Showing papers by "Daniel L. Orange published in 1996"

Biologic and geologic characteristics of cold seeps in Monterey Bay, California

Abstract: Cold seep communities discovered at three previously unknown sites between 600 and 1000 m in Monterey Bay, California, are dominated by chemoautotrophic bacteria (Beggiatoa sp.) and vesicomyid clams (5 sp.). Other seep-associated fauna included galatheid crabs (Munidopsis sp.), vestimentiferan worms (Lamellibrachia barhami?), solemyid clams (Solemya sp.), columbellid snails (Mitrella permodesta, Amphissa sp.), and pyropeltid limpets (Pyropelta sp.). More than 50 species of regional (i.e. non-seep) benthic fauna were also observed at seeps. Ratios of stable carbon isotopes (δ13C) in clam tissues near ∼ 36‰ indicate sulfur-oxidizing chemosynthetic production, rather than non-seep food sources, as their principal trophic pathway. The “Mt Crushmore” cold seep site is located in a vertically faulted and fractured region of the Pliocene Purisima Formation along the walls of Monterey Canyon (∼ 635 m), where seepage appears to derive from sulfide-rich fluids within the Purisima Formation. The “Clam Field” cold seep site, also in Monterey Canyon (∼ 900 m) is located near outcrops in the hydrocarbon-bearing Monterey Formation. Chemosynthetic communities were also found at an accretionary-like prism on the continental slope near 1000 m depth (Clam Flat site). Fluid flow at the “Clam Flat” site is thought to represent dewatering of accretionary sediments by tectonic compression, or hydrocarbon formation at depth, or both. Sulfide levels in pore waters were low at Mt Crushmore (ca ∼ ∼ 0.2 mM), and high at the two deeper sites (ca 7.011.0 mM). Methane was not detected at the Mt Crushmore site, but ranged from 0.06 to 2.0 mM at the other sites.

Seafloor structural observations, Costa Rica Accretionary Prism

Abstract: By studying seafloor morphology we can make associations between near surface deformation, fluid flow and the overall structural framework of accretionary prisms. In February, 1994 a DS/RV ALVIN program to the Costa Rica accretionary prism investigated the relationship of fluid seepage and sediment deformation by using the distribution of chemosynthetic communities and heat flow anomalies as indicators of fluid flow. The active normal faults that cut the hemipelagic section on the Cocos plate may provide conduits for fluids that cause the regional heat flow to be extremely low. These normal faults intersect the toe of the prism at an oblique angle, creating localized regions of increased deformation. Positive heat flow anomalies observed at the deformation front indicate diffuse fluid flow, however, we discovered no seep communities indicative of focused flow. The seaward-most seep communities discovered are in a region of active out-of-sequence thrusts that cut a sediment apron which covers the complex to within 3 km of the prism toe. Vents occur consistently at the base of the fault scarps. Dives on a mud diapir show extensive seep communities, pock marks, and authigenic carbonates. Evidence of fluid release is on the crest which implies a low viscosity fluid migrating upward in the center of the structure. Normal faults on the upper slope can be seen in cross-section in the walls of a submarine canyon. The faults cut the slope apron and displace the seafloor, actively maintaining the critical taper of the prism.

Continental-margin seismic stratigraphy: Assessing the preservation potential of heterogeneous geological processes operating on continental shelves and slopes

Abstract: THE STRATAFORM research program (Nittrouer and Kravitz, 1995) consists of three interrelated areas of multidisciplinary research (Nittrouer and Kravitz, 1996, this issue). The \"stratigraphic sequences project\" is attempting to understand creation of preserved stratigraphy along continental edges as the product of geological processes acting on a wide range of spatial and temporal scales. That goal is to be achieved by linking short-term (i.e., acting over hours to centuries) biological and physical processes (e.g., bioturbation, storms) to sequence stratigraphic architecture and facies, through remote sensing and coring• STRATAFORM's stratigraphic emphasis is on the upper -100 m of section and the last 10 ~ years of Earth's history. The two \"natural laboratories\" chosen by STRATAFORM, the New Jersey and northern California margins, are characterized by different modern depositional regimes. The former study area is relatively quiescent; however, sequence stratigraphic geometries representing a variety of depositional inputs are well-developed for sediments within the space/time interval of STRATAFORM interest. Geometries and associated facies record the history of sea-level variation and continentalmargin progradation. An extensive seismic data-

