Mark L. Holmes

Fluid expulsion sites on the Cascadia accretionary prism: Mapping diagenetic deposits with processed GLORIA imagery

Point-discharge fluid expulsion on accretionary prisms is commonly indicated by diagenetic deposition of calcium carbonate cements and gas hydrates in near-surface (<10 m below seafloor; mbsf) hemipelagic sediment. The contrasting clastic and diagenetic lithologies should be apparent in side scan images. However, sonar also responds to variations in bottom slope, so unprocessed images mix topographic and lithologic information. We have processed GLORIA imagery from the Oregon continental margin to remove topographic effects. A synthetic side scan image was created initially from Sea Beam bathymetric data and then was subtracted iteratively from the original GLORIA data until topographic features disappeared. The residual image contains high-amplitude backscattering that we attribute to diagenetic deposits associated with fluid discharge, based on submersible mapping, Ocean Drilling Program drilling, and collected samples. Diagenetic deposits are concentrated (1) near an out-of-sequence thrust fault on the second ridge landward of the base of the continental slope, (2) along zones characterized by deep-seated strikeslip faults that cut transversely across the margin, and (3) in undeformed Cascadia Basin deposits which overlie incipient thrust faults seaward of the toe of the prism. There is no evidence of diagenetic deposition associated with the frontal thrust that rises from the decollement. If the decollement is an important aquifer, apparently the fluids are passed either to the strike-slip faults which intersect the decollement or to the incipient faults in Cascadia Basin for expulsion. Diagenetic deposits seaward of the prism toe probably consist dominantly of gas hydrates.

Late Quaternary tectonics, northern end of Juan de Fuca Ridge (northeast Pacific)

Abstract Seismic reflection profiles from the northern end of Juan de Fuca Ridge reveal three axial valleys having a basement relief of as much as 2 sec (two-way travel time). A thick sequence, presumably of turbidites, mainly less than 0.7 m.y. old, covers much of the area. The oldest turbidites form the upper part of the fill of a possible Tertiary trench between the ridge and North America. The second turbidite unit extends beyond the trench and once formed an abyssal plain over most of northern Juan de Fuca Ridge and the area west to Explorer Ridge. Following formation of the plain, vertical movements began that broadly uplifted the crest of Juan de Fuca Ridge, block-faulted its northern end, produced faulting along Sovanco Fracture Zone, and upwarped the basement north of the ridge. Younger turbidites have filled the lowlands created by the vertical movements. The present sea floor topography and seismic activity show evidence of continued movements. Our findings indicate that the basement relief of the ridge crest and fracture zone were formed after the time of creation of the magnetic anomalies associated with sea floor spreading. Although abruptly initiated, these vertical movements have continued in a moderated form to the present. The vertical movements are thought to be associated with horizontal plate motions in this complex area and with the episodic release of strain as the northwest-salient block of the Juan de Fuca Plate is possibly being severed from the remainder of the plate.

Sonobuoy Refraction Measurements from Norton Basin, Northern Bering Sea: ABSTRACT

Recent discovery of thermogenic gaseous hydrocarbons seeping from the seafloor 45 km south of Nome, Alaska, suggests that the underlying Norton basin may be an important future petroleum province. The results of 38 sonobuoy refraction profiles obtained in 1977 and 1978 show that Norton Sound and Chirikov basin are underlain by a single sedimentary trough approximately 130 km wide and 350 km long; the basin axis trends west-northwest and extends from Stuart Island to a point 100 km west-southwest of King Island. Although average depth to basement is only 2.5 km, two deeper areas, containing up to 5.5 km of sedimentary section, were discovered 75 to 90 km northwest of the Yukon River delta. Norton basin is floored by an acoustic basement whose compressional velocity is 5.5 to 6.5 km/sec. The basin fill consists of three major units distinguishable on the basis of their compressional velocities; unconformities probably separate each of these units. The basal unit, with a velocity of 4.9 km/sec, is present only in the deeper parts of the basin. A thick (2 to 3 km) section has velocities ranging from 2.3 to 3.7 km/sec and lies on this lower unit and on acoustic basement. Compressional velocities in the 1.2 km-thick upper unit range from 1.6 to 2.1 km/sec. The lower two units are probably Cretaceous and lower to middle Tertiary marine and nonmarine rocks lying on a basement complex of Paleozoic and Mesozoic igneous, metamorphic, and sedimentary rocks similar to those mapped n Seward Peninsula and St. Lawrence Island. The upper unit probably consists of upper Tertiary and Quaternary sedimentary rocks and sediment. End_of_Article - Last_Page 468------------

