The San Pedro del Gallo area 1.8 in the north-central part of Mexico, between 25°30' and 26°00' N latitude, and 104°00' and 104°20' W longitude. It covers approximately 1,900 square kilometers at the western edge of the Sierra Madre Oriental. The oldest rocks exposed in the area belong to the Villa Juarez Formation of possible Late Triassic age. These rocks, of continental origin, comprise siltstone, sandstone, conglomerate, tuff, and intercalated lavas of characteristic red color. Jurassic and Cretaceous rocks form a section with a maximum thickness of about 3,580 m. The sediments were deposited in the Mexican Geosyncline, west of the Coahuila Peninsula of pre-Aptian age. The Villa Juarez Formation is overlain with angular unconformity by orthoquartzite, quartzose sandstone, and limestone lenses of the La Gloria Formation of Oxfordian age. The La Casita Formation, which is stratigraphically above the La Gloria Formation, probably ranges in age from late Oxfordian to early Neocomian. It consists of thin-bedded limestone, black shale, and varicolored sandstone, with intercalations of coal seams and abundant ammonites. The Lower Cretaceous rocks of the area comprise the Coahuila Series, La Pena Formation, Aurora Limestone, and Cuesta del Cura Formation. Neocomian and upper Aptian rocks are represented by thin-bedded limestone, marl, and shale of the Taraises, Las Vigas, Parritas, and La Pena Formations. This sequence of rocks is overlain with apparent conformity by the medium- to thick-bedded bank-type Aurora Limestone of middle to late Albian age. A notable change of facies occurs laterally in the upper Albian and lower Cenomanian rocks, the lithology of the Aurora Limestone grading westward into the thin-bedded limestone with wavy bedding planes and black chert intercalations that characterize the Cuesta del Cura Formation. An erosional unconformity developed on the surface of the Aurora Limestone and Cuesta del Cura Formation is covered by the Indidura and Caracol Formations of Turonian to Coniacian age. These consist or thin-bedded, platy limestone, shale, and marl beds, which grade upward into a thick sequence of poorly fossiliferous calcareous sandstone, siltstone, and shale. A period of intense folding and erosion preceded the deposit of the non-marine Ahuichila Formation, of probable late Eocene and early Oligocene age. This formation was deposited with marked angular unconformity on all the older rocks, including the Villa Juarez Formation. Both, porphyritic and equigranular igneous rocks in the form of dikes, sills, plugs, and stocks intrude the bedded rocks of the area. They range in composition from quartz rhyolite porphyry to andesite and from granadiorite to monzonite. The rocks around the intrusive bodies have been metamorphosed irregularly as far as 100 m from the igneous contacts. Irregular tactite aureoles occur around intrusive bodies in the Descubridora district, west of Cerritos de Los Victorinos, and in the Sierra del Mimbre. Marble and recrystallized limestone is found west of Descubridora and in the Bajio del Bailon, and hornfels has formed in Cerrito de La Cruz and Cerrito de Las Liebres near San Pedro del Gallo, as well as in the southern part of the Sierra del Mimbre. Folowing emplacement of the intrusive bodies mineralizing solutions reacting with sedimentary rocks gave origin to the different mineralized zones in the region. The beginning of the Laramide Orogeny in the San Pedro del Gallo region is marked by Late Cretaceous uplift, which probably continued until middle or late Eocene time. Compressive forces acting in an east-west to northeast-southwest direction folded the Mesozoic strata into a series of narrow asymmetrical anticlines and synclines overturned to the east, with axial, planes almost parallel to the borders of the Coahuila Peninsula. Thrusting and faulting were important in the vicinity of San Pedro del Gallo. There is no conclusive evidence of large-scale post-Triassic and pre-Laramide deformation in the area. Extrusion of lavas ranging in composition from basalt to rhyolite, accompanied by extensive block faulting, occurred during Miocene and Pliocene time. Late Tertiary uplift began during the Pliocene. The streams were rejuvenated and there was increased erosion, which caused, together with a change of climate, the overloading of some streams and local blocking of drainage by alluvial fans in the valleys, giving rise to the Santa Ines Formation. Erosion, more than deposition is the dominant event at present. Contact metasomatic silver, lead, and copper deposits in the Aurora Limestone have been mined in the Descubridora, Parranderas , and Sierra del Mimbre districts. Fluorite and barite have been extracted from veins in the La Gloria Formation and the Aurora Limestone. Thin beds and seams of coal in the La Casita Formation were mined within the town limits of San Pedro del Gallo. No important mining activity exists at present in the area studied. The area has never been tested for oil.

