Comparison of Lower Cretaceous Carbonate Shelf Margins, Northern Campeche Escarpment and Northern Florida Escarpment, Gulf of Mexico
S.D. Locker, R.T. Buffler, 1983. "Comparison of Lower Cretaceous Carbonate Shelf Margins, Northern Campeche Escarpment and Northern Florida Escarpment, Gulf of Mexico", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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The early Mesozoic development of shallow-water carbonate shelf margins and banks is an important lithostratigraphic and structural component of many passive continental margins. While the growth of some carbonate provinces has continued to the present (Bahamas), some Cretaceous carbonate margins were buried by the influx of terrigenous clastics (Gulf coast, Western Atlantic margin) or appear to have drowned by some combination of subsidence, sea-level changes, and tectonic activity (Blake Plateau, Campeche Bank, and Florida Bank). During the Lower Cretaceous, the Gulf of Mexico was almost entirely surrounded by shallow carbonate banks which extended to the Blake-Bahamas region. Our objective here is to compare the structural and stratigraphic characteristics of two Lower Cretaceous margin styles found along the steep Campeche and Florida Escarpments (Figure 1). The origin and evolutionary history of these escarpments are poorly understood.
Five sections of deep penetration multifold seismic reflection data are presented. Three lines across the northern Florida Escarpment, MS-1-EB, 16-3-1, 2, and AG-1 (Figure 2), reveal a fairly simple shelf margin in terms of overall structure and stratigraphy. However, two other seismic sections from the northern Campeche Escarpment, lines NECE-9 and GT3-60 (Figures 3 and 4), reveal a more complex Lower Cretaceous margin characterized by landward (or shelfward) prograding seismic sequences. These two margin types may occur along either escarpment. Geophysical information about each of the lines is contained in the Appendix.
The upper limit of Lower Cretaceous sediments in the Gulf of Mexico is defined by a prominent regional middle Cretaceous unconformity (MCU) (Buffler et al, 1980). On the Florida and Campeche banks, the MCU marks the boundary between shallow-water carbonates and overlying deep water deposits (Bryant et al, 1969; Worzel, Bryant et al, 1973; Mitchum, 1978).
The northern Florida Escarpment Lower Cretaceous shelf margin is characterized by a structural high at the escarpment edge, backed by fairly flat-lying units (Figure 2). The high at the margin edge, interpreted to be a reef barrier (Antoine et al, 1967), is characterized by a weak-reflector/chaotic seismic facies. The deepening of reflecting horizons away from the escarpment high suggests differential compaction of lagoonal versus reef or high energy margin-edge sediments. Three lines are shown in Figure 2 to illustrate variations that occur in the width of the inferred reef barrier zone and thickness of foreslope to toe-of-slope deposits. The weak-reflector facies is up to 10 km (6.2 mi) wide on line MS-1-EB, relative to an approximately 2 km (1.2 mi) width on the other lines, suggesting a broad reef or multiple barrier margin. Line AG-1 indicates that a significant thickness of foreslope sedimentation is possible.
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Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.