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Abstract

Multiple horizons within the Redwater Shale Member of the Oxfordian Sundance Formation of southeastern Wyoming exhibit megascopic features recording repeated submarine cementation on the Jurassic seafloor. Some limestone units were bored by endolithic molluscs and encrusted by oysters and serpulid worms prior to the deposition of overlying units. Other sandstone and limestone layers were lithified, fragmented, reworked, and abraided at the sediment surface, giving rise to laterally-continuous layers of sandstone and limestone cobbles which were also densely bored and encrusted by benthic invertebrates. Still other encrusted and bored units contain lithified limestone clasts demonstrating multiple episodes of cementation during the deposition of individual hardground layers.

Marine cement, truncated at lithoclast surfaces or crosscut by endolithic borings, consists of equant calcite crystals, acicular isopachous crusts, and epitaxial rims, but is predominantly equant inclusion-free spar, identical in habit to Holocene meteoric phreatic low-magnesium calcite. Its presence in Jurassic marine grainstones as a synsedimentary marine phase demonstrates that generalizations which relate cement morphology and composition to cementation environments in modern systems may not be valid when applied to ancient limestones. The variable morphology of early cements, even within individual hardground units, also suggests that calcite crystal habit may more frequently record the effect of kinetic factors, such as rates of crystal growth during cement precipitation, than the often-implied effect of Mg/Ca concentrations in ambient pore fluids.

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