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Over 7000 sq. km of salt and six Plio-Pleistocene biostratigraphic horizons were mapped in the East Breaks and Garden Banks areas using a 12,000 km grid of seismic data and all obtainable well data. Structure mapping of allochthonous Jurassic salt and the six horizons (Globoquadrina altispira, Lenticulina 1, Angulogerina B, Hyalinea B, Trimosina A, and Sangamon Fauna) and isopachs of the intervals between these horizons revealed notable lateral variations in the area underlain by salt, in the degree of salt deformation, and in the size and thickness of associated intraslope basins. East of 94.5° W salt structures occupy 40% of the area and exhibit complex shapes that suggest a high degree of salt deformation. West of 94.5° W salt structures occupy 11% of the area and consist mostly of structurally simple salt stocks. A zone of high-offset north-south trending faults mark the transition between these two areas. Isopach maps of the six Plio-Pleistocene intervals (from 2.9 Ma to the present) reveal major shifts in the rates and locations of sediment accumulation. From 2.9 to 1.0 Ma. sediment-accumulation rates averaged only 0.8-1.3 mm/y with a maximum rate of 2.7 mm/y. From 1.0 to 0.69 Ma. sediment-accumulation rates averaged 5.8 mm/y with a maximum rate of 11.6 mm/y. This interval correlates to sediments deposited between the extinctions of Hyalinea balthica and Trimosina denticulata and recorded a major period of sediment loading/salt withdrawal between 1.0-0.69 Ma. From the end of this time to the present, sediment -accumulation rates averaged 1.7-2.1 mm/y with a maximum rate measured at 6.2 mm/y. Increased sediment influx during 1.0-0.69 Ma coincides with a major third order sea level lowstand and was focused in central Garden Banks. The restriction of such dramatically increased accumulation rates to this area suggests that sediment influx was accompanied by large-scale salt withdrawal. The increase in accommodation space created by salt withdrawal appears to be the most important factor affecting accumulation rates. Salt structural styles found on the upper continental slope are transitional between those found on the lower slope and those on the shelf. The shelf is dominated by isolated, individual salt stocks (km²) surrounded by kilometer thick sedimentary sections. The lower slope is dominated by broad, laterally continuous, allochthonous salt sheets (10³ km²) with moderate to thin sediment cover. The upper slope contains both of these structural styles plus intermediate size (10-10² km²) salt ridges and massifs. Observations made during this study suggest that differential sediment loading is the mechanism causing the changes in structural style. A Loading/Dissection model is presented to explain the formation of the three primary salt structural styles, their genetic relationship, and their observed distribution. Differential loading has dissected large salt sheets into numerous smaller and irregularly shaped ridges and stocks (like those found on the upper slope). Salt found on the upper slope originated in the Jurassic Louann Formation, but is now surrounded by Pleistocene age sediments. To achieve this relationship, it appears that some Jurassic salt has undergone at least two cycles of sediment loading and consequent diapirism. Salt/sediment relationships suggest that virtually all of the mapped salt is allochthonous. Repetitive sediment loading and salt structural development has not been previously documented and represents a step beyond the limits of current salt structural models. / text

The study area is located on the continental slope in the Green Canyon deep-water area of the Gulf of Mexico. This area is being investigated by the Bureau of Economic Geology as part of several active gas hydrate studies across the area. The chirp sonar profiles used in my study were collected with an Autonomous Underwater Vehicle (AUV) utilizing a frequency-modulated seismic (sonar) source that emitted a 2 to 8 kHz sweep (chirp) frequency signal (wavelengths less than 2 meters). The recording time is limited to about 50 milliseconds, with time zero occurring at the altitude of the AUV about 50 meters above the seafloor. The signal images to about 40 meters below the seafloor, and profiles are as long as 5 km. An interpretation of deep-water, near sea-floor geology has been conducted using both chirp sonar profiles and multibeam bathymetry. Seismic reflections from within the shallow sediments are caused primarily by contrasts in density, rather than acoustic velocity. Reflections were successfully simulated using a model with a constant velocity of 1560 m/s and densities of 2.1 g/cc for sand, 1.4 g/cc for mud, and 1.7 g/cc for silty sand. The chirp sonar profiles imaged near-seafloor geology at nearly a meter scale and allowed for detailed interpretation. The interaction of soft sediment deformation, creep movement and neo-tectonic activity related to gas expulsion controlled the actual topography of the sea-floor. The geologic time represented in the 50 milliseconds of chirp data recorded with the AUV, which is about 40 meters of depth, is approximately 0.050 Ma. B.P., consistent with a depositional rate of 0.8 meters per 1000 years. Reflection patterns are interpreted to be related to fluctuations in sea level. High reflectivities (density contrasts) are interpreted to be deposited during the last sea level low stand, and low reflectivities are interpreted as sedimentation during the last high stand. Sediments deposited during the low stand are proximal while others are distal, deposited in deeper water environments. Shallow structures observed in the chirp sonar profiles are mainly related to gas mobilization mechanisms, which shaped the topography of the seafloor in conjunction with soft sediment deformation and creep movement. The geomorphic features are related in some cases to gas expulsion zones such as pockmarks and mud volcanoes. Other structures are related to soft sediment deformation and creep mobilization. Soft sediment deformation is confined to the deepest part of the minibasin, while the other features are not depth dependent. Highly deformed intervals at the bottom of the sequence seem to control subsequent sedimentation. / text

