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45387-B2
Neogene Seep Structures of the Central California Region and Relationships between Fluid Flow and Tectonic Deformation in an Active Transform Margin
During the initial part of the proposed investigation, field and analytical studies of the fossil seepages of fluids and gases exposed in Neogene coastal outcrops of the Monterey Bay region have produced significant improvements concerning our understanding of the relationships between fluid flow, lithology and tectonic deformation in an active transform margin. Significant progress has been made for each of the three main objectives included in the research proposal:
Objective 1 – Field-based, high-resolution geospatial and structural analyses of authigenic carbonates, intrusive dikes and sills of the oil-saturated sands and embedding Neogene rocks are providing a picture of unprecedented detail of the ‘plumbing systems' of hydrocarbon seepages. The geospatial analyses of the seep structures are carried out with Terrestrial Laser Scanning (TLS) a novel technological and methodological approach that can be used to reconstruct and quantify geo-surfaces in different field conditions and to a previously unobtainable accuracy. Using a Trimble VX Spatial Station, a ground-based robotic infrared laser scanner equipped with a high-resolution digital camera, we are accurately measuring orientations, volumes and geometries of seep carbonates, intrusive dikes and the stratihraphic/tectonic characteristics of the embedding rocks units. With TSL we are able to ‘virtually' access outcrops of seep structures (the majority) that are physically unreachable in the field. The preliminary results of this novel approach are indicating 3D correlations at different scales (from cm to hundreds of meters) between the geometry of key morphological features of the seep deposits (e.g. conduits) and the orientations/geometries of geologic structures including faults, joints and folds. Our results show that the fossil seeps are characterized by strong asymmetries, and by fluid conduits that parallel the local fracture and fault fields.
Objective 2 – All seep localities targeted by this study have been sampled for both mineralogic, petrographic and stable isotope analyses. Samples have been chosen to represent all instances of seep precipitates (e.g. carbonate ‘chimneys', carbonate slabs, oil-saturated sand dikes etc.) with the purpose of understanding the composition of the seep precipitates, the relative sequence of precipitation and intrusion of oil-saturated sand, the source of the fluids and the environmental condition under which the seep precipitates formed. The petrographic analysis of a large set of thin sections from the authigenic carbonates is suggesting that the seep precipitates have formed as a result of multiple generations of carbonate precipitation, with each generation being characterized by distinctive paragenesis and textures (e.g. metasomatic replacement of parent rock, micrite v. sparite etc.). Preliminary isotopic data indicate that the authigenic carbonate minerals (mostly low-Mg calcite) have been formed as byproducts of respiration of microbes such as sulfate reducers tapping on the hydrocarbons. These analyses, in conjunction with the above mentioned 3D reconstructions are offering an extraordinary opportunity to understand evolution and structural controls of a subsurface seepage of hydrocarbon-rich fluids.
Objective 3 – This objective concerns the larger-scale regional implications of the fossil seepages of the Monterey Bay and their relationships with the transform tectonics of the San Andreas Fault System (SAFS). The working hypothesis is that the sources of the hydrocarbon-rich fluids that generated the paleo-seeps were located offshore, in deeply buried, organic-rich basins formed during episodes of transtension and that the fluids traveled up-dip along faults to focus in areas of tectonic uplift. To test this hypothesis, we are analyzing and interpreting a large set of seismic profiles and aeromagnetic and gravity anomaly data available for the region. Our preliminary reconstructions show offshore depocenters forming NW-trending, elongated basins separated by basement highs characterized by reduced sedimentary cover. The axes of the basins are not continuous, but are disrupted and/or offset by NE-trending basement structures that segment the continental crust into smaller tectonic blocks. The orientation of these cross-structures matches the orientation of the plumbing system of the Neogene paleoseeps, and several of these structures are located beneath the axes of the submarine canyons of the Monterey Canyon System, where modern activity of fluid seepage has been also documented. Our preliminary reconstructions suggest that these cross-faults could have acted as conduits for hydrocarbon-rich fluids from the basin depocenters (sources) to the seep localities (sinks). We have recently started sampling for paleomagnetism the fossil seep localities to constrain the timing of formation of the seep precipitates by comparing the components of magnetization found in the seep structures with the components of magnetization of the host rocks. With the use of paleomagnetism we also hope to detect possible vertical-axis rotations and restore the orientations of seep and associated geologic structures. This component of the research project has been delayed due to technical problems with the cryogenic magnetometer at the Paleomagnetic Laboratory of the USGS in Menlo Park, where the paleomagnetic measurements will be carried out. The machine has been completely refurbished and has initiated to operate again only at the beginning of 2008.