Continental-Slope Sedimentation: The View from Northern California

Abstract: T H E SLOPE PORTION of the STRATAFORM program sets out to establish a better understanding of the sedimentary processes important in developing the stratigraphic record of continental slopes over the past 10' years. To understand how sediment sequences and their bounding surfaces form on slopes, the project seeks to investigate processes that influence sediment erosion, transport, and accumulation. Project objectives include identification of the sources, fluxes, and mechanisms of sediment transported to and deposited on the slope: determination of the fluxes and mechanisms of sediment remobilized, eroded, and transported within and out of the slope; examinat ion of how the processes identified above interact to form slope strata, unconformities, sequences, and morphology. In this paper we report on the approach to meeting these tasks, some preliminary field results, and future plans.

Surficial evidence of fluid expulsion from the Costa Rica Accretionary Prism

Abstract: The nature and distribution of authigenic carbonates, chemosynthetic bacterial mats, and unique macrobenthic chemosynthetic communities of bivalves and tube worms are important for evaluating and reconstructing present and past fluid venting of accretionary complexes. This paper describes the authigenic carbonates, chemosynthetic fauna, and fluid venting observed at the four tectonic regions of the Costa Rica accretionary wedge in February 1994 during an ALVIN diving program of 20 submersible dives. We found no surficial evidence of highly focused fluid venting at the toe of the prism (outermost 3 km), as implied by the absence of authigenic carbonates and chemosynthetic fauna. The absence of vent communities on the lower 3 km of the prism and the relatively elevated heat flow with respect to the adjacent, incoming Cocos plate (Langseth and Silver, this issue), suggests diffuse, rather than focused flow through the toe of the prism. Twelve active and relict vent sites marked by small clusters of live vesicomyid clams are localized at the bases and tops of out-of-sequence-thrusts, implicating fracture permeability as the fluid conduit in the lower slope region (but upslope from the toe). Vast authigenic carbonates and seven active and relict vent sites marked by large, dense clusters of chemosynthetic organisms predominate the largest mud diapir in the mid-slope region. Fluid expulsion appears to be more restricted on the upper slope, with only 2 small but dense vents marked by chemosynthetic fauna observed at one wall of one canyon.

accretionary prism expulsion from the Costa Rica

Abstract: The nature and distribution of authigenic carbonates, chemosynthetic bacterial mats, and unique macrobenthic chemosynthetic communities of bivalves and tube worms are important for evaluating and reconstructing present and past fluid venting of accretionary complexes. This paper describes the authigenic carbonates, chemosynthetic fauna, and fluid venting observed at the four tectonic regions of the Costa Rica accretionary wedge in February 1994 during an ALVIN diving program of 20 submersible dives. We found no surficial evidence of highly focused fluid venting at the toe of the prism (outermost 3 km), as implied by the absence of authigenic carbonates and chemosynthetic fauna. The absence of vent communities on the lower 3 km of the prism and the relatively elevated heat flow with respect to the adjacent, incoming Cocos plate (Langseth and Silver, this issue), suggests diffuse, rather than focused flow through the toe of the prism. Twelve active and relict vent sites marked by small clusters of live vesicomyid clams are localized at the bases and tops of out-of-sequence-thrusts, implicating fracture permeability as the fluid conduit in the lower slope region (but upslope from the toe). Vast authigenic carbonates and seven active and relict vent sites marked by large, dense clusters of chemosynthetic organisms predominate the largest mud diapir in the mid-slope region. Fluid expulsion appears to be more restricted on the upper slope, with only 2 small but dense vents marked by chemosynthetic fauna observed at one wall of one canyon.