Abstract

Mississippi Canyon Federal Lease Block 118 has been selected as the location for the Gulf of Mexico Hydrates Research Consortium's sea floor observatory. The purpose of the observatory is to monitor activities in the gas hydrate stability zone in the northern Gulf of Mexico. Consortium members have been engaged in the design and development of sensors and systems that will comprise the observatory, but little is known of the geology of the site. The Consortium conducted three coring cruises to MC118 in 2005. Analyses of core samples recovered from the site have begun and the geology and history of MC118 are beginning to unfold.

Preliminary conclusions are that occurrences of hydrate as well as gas venting are localized, concentrated in the vicinity of a large mound in the south-central portion of the block, that venting is likely related to fracturing in the vicinity of the mound, that the hydrate is structure II thermogenic hydrate associated with oil and gas at depth. At <50cm depth, microbial processes appear to dominate. Sedimentation rates here average ~10-31cm/ky and sediments are, for the most part, fine-grained, with minor foraminiferal sand in the Holocene deposits.

Background

When the Gulf of Mexico Hydrates Research Consortium selected Federal Lease Block Mississippi Canyon 118 as the location for the sea floor observatory to monitor the gas hydrate stability zone, little was known of the geology of the site. MC118 was chosen largely because hydrates and gas seeps had been documented on the sea-floor there by Sassen and Roberts1 and there was no leaseholder on the block. Since that time, the Mississippi Mineral Resources Institute's (MMRI) Center for Marine Resources and Environmental Technology (CMRET) at the University of Mississippi has launched three cruises on which sediment samples from the seafloor and shallow subbottom have been recovered for various research projects conducted by Consortium members. The overarching goal of this ongoing effort is to improve our understanding of the geology, both past and present, at the site of the monitoring station and, further, to determine what role the hydrates have played in the development of the site. This discussion is an attempt to summarize what we have learned, thus far, from the samples collected from MC118.

Introduction

MC118 is located east of the Mississippi Canyon about 150km south of Pascagoula, MS, on the continental slope of the northern Gulf of Mexico (Figure 1). Water depths range from 820 - 990m. The topography of the continental slope in the northern Gulf is dominated by salt features including diapirs, salt ridges, and minibasins. The sea-floor slopes generally to the southeast, and is featureless except for the regional canyon, which cuts across the northeastern corner of the block, and the mound, identified as a sea floor geophysical anomaly (bright amplitudes, unpublished data) by the Minerals Management Services during a 2004 survey of the central and western continental slope of the Gulf.

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