Crustal seismic measurements in southern Mississippi

Abstract

A seismic refraction survey was made in Mississippi along a line trending north from Ansley to Oxford. About 200 seismograms were recorded from 22 chemical explosions at 5 shot points spaced at intervals of about 50 km between Ansley and Raleigh. The Salmon nuclear event, fired in the Tatum salt dome, was also recorded at several locations. A measured average velocity of 3.0 km/sec, to approximate all of the shallow sedimentary rocks, was used in depth calculations. The first strong refraction arrival represents a layer of 5.0 km/sec at a depth ranging from 3.1 to 3.7 km, just under the unconformable boundary between the Upper and Lower Cretaceous sedimentary rocks. The crustal structure is complex below the Lower Cretaceous sedimentary rocks. A curved line could be fitted to the first arrivals between 15 and 150 km on each travel-time plot, increasing in apparent velocity from 4.9 to 6.9 km/sec. All first arrivals between the 5.0-km/sec lower sedimentary rocks and the higher-velocity lower crust are included in one layer, giving an average velocity of 5.9 km/sec for ‘basement’ The basement structure shows a low point corresponding to the trough of the Mississippi salt basin between the Tatum dome and Raleigh and a broad high feature cresting south of McNeill. First arrivals determine an intermediate layer in which the velocity is 6.9 km/sec, typical of basalt, and which rises from a depth of 19 km at McNeill to 13 km near Raleigh. Strong events are apparently reflected from the M discontinuity at critical distances and beyond, and it is necessary to supplement meager first arrivals with the reflections to arrive at a delineation of the upper mantle. The average velocity in the upper mantle is 8.4±0.3 km/sec, the average crustal thickness is 35 km, and the M discontinuity dips 3° toward the south from Raleigh to Ansley. The complicated crustal structure may be due to the intermingling of two major geotectonic trends, the Appalachian and the Ouachita. Low to zero values of regional free-air gravity anomaly indicate that the region is near isostatic equilibrium. For our crustal model to be in isostatic equilibrium, the density must increase in the upper mantle below southern Mississippi. A hypothesis is made that increasing density in the mantle may be the mechanism of formation of the Gulf Coast geosyncline.