Origin, Distribution, And Formation Of Atypical Map-Scale Dolomite Bodies In The Monterey Formation, Central California

Abstract

Dolomite within California's Monterey Formation is most commonly known to occur as laminations, beds, nodules, veins, and often replaces other lithologies. However, in central California, enigmatic, massive, isolated, map-scale dolomites or other carbonates exist locally throughout the Monterey, especially associated with cross-cutting fault zones or at disconformities at the base of the formation. Hydrocarbons are commonly included in complexly fractured and re-cemented massive dolomites and also form nuclei of individual dolomite crystals. These occurrences closely, if not directly, link dolomite-forming brines to initial or intermediate reservoirs and migration paths of hydrocarbons, making dolomite occurrence and inclusion a proxy record for hydrocarbon migration associated with basinal or even deep-crustal dewatering. Little is understood of these non-stratigraphic dolomite bodies of mappable proportion. This study will test the hypothesis that these bodies are largely of hydrothermal origin associated with normal or detachment faulting. Identifying the origin, timing, and distribution of the atypical, map-scale dolomites will offer more insight into how fluids (hydrocarbons and brines) were able to migrate through the Monterey Formation, giving a more complete understanding of how the Monterey basins evolved. Geologic maps have been scoured in order to establish the distribution of exposures of massive dolomite/carbonate bodies. Subsurface records from Central Coast oil fields will be examined, some of which are known to contain volumes of intense or complete dolomitization. Available outcrops and cores will be analyzed for rock for rock fabric and to identify similarities or differences between occurrences. Samples will be analyzed geochemically and petrographically for microfabric and replacement relationships pertaining to their genesis. Isotopic analysis will include Δ18 O values (possibly also clumped isotopologues) to determine the temperature at which the dolomite formed and Δ13 C to provide insight into compositional differences or similarities of geochemical environment between locations. These data will be integrated to assess the varied or consistent origin of these enigmatic, non-stratigraphic rock masses.

AAPG Datapages/Search and Discovery Article #90215 © 2015 Pacific Section AAPG Convention, Oxnard, California, May 3-6, 2015