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Page C. Valentine, U.S. Geological Survey and Joseph R. Uzmann and Richard A DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP MF-1531 U.S. GEOLOGICAL SURVEY SUBMARINE TOPOGRAPHY, SURFICIAL GEOLOGY, AND FAUNA OF OCEANOGRAPHER CANYON, NORTHERN PART By Page C. Valentine, U.S. Geological Survey and Joseph R. Uzmann and Richard A. Cooper, National Marine Fisheries Service 1984 INTRODUCTION Block 312 (fig. 1, table 1) were performed using a rapid sediment analyzer (Schlee, 1966) for the sand fraction Oceanographer Canyon has been investigated by and a Coulter Counter Model TA n1 for the silt and the National Marine Fisheries Service (NMFS) and the clay fractions. Shelf, lower slope, and upper rise U.S. Geological Survey (USGS) since 1974 in a series of sediments reported on by earlier studies (USGS dives with research submersibles. The objectives of stations) were processed using a rapid sediment the long-term study are to describe the canyon fauna, analyzer for sand and pipette analysis for silt and clay to document the distribution, abundance, and substrate (Hathaway 1971; Schlee, 1973). The gravel-size dependence of benthic invertebrates and demersal fraction in all samples was determined by sieving. fishes, to describe the surficial geology, including Statistical parameters of grain size were calculated unconsolidated sediment and outcropping rocks, and to using the formulas of Folk (1980), including the graphic study sedimentary features and erosional processes. A mean (Mz) the inclusive graphic standard deviation synthesis of our observations as of the 1978 field (tfj), the inclusive graphic skewness (Skj), and the season has been published (Valentine and others, graphic kurtosis (KQ); phi values were obtained from 1980 a). cumulative curves (weight percent) plotted on Recently, oil and gas exploration on Georges probability paper. Bank and the proposed leasing of the Continental Slope Our purposes in this study are (1) to describe have prompted studies to assess and monitor the the submarine topography based on our new biological and geological environment of the shelf and bathymetric survey; (2) to describe and map the canyons, and in 1980 the NMFS established several bottom types and sedimentary features and processes permanent stations on Georges Bank and in in the canyon, and to compare the canyon sediments to Oceanographer Canyon that are visited annually (fig. those from the surrounding shelf and slope 1). Since 1978, we have continued our observations in environments; and (3) to identify the benthic Oceanographer Canyon with the submersibles Alvin invertebrates and demersal fishes that characterize (1980, 1982) and Johnson-Sea-Link (1981, 1982, 1983). particular substrates and depth intervals. During these cruises, we made a new bathymetric The cruises to Oceanographer Canyon since survey of the northern part of the canyon using a 12 1974 were sponsored by the U.S. National kHz echo sounder that is the basis for the map Oceanographic and Atmospheric Administration presented here. Navigation was based on Loran-C (NOAA), Manned Undersea Science and Technology time-delays (9960 chain) with fixes taken 1 to 5 Office. We gratefully acknowledge the support of the minutes apart using a Northstar 6000 loran receiver. crew of the Johnson-Sea-Link and support ship RV The survey lines are spaced 500 m or less apart, and Johnson, Harbor Branch Foundation; the crew of the profiling is most dense in the canyon head and on the Alvin and support ship RV Lulu, Woods Hole east rim. Latitude and longitude on our map are based Oceanographic Institution; and the crew of the Nekton on Northstar 6000 conversions at sea and are therefore Gamma and support ship Atlantic Twin, General approximate. We find that accurate, large-scale Oceanographies, Inc. We also thank Maura Connor, (1:6,000) navigation and bathymetric charts are Leslie Reynolds, Donald McElroy, Lawrence Poppe, essential for conducting our submersible operations Dann Blackwood, Patricia Forrestel, and Jeffrey and for interpreting our observations. The maps Zwinakis for their technical assistance. presented here have been reduced for publication. Observations from the submersibles were SUBMARINE TOPOGRAPHY documented by voice tapes and by hand-held and externally mounted photographic and video cameras. Oceanographer Canyon is the largest submarine Sediment samples were collected with devices canyon on the southeastern edge of Georges Bank (fig. operated by mechanical arms, which included a jaw- 1). The canyon's path is sinuous; it incises the shelf for like grab sampler, a canvas bag sampler attached to a some 22 km, and from its complexly dissected head in metal frame that was scraped across the sea floor, and the north, it crosses the shelf in two large meanders. core