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Attributes of Carbonate Platforms Associated With Passive Salt Diapirism

Abstract

Older concepts of carbonate deposition around passive salt diapir highs envision atoll-like geometries with interior hypersaline lagoons that experienced frequent subaerial exposure events and extensive meteoric diagenesis. Recent detailed depositional & diagenetic facies analysis of outcropping diapir-related carbonate platforms suggest atolls and extensive subaerial exposure are an exception rather than the rule. Outcropping salt diapir-related carbonate platforms in strata of Neoproterozoic-Cambrian, South Australia; Cretaceous-Paleogene, Mexico; and Cretaceous, Spain, are stratigraphically contained within both hook and wedge-type halokinetic sequences (HS), however, their depositional facies distribution & structural geometry varies between the two types. Platforms within hook-HS are typically, isolated, thinner and less areally extensive than in wedge-HS. Halokinetic drape folds are very steep, near 90 degrees, and tight (folding extends >150m from diapir). Hook-HS contain basal debris flow facies with clasts of both diapir-roof and diapir–derived detritus and fauna tolerant of moderate fluctuations in salinity. The overlying normal marine boundstones/grainstones display steep depositional gradients away from the diapir, windward-leeward facies relationships, but lack interior lagoon facies. Locally boundstones may be highly fractured with extensive early marine fracture-filling cements. The high-energy carbonate facies are overlain by deeper water shale or carbonate mudstone platform drowning facies. In contrast, wedge-HS are typically downdip from highly productive regional carbonate platform systems and contain thicker, more widespread, lower gradient boundstone & grainstone facies than hook-HS. Halokinetic drape folding is low angle (<20 degrees) and forms broad, open folds (folding extends up to 1.5km from the diapir). Basal debrites and halokinetic-induced fracturing are absent. In both types of halokinetic sequences, carbonate production on the diapir-related platform progressively decreases due to relative sea-level rise so that local sediment accumulation rate versus diapir rise rate decreases, resulting in increased topographic relief over the diapir and subsequent diapir roof erosion forming a local angular unconformity (HS boundary). The carbonate facies lack evidence of hypersaline conditions, dolomitization, extensive meteoric diagenesis or exposure, and are not associated with caprock formation.