Monterey Fan Research Articles

Quaternary Styles of California Submarine Fans: ABSTRACT

The morphology and sedimentation patterns of large, deep-ocean submarine fans along the California coast, like Monterey fan, differ from that of smaller borderland or slope-basin submarine fans like the Navy and La Jolla fans. Small borderland fans feature areas of channels, isolated depressions, and a convex upward profile which are characteristic of the area defined as a suprafan and regarded as the location of active sand deposition. Large submarine fans are not simply scaled-up versions of small fans but seem to have certain features that suggest a composite of many small fans. On large fans, the basin shape and basin topographic features are a significant factor in the location of active turbidite depositional areas and the duration of these areas as principal sites of deposition. Bathymetric highs act as dams, restricting fan progradation, or deflecting the transport of fan sediments. Continued deposition commonly results in breeching topographic barriers, producing a rapid shift of primary depocenters to more distal regions. What was formerly a lower-fan environment may become the site of a middle-fan depositional lobe, and lower-fan deposition moves further seaward. Owing to these abrupt large-scale changes o fan deposition, it is difficult to recognize classic middle-fan environments on large submarine fans. Extensive channel End_Page 443------------------------------ systems result in sediment bypassing the old middle-fan region. At the lower end of the main valleys on Monterey fan, channels and isolated depressions are observed but the characteristic morphology of suprafans is not clearly developed. Large sediment waves are present on the upper fan valley levee and at various locations on Monterey fan, but are absent on the smaller Navy and La Jolla fans. End_of_Article - Last_Page 444------------

Potential Petroleum Reservoirs on Deep-Sea Fans Off Central California: ABSTRACT

A variety of potential petroleum reservoirs are indicated in subbottom seismic profiles or implied by the depositional history of the deep-sea fans off central California. The size and extent of both the stratigraphic and tectonic traps off California are large compared to terrestrial analogs as seen only through the crude filter of acoustic profiling. Stratigraphic traps such as buried deep-sea channels, sand lobes, and updip pinch-out sands are produced as a normal consequence of the formation of deep-sea fans. Such stratigraphic traps can be expected on any submarine fan if the sand budget and porosity are sufficient. Slumps of sediment from the continental slope cover large areas of the deep-sea fans, and slumped sediment may isolate and bury channel segments and asso iated sand bodies. Tectonic traps resulting from folding or faulting are rare in deep-water fans. Faulting and folding are more commonly observed in fans from slope basins and from the California borderland and produce both tectonic traps and stratigraphic traps by altering configuration of the basin. Large deep-sea fans are built over irregular oceanic crustal topography that has as much as 2 km of relief. As a result, many localized basins on the middle and outer fan are substantially thicker than much of the adjacent fan. On Monterey fan, for example, these local basins include valleys between abyssal hills and a large fracture-zone trough. End_of_Article - Last_Page 803------------

Channel Sands and Petroleum Potential of Monterey Deep-Sea Fan, California

The possibility of substantial petroleum resources in deep water adjacent to the continents in areas of hemipelagic sedimentation has been suggested by numerous authors. One such area adjacent to the United States is the continental rise off California between 33 and 40° N lat. The rise covers 200,000 sq km and consists of three major submarine fans, the Arguello, Monterey, and Delgada, with sediment thicknesses up to 3 km in water depths of 3,000 to 4,500 m. The fans are composed primarily of continental debris carried down submarine canyons and deposited on the fan through a system of branching and meandering submarine valleys and channels. During the development of the fan, the active channelway shifts periodically in response to locally increased gradients, produ ing abandoned channels. The coarser sediment naturally confined to the channel becomes covered, after abandonment, by finer grained material because of slumping, reworking, and subsequent fan deposition. Such channels, after sufficient burial, would make excellent stratigraphic traps for hydrocarbons, especially if abandonment occurs on the upper fan where the grain size of channel-fill sediments is coarser and the channel width and relief are greater. On the Monterey fan, potential stratigraphic traps would be seaward of the mouths of the Monterey-Carmel, Ascension, and Pioneer Canyons as shown by subbottom profiling. The proximity of such potential reserves to the United States and to existing refineries and markets in coastal areas makes buried channels on large deep-sea fans such as he Monterey fan particularly attractive prospects.

