ABSTRACT The Santiago Basin of the northern Peruvian sub-Andes is a structurally complex region related to a combination of thin- and thick-skinned deformation and the impact of salt tectonics during Andean deformation. Oil shows in this basin are very common, and even though the first exploration campaigns started in the 1940s, no commercially exploitable hydrocarbons have been discovered yet. We present three basin-scale structural transects and refined structural interpretations, based on vintage 2-D seismic data and well tops that help elucidate the relationships between thin-skinned and deep-seated, thick-skinned structures. Two dip sections were kinematically restored to the top of the Yahuarango formation, one of the youngest pre-Andean units. We calculated the depth to the intra-basement detachment to be approximately 20 km (12 mi), a value that correlates with other thick-skinned detachments and earthquake hypocenters from the region. We recognized a varied inventory of salt-related structures, which we interpret to be part of the approximately 800-km (500-mi)-long Peruvian Salt Belt. The onset of salt movement occurred soon after salt deposition, likely through sediment loading. Our data suggest that Miocene-Pliocene basin deformation starting at 5.3 Ma has been sustained until the present-day. Shortening ranges from 7.31 km (4.54 mi) to 7.56 km (4.70 mi) (5.9% and 6.1%, respectively), corresponding to Miocene-Pliocene deformation rates of 1.3–1.4 mm/yr. These values are significantly lower than those of adjacent regions in the sub-Andes. This may be related to the combined effects of pressure solution, strain accommodation, or deflection by crustal-scale faults farther west.

ABSTRACT The quartz-rich sandstones of the Upper Cretaceous Vivian Formation constitute the most important reservoirs in the prolific Marañón foreland basin of northern Peru. The Vivian sandstones are largely fluvial in the northeast and transition to a marine shoreface and open shelf in the west. The formation consists of two sand-rich units (Lower and Upper Vivian) with better reservoir characteristics in the Lower Vivian. Porosity versus depth analysis for the dominantly fluvial Vivian sandstones shows a simple trend of decreasing porosity with depth, with facies-dependent variations for wave-reworked facies. This simple linear trend (correlation coefficient R2 > 0.77) indicates that overburden stress is the dominant factor that determines the porosity reduction. Further west, in the wedge-top Santiago Basin, the Vivian sandstones exhibit anomalously low porosities at relatively shallow depths, a distinct diversion from the regional trend. The depth difference between these low porosities and the porosities of the regional trend was used as a proxy for the uplift that the basin has suffered during the Andean deformation in the latest Miocene. The resulting values (up to approximately 2600 m [8530 ft]) are consistent with uplift estimates derived from other methods and suggest that parts of the foredeep have been partially uplifted.

ABSTRACT The tectonic evolution of the Marañón Basin and its related basins, the Huallaga and Santiago Basins, in northern Peru, spans more than 250 m.y. of Mesozoic–Cenozoic subsidence. Basin evolution began with an initial rifting in the Late Permian–Early Triassic. This period of extension was accommodated by inherited structural inhomogeneities and a southwest-oriented extension, which dissected the Paleozoic sequences into a series of roughly northwest-southeast-trending grabens and half grabens filled with volcanic and continental-derived sediments. Fault-controlled subsidence was followed by regional postrift subsidence, and a thick section of Triassic to Jurassic marine to transitional sediments was deposited over the preexisting extensional features. These included one of the potential source rocks for the western part of the basin (the Aramachay Formation). The Late Jurassic to Early Cretaceous Jurua orogeny and later peneplanation produced a major regional unconformity. Subsequent Cretaceous sedimentation, mostly controlled by eustatic processes and related regressive–transgressive cycles, is composed of a thick section of continental to proximal and shallow marine deposits, comprising the main reservoirs and the main source rock for the basin, responsible for most of the oil discoveries in the northeastern region. Several compressional and structural inversion episodes, related with the subduction of the Nazca plate in Late Cretaceous and that culminated in the Miocene, have modified the basin and isolated the Huallaga and Santiago subbasins to the west, both structured by complex thrust systems. Cenozoic deposits constitute the foreland basin system infill and contain more than 4000 m (13,120 ft) of mostly fluvial and deltaic deposits with minor marine incursions. In the sub-Andean zone they constitute the “molasses” from the rising Andean cordillera to the west. This history of tectonic evolution is reflected in a complex structural framework in the western part and a well-developed foreland system to the east with a broad topographic high, known as the Iquitos Arch, which corresponds to the present forebulge.

