Geologic History Of Georges Bank As Determined By Submersible Exploration Of Submarine Canyons

Abstract

ABSTRACT Seven ALVIN submersible dives were made into three submarine canyons that incise the slope region of Georges Bank off the New England coast. Forty-eight samples were collected and detailed dive tracks reconstructed. Our visual observations coupled with our sampling indicate that older Lower Cretaceous carbonate and mixed carbonate clastic platform facies rocks are exposed in portions of the canyons. This older sequence of rocks has been eroded and incised by canyons prior to the Middle Eocene and possibly prior to Maastrichtian times. Canyon cutting has been a process that has been reactivated over spans of tens of millions of years. The regions of the canyons are characterized by a complexity of stratigraphic/erosional discontinuities, various sequences of older canyon fill deposits that exhibit rapid facies changes, and chaotic mixing of units that would be impossible to understand without the submersible tool. INTRODUCTION During a ten day expedition in the summer of 1977 we explored three selected New England submarine canyons using the research submersible ALVIN. The submersible is self-contained, battery powered, has a depth capability to 3650 meters and carries two observers in addition to the pilot. Our program was designed to traverse the narrow axial tha1wegs of the canyons in the regions where they deeply incise the continental slope. We anticipated that we would encounter bedrock outcrop of old continental shelf strata that had been exposed by the retreat of the slope, principally as the consequence of intense phases of regional erosion. Several dives were made up a wall of each canyon on tracks more or less perpendicular to the thalweg. These reconnaissance routes gave us a chance to observe if the present sculpturing of the canyons was pervasive on the slope or was restricted to the region of the valley floor. Submarine canyons on the continental slope of the Atlantic side of North America (Fig. 1) were discovered and initially sampled some four decades ago (Shepard et a1.. 1934; Veatch and Smith, 1939). Down cutting in the canyon regions exceeds one kilometer giving rise to deep gorges along a branching dendritic-type drainage network. Some geologists have matched this morphologic pattern to counterparts on land. While speculating about a hypothetical sub aerial origin prior to the subsidence of the continental edge (Stetson. 1936; Shepard, 1952). Upon arrival at our dive sites, each canyon was surveyed with 12 kHz echo sounding in systematic grids, both previous to and between our dives. Navigation was obtained with Loran-C at approximate 15 minute intervals and at all course changes. Our bathymetric maps were constructed with our survey data supplemented by other available tracks, all shown by dotted lines in Figs. 2,3 and 4. The canyons we visited are of different sizes and shapes. Oceanographer Canyon (Fig. 2) is the largest and cuts the furthest back into the shelf edge. Its lower part exhibits large right angle bends. The "tributaries" of this canyon are fairly linear and form two orthogonal systems suggesting control by pre-existing structures in the bedrock.