tubes. Sediment size analyses of samples from This study is focused on the northern 15 km of the Oceanographer Canyon and NMFS stations 2 and 3 and canyon, from about 40°24' to 40°30'N (sheet 2). Eleven " Use of trade names is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey. cross sections drawn at true scale (sheet 1) reveal the observed gullies, ravines, or small valleys on the broad, shallow nature of the canyon in this region; it canyon walls during our dives; however, our traverses increases in width from 2.7 km in the head to 5.5 km in have been up the walls rather than along them and thus the south, and in depth from 210 to 700 m. The canyon not designed to search for such features. floor, where sand and dunes are present (sheet 3), is The canyon walls display a gently undulatory only 100 to 300 m wide. The slope of the thalweg is character. Depressions, or valleys, are broad and very gentle throughout this part of the canyon varying shallow, and quite often do not extend from the shelf from 1 to 1.5°; at its steepest, it slopes 3° in the break to the canyon floor. The walls generally vary in western arm of the canyon head and in the meander. slope from 5 to 20°. The western canyon wall attains The large meander extends from about 40°28T to the slopes of 15 to 20° and generally is less steep than the base of the L.ap, and the steepest walls are present east wall where maximum slopes reach 20 to 38°. The between 40°24'N and 40°25'N on the eastern wall steepest part of the east wall in the northern part of (section 1-1' on sheets 1 and 3). North of 40°28'N, the the canyon is in a large meander (section 1-1' and dive canyon swings to the north for a short distance and 25 on sheets 1 and 3). Here the east wall, from its then turns sharply to the west. base to about 500 m, is covered by dislocated, angular The canyon head is more complex blocks of semi-consolidated silt that measure up to morphologically than the main canyon. There are six 4 m across. Above the talus, a series of subvertical major valleys draining into it from the surrounding cliffs and spalled blocks of silt, separated by narrow shelf, and they are best developed in the western arm ledges and overhangs, extends up to 375 m where the of the canyon head. The steepest and narrowest valley slope decreases and mass-wasting is less spectacular. enters from the southwest, and it extends all the way South of the canyon head, the shelf steepens to the canyon floor (sections b-b' and c-c' on sheets 1 markedly into the canyon at approximately 150 to 170 and 3). The western valley, at the termination of the m. We speculate that this feature may delineate a canyon, is subcircular and appears to drain a large area resistant, partially eroded siltstone stratum similar to from the south, west, and north. The northern and the beds that crop out at 150 m in the western arm of southern walls of the canyon at the mouth of this the canyon head. valley are steep, and although the average inclination of the southern wall is about 38° (section a-a' on sheets 1 and 3), it is almost vertical in some places. SEDIMENT TEXTURE AND DISTRIBUTION The walls are armored with talus blocks that have been eroded from a resistant siltstone layer that crops out Our observations reveal the presence of seven at about 150 m; the siltstone retards erosion and is major bottom types in northern Oceanographer Canyon responsible for the steepness of the walls. In the and on the adjacent Continental Shelf (sheets 3 and western valley, the siltstone has been breached. A 4). The shelf around the canyon is covered with (1) broad, gently sloping valley floor that is covered with quartzose sand. At the shelf break, the sediment sand leads out of the canyon to the shelf, and only a texture changes abruptly to (2) pebbly sands and gravel small exposure of dislocated siltstone blocks was and (3) gravel pavements that lie in a band along the observed at 143 m (dive 16 on sheet 3). In contrast, canyon rim. The canyon is underlain by (4) a the southwestern valley is much narrower, and its semiconsolidated sandy silt or mud. On the canyon development is constrained by the resistant layer on walls, the silt is covered intermittently by a thin the canyon rim. The valley profile (section c-cf on veneer of (5) rippled sand; and (6) large dune-like sheets 1 and 3) flattens between 155 and 160 m where features of sand up to 3 m high lie on the canyon a steep outcrop of burrowed, semiconsolidated silt is floor. In the western arm of the canyon head, (7) capped by siltstone outcrop and talus (dive 14). Above siltstone outcrops and talus are present on the canyon this level, the head of the valley terminates at a steep rim and walls; and similar talus is commonly present at wall of silt that is, in part, bioeroded and collapsed.
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