Abstract:

High-resolution sedimentological and biostratigraphic data recently recovered from Upper Cambrian strata in the northern Rocky Mountain and central Appalachian regions reveal that meter-scale cycles of very different character developed synchronously during deposition of the basal subzones of the Ibexian Series in both the carbonate facies belt and the distal part of the inner detrital, mixed siliciclastic–carbonate facies belt of the Laurentian paleocontinent. A typical mixed siliciclastic–carbonate cycle in the Snowy Range Formation of Wyoming and Montana consists of the following lithofacies in ascending order: shale, shale with very thin grainstone interbeds, grainstone with subordinate thin shale interbeds, flat-pebble conglomerate, and (in some cycles) thrombolitic bioherms. Internal transitions between the constituent lithofacies are mostly gradational, but cycle boundaries are sharp with shale directly overlying flat-pebble conglomerate and/or thrombolitic boundstone. We postulate that these mixed siliciclastic–carbonate cycles developed in response to different depositional dynamics than meter-scale cycles developed in carbonate-belt facies. Specifically, cycles in the carbonate belt were produced largely by temporal variation in accommodation space, and associated filling of that space through progradation of the carbonate system and aggradation to sea level. In contrast, cyclic deposition within the inner detrital belt was controlled by variations in terrigenous sediment input and resulting effects on carbonate productivity. In this case, enhanced delivery of siliciclastic mud by rivers was triggered by regression and/or a shift to more humid climatic conditions. An upward increase of carbonate in the main part of the cycles reflects enhanced carbonate production in response to the reduction of terrigenous clay supply during shoreline retreat. Widespread cementation of fine grainstone substrates led to deposition of extensive beds of flat-pebble conglomerate, possibly in association with elevated sea-surface temperatures and enhanced storm intensity. Thrombolites formed at times of maximum transgression as microbial communities thrived in the absence of turbidity, colonizing the irregular surfaces of flat-pebble conglomerate beds. We thus interpret these wholly subtidal cycles to record full changes in paleobathymetry with the bulk of the cycles recording upward deepening, rather than shoaling associated with regression. Comparison with age-equivalent shoaling cycles of the Conococheague Formation of the carbonate belt of the Appalachian Mountains highlights the different depositional dynamics in the mixed siliciclastic–carbonate systems, and underscores the peril in extrapolation of depositional models derived from carbonate systems to mixed systems, regardless of age or paleogeographic setting.

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