Marine Geophysical Measurements in Central Puget Sound, Washington: ABSTRACT

Marine seismic refraction (sonobuoy and OBS) and gravity data obtained from the Puget Sound main basin and Lake Washington show a major discontinuity in both seismic velocities and rock densities across a steep (15 mgal/km) gravity gradient striking generally westward through Seattle from the Cascade Range foothills to Hood Canal. North of this gradient is a -129 mgal Bouguer gravity minimum centered over Lake Washington. A least squares inversion analysis of the residual Bouguer gravity field was combined with the refraction velocities to model the subsurface density distribution beneath the central Puget Sound lowland. The data suggest the existence of a 7 to 8-km deep sedimentary basin beneath the gravity minimum north of the steep gradient. The basin is filled with probable Tertiary and Quaternary rocks having densities ranging from 2.0 to 2.6 g/cc. Modeled rock densities beneath the basin (2.7 to 2.8 g/cc) may indicate the presence of volcanic basement rocks. South of the gravity gradient, Tertiary volcanic and intrusive rocks are overlain by Tertiary and Quaternary sedimentary rocks up to 2 km thick. The gravity gradient appears to mark a steep fault or faulted flexure forming the southern boundary of the Tertiary basin lying beneath Lake Washington and Seattle. The gravity model suggests that much of the steepness in the gradient across this feature is due to a near-surface density contrast between a west-trending End_Page 1690------------------------------ belt of high density Tertiary igneous rocks and low density tuffs or pyroclastic volcanic and sedimentary rocks along the southern margin of the basin. End_of_Article - Last_Page 1691------------

Continental Shelf Structure and Sediments of Gulf of Anadyr: ABSTRACT

Interpretation of continuous seismic reflection profiles indicates that the continental shelf of the northwestern Bering Sea is underlain by up to 1 km of Tertiary End_Page 2492------------------------------ and Quaternary sediments that were deposited in a basin formed by the subsidence of a lithified and folded Mesozoic basement complex. This Anadyr basin is evidently an extension of the depression containing up to 3 km of Tertiary strata below the Anadyr Lowland on the west of the Gulf of Anadyr. A basement sill extends eastward from the southwest corner of the gulf; apparently, the sill is the continuation onto the shelf of the Koryak Mountain structural province. Tertiary layers generally are little deformed over the basement sill, thus it is suggested that this broad arch has been in existence throughout the Tertiary. If the thick Tertiary deposits of the Anadyr, Bristol, Norton, and Chukchi basins are shallow marine in origin, the extent of an exposed Bering Sea shelf during the Te tiary may not have been as great as previously thought. Some draining of the shelf is suggested, however, near the end of Tertiary time by a distinct erosional unconformity below Quaternary sediments in the northwestern part of the Gulf of Anadyr. On seismic profiles Quaternary sediments appear to be highly deformed across parts of the Anadyr shelf. These deformed Quaternary deposits are ascribed to continental ice encroachment on the shelf, and they define the outer limits of ice advance during the maximum glaciation as lying approximately along a line from St. Lawrence Island to a point 150 km east of Anadyr Bay, then south roughly parallel with the coast. Buried erosion surfaces and stream channels within the Quaternary sediments are evidence of sea-level fluctuations during that time. Present bathymetry and the sediments and fauna of 1-m cores suggest a sea level stillstand at approximately -75 to -80 m. Lack of suitable material precludes dating of this stillstand. End_of_Article - Last_Page 2493------------

Holocene Oceanography of Chukchi Sea: ABSTRACT

Piston cores from the southeastern Chukchi Sea have permitted differentiation of modern from Holocene sediments deposited when sea level stood about 20 m lower. Because microfossil distributions in modern sediments are associated closely with ice-free oceanographic conditions, the following trends appear to be related to Holocene oceanographic conditions. Eggerella advena, indicative of warm, dilute Alaskan coastal water, is as abundant in Holocene as in modern sediments, but Reophax arctica, indicative of central shelf water, and Spiroplectammina biformis, indicative of cold bottom water in the northern Chukchi Sea, are much less abundant in Holocene sediments. Frustules of the planktonic diatom Coscinodiscus, presently displaced northward by the Bering Strait current fr m regions of maximum phytoplankton concentrations in the overlying water, are more abundant in Holocene than in modern sediments in cores directly north of Bering Strait. The northward-flowing Bering Strait current controls conditions in the southeastern Chukchi Sea. This flow was reduced during the Holocene because the cross-section area of the strait was smaller; apparently, however, the flow was reduced at the expense of central shelf water as Alaskan coastal water filled the southeastern Chukchi Sea. Although currents were slight in the central part of the southeastern Chukchi Sea, waters still piled up against the coast near the present settlement of Kivalina and produced a high-velocity northwest current. As evidence of this, Holocene sediments northwest of Point Hope contain more plant fragments and sand than nearby areas, and these presumably were deposited from the current. End_of_Article - Last_Page 716------------