In September 1980, the U.S. Geological Survey conducted a seismic refraction investigation of the northern Mississippi Embayment. During the investigation, 34 shots from nine shotpoints were recorded along a series of profiles. The profiles were parallel to and across an inferred Precambrian rift zone which is outlined by a series of magnetic anomalies and covers an area at least 200 km (125 mi) long and 70 km (45 mi) wide.

Along the northeast-southwest trending axis of the rift, a 0.7 to 1.1-km (2,297 to 3,609-ft) thick 1.8-km/sec layer representing the unconsolidated Tertiary and Cretaceous sediments overlies a 2-km (6,562 ft) thick 5.95-km/sec layer representing a Paleozoic carbonate sequence. Beneath the carbonates is a 3-km (9,843-ft) thick low-velocity layer which probably consists of late Precambrian clastics. The presence of this low-velocity layer is indicated on the seismic profiles by an abrupt cutoff of the 5.95-km/sec arrivals at about 20 km (12 mi) from the shotpoints. This cutoff is followed by a 30 to 40-km (20 to 25-mi) long zone of no arrivals beyond which are seen delayed arrivals from a 6.2-km/sec crystalline basement. The 11-km (6.8-mi) thick 6.2-km/sec basement overlies a lower crust composed of a 6.6-km/sec layer whose thickness decreases to the northeast owing to its replacement by an anomalous high-velocity 7.3-km/sec basal layer. The base of the crust ranges from a 39 km (24 mi) depth at the southern end of the profile to a 46 km (29 mi) depth at the northern end where the 7.3-km/sec layer is 20 km (12 mi) thick. Below the base of the crust (Moho), the upper mantle has a velocity of 8.0 km/sec.

The velocity structure beneath the west flank of the embayment is simpler than that of the axis: 5.95, 6.2, 6.6, 7.3, and 8.0 km/sec. The low-velocity layer is absent and the anomalous 7.3-km/sec layer is thinner than along the axial profile. The profiles perpendicular to the rift show that the low-velocity layer is restricted to the axial zone and that the anomalous 7.3-km/sec basal crustal layer reaches maximum thickness there.

The tectonic model proposed to explain the origin of this embayment velocity structure includes a late Precambrian mantle plume that intruded the lower crust of the northern embayment, causing uplift (and/or crustal stretching) and subsequent rifting of the axial area. This was followed by erosion, subsidence, and subsequent deposition of sediments in the resulting trough.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.