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A coastal geosyncline (paraliageosyncline) extends more than 4,000 miles along the eastern margin of North America from Newfoundland to British Honduras. About half of this element is covered by waters of the Atlantic Ocean and Gulf of Mexico. In the uncovered part, some Jurassic--but mostly Cretaceous and Cenozoic--strata crop out in belts which are progressively younger seaward. Rocks and features which have been considered to be inherent features of either miogeosynclines or eugeosynclines are found in the province.
The geosynclinal sedimentary mass, lithically variable and roughly lenticular in cross section, is built on a basement of differing Precambrian and Paleozoic rocks. Maximum thicknesses of the sedimentary materials are on the order of 25,000 feet in the Atlantic segment and 50,000 feet in both the northern and southern Gulf of Mexico. These occur generally near the margin of continental (sialic) material.
Character and degree of deformation vary appreciably but they appear to be principally due to vertical stresses resulting from isostatic and gravity adjustments, density differentials, igneous emplacements, sedimentary loading, subsidence--whatever the cause, crustal thickening and flow, bulging around the perimeter of depocenters, or combinations of these. Segments of the geosynclinal mass, involved in orogenic activity which created the Sierra Madre Oriental of Mexico, constitute the Coahuila and Chiapas marginal folded provinces. Here, structural types normally attributed to compressional stresses, or to large-scale gravity sliding, predominate. Elsewhere, the presence of intrusions, volcanic flows, and tuffaceous materials discloses episodes of Triassic, Late Cretaceous, and Terti ry igneous activity.
Large structural features, such as the Ocala, Sabine, Tamaulipas, and Teziutlan uplifts, and the Mississippi and Rio Grande embayments, are mobile "sags and swells" or "welts and furrows" in the developing geosynclinal province.
Major systems of normal strike faults, or of pronounced monoclinal flexures are common. They are closely related to (1) fracturing of the continental margins in the early stages of the geosynclinal development, and (2) subsidence associated with rapid sedimentation. Other normal faults of divergent orientations are also numerous. Reverse and thrust faults predominate in the more deformed areas of Mexico and Guatemala.
Salt structures, many of them diapiric, are common in the salt basins of Mississippi, Arkansas, Louisiana, Texas, Tamaulipas, Veracruz, and Tabasco. Clusters of them reflect a close association of thick salt and superjacent thick sediments and, to less degree, zones of faulting and flexing.
The oldest strata which can be considered an integral
part of the geosyncline are thick, probably Triassic redbeds. Superjacent strata consist of widespread evaporites, notably halite, and redbeds which may be Triassic, Jurassic, or both, in age. Younger Jurassic, Coahuilan, Comanchean, and early Gulfian rocks vary lithically from coarse, marginal, or continental clastics to basin and platform or shelf carbonates. Thick evaporites, predominantly anhydrite, are present in the Late Jurassic, Coahuilan, and Comanchean of the Gulf region. Extensive, calcareous-argillaceous, marine Cretaceous beds constitute evidence of the maximum spread of oceanic waters upon North America during the Mesozoic. Subsequently, in the Tertiary, waves of clastic sediments progressed into the geosynclinal province. Paleogene sedimentation centered in the Rio Gran e region of southern coastal Texas whereas Neogene deposition was concentrated in southern Louisiana and adjacent offshore area. Tertiary thicknesses in each area exceed 25,000 feet. In the Florida and Yucatan platforms, on the other hand, deposition of carbonates predominated during the entire Mesozoic and Cenozoic.
Major sedimentary units of the geosynclinal mass are arranged in belts roughly concordant with the general trend of the modern shore. In gross form, they resemble flattened link sausages, the thick portions corresponding with centers or loci of deposition (depocenters) of particular sedimentational epochs. Several depocenters of the same age constitute a regional axis of deposition (depoaxis), the locations of which have gradually shifted gulfward since the Jurassic. Major reversals of this gulfward migration occurred in the Cretaceous and Tertiary and resulted in widespread cyclic sedimentary sequences (cyclothems).
Filling of the Gulf basin occurred by seaward building (prograding) of sedimentary masses. With progradation, depression of the continental edges continued; the sedimentary masses increased generally in thickness, possibly because of concentration in belts of less width; and tilting and uplift of the landward margins of the older rocks of the geosyncline occurred. Sags, swells, faults, salt structures, and so on, measurably deformed and affected the sedimentary materials. These processes of accretion and deformation continue today, gradually adding girth and natural resources to the continental area.
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