V. Cold Seeps
(also see Cold Seep Communities section)
Recent investigations are uncovering evidence of past and present offshore
fluid flow. These fluid seep sites are identified by the existence of chemosynthetic
communities as well as by the buildup of carbonate deposits on the MBNMS
seafloor ((Figure 4); Barry et al. 1993, Greene et al. 1993).
Regional geology has been correlated to the faunal composition of cold seeps
(Barry et al. 1993; Greene 1990, Greene et al. 1993, 1994; Orange et al.
1993, 1994, 1996, in press). In Monterey Bay at least three lithologically
distinct settings provide conditions suitable for the establishment and
persistence of chemosynthetic communities. Aquifer-related fluids released
at densely-faulted exposures in the Monterey Canyon appears responsible
for seeps in the sandstones of the Purisima Formation, as well as in the
hydrocarbon-rich Monterey Formation. Tectonic compression of hemipelagic
sediment may account for methane- and sulfide-rich fluid expulsion in the
Monterey Formation, and outgassing of relict organic debris deposits is
hypothesized to support chemosynthetic communities in the canyon axis at
the Monterey-Ascension Fan site (Embley et al 1990; Greene 1990; Orange
et al. in press).
Interstitial fluids which migrate through sediments and seep out along faults
exposed on the seafloor (Greene et al. 1993, 1996; Orange et al. 1993, 1994)
may be caused by compression, which is in turn due to several interacting
forces. The Palo Colorado-San Gregorio fault zone juxtaposes Tertiary marine
sedimentary rocks and their underlying Mesozoic basement units. West of
the fault zone, continental slope sediments are subjected to transpressional
forces associated with the oblique convergence of the Pacific Plate against
the North American Plate (Greene 1990). Here an apparently elevated ridge
(Smooth Ridge) is being uplifted by the oblique fault motion associated
with the Palo Colorado-San Gregorio fault zone (Figure
4).
East of the Palo Colorado-San Gregorio fault zone, evidence of fluids seeping
out along faults of the Monterey Bay fault zone consists of bacterial mats
and chemosynthetic communities (Barry et al. 1993; Greene et al. 1993). These
fluids may be sulphide-rich aquifer waters that originate in the Santa Cruz
Mountains northeast of the canyon (Greene et al. 1993). Investigations into
the relationship of structure, stratigraphy, hydrology and biology associated
with these fluids are underway. The chemistry of these fluids is also being
investigated (Feriole et al. 1994, Martin et al. 1995).
Next - Section VI. Coastal Erosion
Geology Table of Contents
