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M.E. Field

Publications and source records attributed to M.E. Field.

53 records · Page 3Linked to original sources
USGS1991

Observations of gas hydrates in marine sediments, offshore northern California

Biogenic gas hydrates were recovered in shallow cores (< 6 m deep) from the Eel River basin in offshore northern California between 40°38′ and 40°56′N. The gas hydrates contained primarily methane ( δ 13 C = −57.6 to −69.1‰) and occurred as dispersed crystals, small (2–20 mm) nodules, and layered bands within the sediment. These hydrates, recovered in sediment at water depths between 510 and 642 m, coincide nearly, but not exactly, with areas showing bottom-simulating reflectors (BSRs) on seismic-reflection records. This study confirms indirect geophysical and geologic observations that gas hydrates are present north of the Mendocino Fracture Zone in sediment of the Eel River basin but probably are absent to the south in the Point Arena basin. This discovery extends the confirmed sites of gas hydrates in the eastern Pacific region beyond the Peruvian and Central American margins to the northern California margin.

Marine Geology↗
USGS1991

Valencia gorge: Possible Messinian refill channel for the western Mediterranean Sea

A deeply incised gorge is buried nearly 800 m beneath the floor of the Valencia Trough in the western Mediterranean Sea. The gorge is steeply cut more than 200m into Messinian evaporite deposits and has been mapped for more than 40 km. Published and unpublished seismic profiles show the total length to be nearly 300 km. The gorge is not aligned with any known large fluvial systems, and its orientation indicates that it may have been connected near Valencia with the Betic Strait, the portal in southern Spain proposed to link the Atlantic and the western Mediterranean. The Betic Strait was probably open at least through early Messinian time. Conservative estimates show that sea water flowing through the Valencia gorge at rates of 0.5 to 2 m/s could have refilled the western Mediterranean basins below 2000 m in less than 50 yr.

Geology↗
USGS1990

Sedimentary evolution of the Pliocene and Pleistocene Ebro margin, northeastern Spain

The Pliocene and Pleistocene deposits of the Spanish Ebro margin overlie a regional unconformity and contain a major disconformity. These unconformities, named Reflector M and Reflector G, mark the bases of two seismic sequences. Except for close to the upper boundary where a few small channel deposits are recognized, the lower sequence lacks channels. The upper sequence contains nine channel-levee complexes as well as base-of-slope aprons that represent the proximal part of the Valencia turbidite system. Diverse geometries and variations in seismic units distinguish shelf, slope, base-of-slope and basin-floor facies. Four events characterize the late Miocene to Pleistocene evolution of the Ebro margin: (a) formation of a paleodrainage system and an extensive erosion-to-depositional surface during the latest Miocene (Messinian), (b) deposition of hemipelagic units during the early Pliocene, (c) development of canyons during the late Pliocene to early Pleistocene, and (d) deposition of slope wedges, channel-levee complexes, and base-of-slope aprons alternating with hemipelagic deposition during the Pleistocene. Sea-level fluctuations influenced the evolution of the sedimentary sequences of the Ebro margin, but the major control was the sediment supply from the Ebro River.

Marine Geology↗
USGS1987

Seafloor gas seeps triggered by a northern California earthquake

A series of marine surveys using high-resolution seismic-reflection and side-scan sonar profiling was conducted on the Klamath River delta from 1977 to 1985. In November 1980, near the middle of this 8-year time period, a major earthquake (magnitude 7) occurred in the area. The earthquake caused liquefaction of shelf deposits off the Klamath River, and while mapping the sediment failures, we observed an inordinate number of gas vents and acoustic anomalies in the sediment (gas charging) and water column (gas clouds). Our analyses indicate that a change in gas seepage occurred after the earthquake. There was a marked increase in gas abundance immediately after the earthquake, a gradual decrease over the next year, and stabilization at pre-earthquake gas levels within five years or less. The acoustic anomalies were observed in pre-Quaternary rocks as well as in surficial Holocene deposits, and gas seeps overlay truncated anticlines, faults, and syncline exposures. Thus, the gas released may have been in part thermogenic in origin. A large area (20 km 2 ) on the delta failed during the earthquake, and the abundance of gas seeps and craters in the area suggests that sediment degassing may have been an important process contributing to failure.

