, which are consistently from north-northeast to northwest. Centimeter- to decimeter-scale internal cross-bedding shows bimodal to polymodal paleocurrent orientations and reflects the migration directions of superimposed dunes. Sedimentological and stratigraphic analysis indicates that the dunes are part of condensed deposits that formed as latest TST deposits.

The underlying Sawatch Formation provides insight into the nature of TST's in epicontinental settings. The lack of valley fill or coastal plain deposits at the base of the Sawatch suggests transgression over a locally unchannelized, low-relief hinterland. This quartz-rich unit presumably formed from reworking of a thick regolith that sat on deeply weathered Precambrian crystalline rock. No remnants of this regolith remain in the Manitou Springs region. The extremely thin nature of the TST and the presence of numerous ravinement and marine erosion surfaces is the expected signature of TST deposits in epicratonic settings.

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