The Huizachal-Peregrina Anticlinorium is a large NNW-trending structure in the front ranges of the Sierra Madre Oriental of Mexico (23° 45ʹ N; 99° 10ʹ W). The breached core of the anticlinorium exposes three major geologic terranes: (1) . A late Precambrian granulite terrane (Novillo Gneiss) remarkably similar in composition, appearance, grade and age of metamorphism to rocks of the Grenville Province, especially the Adirondacks; (2). A mid-Paleozoic low-grade metamorphic complex (Granjeno Schist) of volcano-sedimentary origin with ophiolite rock assemblages, that resemble rocks of the Ouachita-Appalachian inner zones; and (3). A strongly folded and faulted section of Paleozoic fossilifireous sedimentary strata, more than 1500 m thick, similar to the rocks of the Ouachita frontal zone exposed in the Marathon region of Texas. Except for an extensive subcrop terrain of Permo-Triassic granitic intrusives, the terranes in the area studied represent "unique samples" of the Pre-Mesozoic basement framework of eastern Mexico. These terranes belong to two ancient superposed orogenic systems: the Late Precambrian Oaxacan (Grenville) and the Paleozoic Huastecan (Ouachita-Appalachian) structural belts. Based on the geologic study of these pre-Mesozoic terranes at Huizachal-Peregrina, and compared with the widely spaced and limited outcrops (and sub-crops) of equivalent rock units of eastern Mexico, a tectonic model is proposed which interprets the granulite terrane as representative of continental crust, and the low-grade metamorphic terrane as rocks that accumulated on top of the ocean crust of a marginal basin. This latter sequence experienced a complex history of deformation and metamorphism as it was subducted towards the east, culminating 330 m.y. ago. The onset of Carboniferous-Permian orogenic flysch sedimentation is interpreted to have occurred when the marginal basin became closed by an arc-continent collision. The Permo-Triassic granitic rock terrane of the subsurface of the Gulf Coastal Plain represents the magmatic roots of that volcanic arc. The pervasive NW to NNW-trending structural grain, of the Oaxacan and Huastecan structural belts south of Huizachal-Peregrina through Oaxaca, when compared to the NE-trending distribution of Precambrian and Paleozoic terranes in the United States are compatible with the existence of a proposed zone of large left-lateral displacement across northern Mexico. / text

Utilizing 887 kilometers of multi-stacked seismic reflection profiles and data from 27 exploratory wells, five salt domes arranged along two intersecting salt ridges, bounding and separating localized depocenters , were delineated by subsurface mapping of the structure, sediment distribution, and major sand trends of a 1621 square kilometer area on the Louisiana continental shelf. Pervasive normal faulting in the study area developed to accommodate lengthening of sedimentary units resulting from relative vertical displacement caused by uneven deposition and salt dome growth. These faults are domal (crestal and radial), bounding, and transverse in geometry. The genesis of the bounding faults can be visualized by two models; hinge faulting and collapse faulting. The dominant structural features of the study area began developing in early Pliocene. These features developed coincident with, and as the result of, a huge influx of paralic sediments associated with the progradation of the shelf edge across the study area between early Pliocene and early Pleistocene. The two major dip-oriented sand trends within this sediment wedge represent high constructive delta systems. The positions of the two systems were locally controlled by growing salt structures and sediments were therefore concentrated in the evolving depocenters . A hint of the deep structure which predated the influx of paralic sediments in Pliocene and early Pleistocene is revealed by a structural reversal present on seismic profiles at about 4.0 seconds in the northwest part of the study area. Reconstruction of the paleostructure of this area reveals the flank of an ancestral structural and stratigraphic basin. This basin may represent an ancestral depocenter which began developing very early on the abyssal plain. / text

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