Sedimentary and crustal structure of the northwestern Gulf of Mexico

Abstract

We conducted a large-offset seismic experiment in the northwestern Gulf of Mexico using largecapacity air guns and digital ocean-bottom seismographs to determine the velocity structure of the sediments, crust, and upper mantle. Five multiple-detector split-profile lines were shot over an area extending from the shallow midshelf south of Galveston to the continental rise just south of the Sigsbee escarpment. The data were of adequate spatial density and quality to allow combined use of near-vertical reflections, wide-angle reflections, and refractions for interpretation. Several techniques were used to obtain velocity-depth functions: (1) conventional constant-velocity-layer analysis, (2) interval velocities from moveouts of the precritical reflections, (3) analysis in the tan-p domain to determine extremal depth bounds, (4) estimation of the thickness of the allochthonous salt from the limiting distance of salt refractions, and (5) forward modeling using two-dimensional (2-D) ray tracing. A sedimentary sequence of nearly constant thickness (13 to 15 km) covers the study area. Various mobilized salt features exist within the sediments in the slope area, ranging from deeply buried layers and diapirs under the inner slope to shallow, thin, allochthonous bodies under the outer slope. In contrast to the sediments, the crust shows considerable variation in thickness, from normal oceanic crustal thickness beneath the continental rise to nearly normal continental thickness beneath the shelf. The transition under the slope, however, is not monotonic but includes thinning to nearly oceanic thickness under the midslope, possibly suggesting an incipient rift that failed to materialize during the opening of the gulf. 2Now at Paper number 6B6309. 0148-0227/88/006B-63 09505.00traces enable us to adopt modem processing and analysis techniques for interpretation. In this paper we will use the term "crust" to refer to the material beneath the Mesozoic and younger sediments and above the mantle. Basement here means the top of the crust. GEOLOGICAL SETrINGThe Gulf of Mexico (Figure 1) is a relatively small sea enclosed by continental and insular land masses except for the Straits of Florida and the Yucatan Channel. The West Florida and the Yucatan platforms rise steeply from the abyssal depths to the east and south, respectively. To the north and west are relatively gentle dips of the Texas-Louisiana slope and the undulating Mexican Ridges. In the central Gulf is the relatively flat basin, the Sigsbee plain, where water depths reach about 3.6 km. The central Gulf basin is underlain by a thick sequence of Jurassic-to-Recent sedimentary rocks overlying oceanic crust [Ewing et al., 1960, 1962; Antoine, 1972]. This sequence of sedimentary rocks can conveniently be subdivided into two major depositional sequences by the middle Cretaceous unconformity (MCU), a prominent erosional surface which represents a major period of nondeposition. The pre-MCU sequence reveals a complex history of the early evolution of the Gulf basin [Buffler, 1984,...

Structure and origin of the rifted margin of the northern Gulf of Mexico

“…In later years, air-gun arrays offered much denser shot coverage in marine seismic-refraction experiments. However, the acoustic source volume in these second-generation seismic-refraction studies of the northern Gulf of Mexico was small, which resulted in OBS refraction data of limited quality (Ebeniro et al, 1988). The 2010 GUMBO study benefitted from a large acoustic array as well as a large number of OBS instruments.…”

“…Based on the available geophysical data, several scientists suggest that thinned continental crust extends offshore Louisiana (Buffler and Sawyer, 1985;Ebeniro et al, 1988;Pindell and Kennan, 2009;Nguyen and Mann, 2016). Conversely, others prefer a model where the Houston magnetic anomaly and Louisiana magnetic anomaly (Fig.…”

Gravity modeling in the western Gulf of Mexico to support seismic imaging

“…The other sources of gravitational anomalies are the bathymetry interface (known from seismic data and constrained during modeling), density variations in sedimentary section, autochthonous salt (generally, poorly imaged in seismic data), the basement, and the base of the crust (Moho boundary). The two latter horizons are not observed in seismic data, so they must be inferred from gravity and public domain refraction data Ebeniro et al, 1988;Eddy et al, 2013).…”

“…The initial model consisted of the flat basement at the depth of 50,000 ft. As the gravity model progressed, the basement boundary evolved to ensure a good regional match.  Moho discontinuity -Although this boundary is relatively deep Ebeniro et al, 1988;Eddy et al, 2013), it is a source of high amplitude, low frequency gravity anomalies because of significant density contrasts at this boundary. The attenuated continental crust was included in the model; the crustal thickness in the seismic re-processing area varied from 35,000 ft to 45,000 ft.…”

Earthquake activity in the Southern Vanuatu arc recorded by the Texas digital OBS

“…Although the Texas digital ocean bottom seismograph (OBS) has been used previously for refraction research (Nakamura et al, 1987Ebeniro et al, 1988), there had been no previous successful efforts to configure the Texas digital OBS instruments in a triggered mode to record local earthquakes. However, between August and October of 1986 the University of Texas Institute for Geophysics (UTIG) and the Institute Frangais de Recherches Scientifique pour le Developpement en Coop6ration (ORSTOM) undertook a joint program involving both digital and analog OBS in southern Vanuatu (formerly, the New Hebrides).…”