Marine Geology↗
USGS1987

Sea-floor gouges caused by migrating gray whales off northern California

Side-scan sonar records collected during March and April 1981 and 1982 off northern California contain elongate depressions whose sizes and shapes are similar to sea-floor gouges made by feeding gray whales (Eschrichtius robustus) in the northern Bering Sea and in shallow embayments off British Columbia. The discovery of the whale gouges in the sonar records was unexpected, and supports some of the previous speculation that gray whales feed opportunistically during migration. Gouges occupy about 0.032% of the 7.6 km2 of sea floor that was surveyed, which represents about 575 metric tons of excavated material. Although seemingly minor in amount, the total amount of bottom sediment removed from the central and northern California continental shelf by gray whale activities year after year represents macroscale biologically induced erosion and could have significant geological implications in shelf erosion and depositional schemes. This is the only published evidence of benthic feeding by gray whales along their migration route off northern California.

Continental Shelf Research↗
USGS1987

Crescentic dunes on the inner continental shelf off northern California

Large crescentic dunes that resemble barchans have been discovered within elongate scour depressions on the northern California inner continental shelf by using side-scan sonar. These dunes appear to be migrating obliquely to the regional shelf gradient; a preferred offshore direction of transport is indicated by the extended southern wings of many dunes. The isolated dunes and the scour depressions that contain them are located seaward of sea-floor outcrops off rocky capes and sea stacks. Repeated side-scan sonar records spanning four years (1981–1985) indicate that the dunes and bounding sidewalls of the scour depressions were somewhat modified, but the overall alteration to the bottom morphology during this period was only moderate to undetectable. The apparent low height of the dunes is consistent with a low migration speed. Over longer time periods (decades), the seaward transport of fine to medium sand in the crescentic dunes is probably an important way by which sand escapes the shallow part of the continental shelf in this region and mixes with the muddy deposits of the central shelf.

Geology↗
USGS1982

Sonographs of submarine sediment failure caused by the 1980 earthquake off northern California

In 1980, a large earthquake caused extensive sediment failure on the shallow continental shelf off the Klamath River in northern California. Side-scan sonography was used to complement detailed geophysical profiling in identifying specific features and resolving modes of failure. The features include a nearly flat failure terrace mantled with sand boils, collapse craters and sediment flows, and bounded on the seaward side by a meandering continuous toe ridge. Seaward of the terrace lies a compression zone delineated by small pressure ridges. Our findings indicate a temporal progression of failure from lique-faction of shallow subsurface sand to lateral spread of intact blocks to sediment collapse and flow. ?? 1982 A. M. Dowden, Inc.

Geo-Marine Letters↗
USGS1982

Earthquake-induced sediment failures on a 0.25o slope, Klamath River delta, California.

On Nov. 8, 1980, a major earthquake (magnitude 6.5-7.2) occurred 60 km off the coast of N California. A survey of the area using high-resolution seismic-reflection and side-scan sonar equipment revealed the presence of extensive sediment failure and flows in a zone about 1 km wide and 20 km long that trends parallel to the shelf on the very gently sloping (less than 0.25o) Klamath River delta.-from Authors

Geology↗
USGS1982

Variation in sand body types on the eastern Bering Sea epicontinental shelf.

The eastern epicontinental shelf of the Bering Sea is characterized by variations in river and glacial sediment supply, wave energy, tidal range (microtidal to mesotidal), and tidal, geostrophic, and storm-induced currents. These factors, combined with the effect of the Holocene rise in sea level, have resulted in the formation of a complex assemblage of generally linear sand bodies of similar morphology anad lithology, but different origins. The sand bodies are large features found from the present shoreline to tens of kilometers offshore, in water depths up to 50m. -from Authors

Geologie en Mijnbouw↗
USGS1981

Storm-built sand ridges on the Maryland inner shelf: a preliminary report

Several aspects of the Maryland ridge field are pertinent to the problem of ridge genesis in response to Holocene sea-level rise. There is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges, which reflects the changing hydraulic regime. Grain size is 90?? out of phase with topography, so that the coarsest sand lies between the axis of each trough and the adjacent seaward ridge crest, while the finest sand lies between each ridge crest and the axis of the adjacent seaward trough. Finally, analysis over a 43-year period on an outer ridge reveals a systematic pattern of landward flank erosion, seaward flank deposition, and seaward crest migration. These relationships support a model which explains the ridges as consequences of the up-current shift of maximum bottom shear stress with respect to the crests of initial bottom irregularities. The oblique orientation of the ridges with respect to the beach may be at least partly due to the more rapid migration rate of the ridges' inshore ends. ?? 1981 A.M. Dowden, Inc.

Geo-Marine Letters↗