Holocene sedimentation history of the major fan valleys of Monterey fan

There are three major fan valleys on upper Monterey fan. Deep-tow geophysical profiles and 40 sediment cores provide the basis for evaluation of the sedimentation histories of these valleys. Monterey fan valley leads from Monterey canyon to a major suprafan and is bounded by levees that crest more than 400 m above the valley floor. The valley passes through a large z-bend or meander. Monterey East fan valley joins Monterey fan valley at the meander at about 150 m above the valley floor, and marks an earlier position of the lower Monterey fan valley. Ascension valley, a hanging contributary to the Monterey fan valley, appears to have once been the shoreward head of the lower part of the present Monterey fan valley. The relief of Monterey fan valley appears from deep-tow profiles to be erosional. The valley is floored with sand. Holocene turbidity currents do not overtop the levees 400 m above the valley floor, but do at times overflow and transport sand into Monterey East valley, producing a sandy floor. An 1100 m by 300 m dune field was observed on side scan sonar in Monterey East valley. Ascension fan valley was floored with sand during glacial intervals of lowered sea level, then was cut off from its sand source as sea level rose. A narrow (500 m), erosional, meandering channel was incised into the flat valley floor; the relief features otherwise appear depositional, with a hummocky topography perhaps produced in the manner of a braided riverbed. The sand is mantled by about 6 m of probable Holocene mud. Hummocky relief on the back side of the northwestern levees of both Ascension and Monterey valleys is characteristic of many turbidite valleys in the northeast Pacific. The hummocky topography is produced by dune-like features that migrate toward levee crests during growth.

Distribution and scanning electron microscope (SEM) observations of authigenic pyrite from a Pacific deep-sea core

Authigenic pyrite in a variety of forms occurs in a deep-sea core taken on Monterey Fan off the coast of central California. The pyrite under scanning electron microscope examination has been shown to occur in a hierarchy of forms; the larger forms are spherical while the smallest may be spherical, cubic, octahedral, or pyritohedral. Pyrite apparently concentrated at the Holocene-Pleistocence boundary appears to reflect burrowing activity and/or differing organic content during these two epochs.

The channelized flow of turbidity currents with application to Monterey Deep-Sea Fan Channel

Whenever a turbidity current is confined to a channel its upper surface has a cross-channel slope, owing to the combined effects of the Coriolis and centrifugal forces. This cross-channel surface slope for channel full flows may cause a difference in the heights of the levees that have developed on opposite sides of the channel. An equation is developed that balances the Coriolis and centrifugal forces against the pressure force that results from the surface slope. This equation can be used to calculate curves of average velocity versus density, the two variables in the equation, and the two principal unknowns of turbidity current flow. The autosuspension mechanism of turbidity current flow provides a lower limit on the range of permissible velocities and, hence, by way of the equation, on the range of densities. Grain-to-grain friction becomes prohibitively large for densities greater than about 1.18 g/cm3 so that an upper limit is placed on the range of permissible densities and velocities. The slope equation is applied to fathograms of twelve crossings taken along a 100-km stretch of the channel that extends from Monterey submarine canyon off the coast of central California and crosses Monterey fan. This portion of channel consists of a large meander curving to the right followed by a broad curvature of the channel to the left. The cross-channel surface slope of the flow, as reflected in the difference in the heights of the levees, behaves as expected in response to the combined Coriolis and centrifugal forces. Solutions of the slope equation for the twelve crossings, in view of the permissible densities, give velocities ranging between 600 and 2000 cm/sec. This agrees in magnitude with the velocities obtained from ordered sequences of submarine cable breaks. Calculations of curves of velocity versus density with a modification of the Bagnold [1962] equation, based on the concept of autosuspension and utilizing different channel parameters, agree favorably with the results obtained with the cross-channel slope equation. Using velocities deduced from the levee height differences, it is found that a reasonable range of Pleistocene slump volumes in Monterey Canyon is capable of generating channel-full turbidity currents of adequate length and time of passage. The simultaneous solution of the cross-channel slope equation and the Bagnold equation, assuming a wide range of sediment discharge rates, indicates that there has not been a substantial amount of erosion of the channel since the levee height differences were developed.