Abstract: Early Cementation of Sandstones: Reflections of Cyclic Sedimentary Processes
Laura J. Crossey
A thorough description of early diagenetic processes is inextricably linked to and limited by our understanding of the nature of sedimentation and early hydrologic flow systematics. Interpretations for the early diagenesis of carbonate successions have benefited from modeling the geochemical and mineralogical system in the context of cyclic depositional patterns and associated changes in pore-fluid chemistry. Similar application to clastic systems is challenging, in part because the diagenetic response to depositional changes is prolonged over a greater range of burial depth due to the permeable nature of shallow-marine clastic units (e.g., beach and shoreface deposits). Complex initial mineral assemblages and overprinting by subsequent deeper burial diagenetic effects ar additional complicating factors. The initial depositional setting imparts distinctive characteristics at several scales important for subsequent diagenesis, from microscopic (texture and fabric) to regional (sand/shale ratios and sand body geometry). These characteristics control later access to pore fluids, hence opportunities for further diagenetic modification. Burial rates, also an aspect of the depositional system, are critical in controlling the nature and extent of early cementation. The marginal-marine setting is a dynamic geochemical environment, with diverse pore-fluid histories (ranging from meteoric to marine). Recent hydrologic models for nearshore marine environments during relative sea level changes predict dramatic shifts in pore-fluid chemistry. Outcrop diagenetic studi s using excellent three-dimensional exposure have demonstrated systematic cementation patterns related to cyclical sedimentation (meter to decimeter scale). Specific carbonate signatures (mineralogical and geochemical) can record mixing-zone processes. Authigenic clay mineral assemblages also provide clues. Case studies from diverse settings highlight the general patterns of early cementation of clastic units in a dynamic stratigraphic setting.
AAPG Search and Discovery Article #90953©1995-1996 AAPG Distinguished Lecturers