Abstract The Recôncavo basin, on the Atlantic Coast near the city of Salvador, has an area of about 10,000 km 2 and is the principal petroleum province of Brazil. Since 1939, approximately 255 wildcats have been drilled and have discovered 43 accumulations which total 942 million bbl of producible oil and 992 billion ft 3 of gas. API gravity of most oil ranges from 35 to 40°. The Bahia Supergroup, the main objective for petroleum exploration, has a maximum thickness of 6,500 m. This nonmarine unit ranges in age from Late Jurassic(?) to Early Cretaceous. The Upper Jurassic(?) typically consists of a redbed sequence (Alianç a Formation) overlain by a blanket sandstone (Sergi Formation). The Sergi is the best reservoir rock of the basin. The Lower Cretaceous (Wealden) strata are mainly dark-gray and grayish-green shale of the Itaparica, Candeias, and llhas Formations and are considered to be the oil and gas source rocks. The A Sandstone, the lenticular sandstone bodies of the Candeias Formation, and the São Paulo and Santiago Sandstones of the llhas Formation are the best reservoir rocks of the Lower Cretaceous section. The Recôncavo basin is an intracratonic half gaben. Intensive faulting occurred during the deposition of Candeias and lower llhas when the basin became a rapidly sinking trough. Accelerated growth of the Salvador and Mata-Catu uplifts, the most prominent structural features of the basin, produced the two principal (NE- and NW-trending) sets of normal faults. A late post- Sao Sebastião tectonic phase reactivated ancient faults and caused new ones to form. Consequently, the basin is characterized by a complex system of faulted blocks. The six major fields, containing 96 percent of the total producible oil, are related to the structural evolution of the basin. It is believed that the early period of faulting, contemporaneous with the deposition of the Candeias and lower llhas, was a decisive factor in the control of petroleum migration to, and accumulation in, the Sergi and A sandstones. The horst blocks of Agua Grande, Buracica, and Dom João fields (the first two having been partly uplifted during this tectonic phase) trapped about 623 million bbl of recoverable oil in these two sandstone bodies. Accumulation in llhas reservoirs was controlled mainly by the later, post-São Sebastião, phase of faulting. Folds developed in the downthrown blocks of normal faults, but the folding was not caused by compressional stress. These folds form traps for accumulations in the São Paulo, Santiago, and other llhas sandstones. Examples of such traps, in which about 187 million bbl of producible oil accumulated, are the Miranga and Taquipe fields. The genesis of the lenticular sandstone reservoirs in the Candeias field—a strati- graphic trap—is related to syntectonic deposition of the Candeias Formation; fractured shale and limestone also are reservoirs in this field. Candeias trapped about 93 million bbl of producible oil.

Improved depositional age constraints and stratigraphic description of rocks in San Diego require designation of a new Upper Jurassic formation, herein named the Peñasquitos Formation after its exposures in Los Peñasquitos Canyon Preserve of the city of San Diego. The strata are dark-gray mudstone with interbedded first-cycle volcanogenic sandstone and conglomerate-breccia and contain the Tithonian marine pelecypod Buchia piochii. Laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon 206* Pb/ 238 U ages of 147.9 ± 3.2 Ma, 145.6 ± 5.3 Ma, and 144.5 ± 3.0 Ma measured on volcaniclastic samples from Los Peñasquitos and Rancho Valencia Canyons are interpreted as magmatic crystallization ages and are consistent with the Tithonian depositional age indicated by fossils. Whole-rock geochemistry is consistent with an island-arc volcanic source for most of the rocks. The strata of the Peñasquitos Formation have been assigned to the Santiago Peak volcanics by many workers, but there are major differences. The Peñasquitos Formation is marine; older (150–141 Ma); deformed everywhere and overturned in places; and locally is altered to pyrophyllite. In contrast, the Santiago Peak volcanics are nonmarine and contain paleosols in places; younger (128–110 Ma); undeformed and nearly flat lying in many places; and not altered to pyrophyllite. The Peñasquitos Formation rocks have also been assigned to the Bedford Canyon Formation by previous workers, but the Bedford Canyon is distinctly less volcanogenic and contains chert, pebbly mudstones, and limestone olistoliths(?) with Bajocian- to Callovian-age fossils. Here, we interpret the Peñasquitos Formation as deep-water marine forearc basin sedimentary and volcanic strata deposited outboard of the Peninsular Ranges magmatic arc. The Upper Jurassic Mariposa Formation of the western Sierra Nevada Foothills is a good analog. Results of detrital zircon U/Pb dating from an exposure of continentally derived sandstone at Lusardi Creek are consistent with a mixed volcanic-continental provenance for the Peñasquitos Formation. A weighted mean U/Pb age of 144.9 ± 2.8 Ma from the youngest cluster of detrital grain ages is interpreted as the likely depositional age. Pre-Cordilleran arc zircon age distributions (>285 Ma) are similar to Jurassic deposits from the Colorado Plateau, with dominant Appalachian-derived Paleozoic (300–480 Ma), Pan African (531–641 Ma), and Grenville (950–1335 Ma) grains, consistent with derivation either directly, or through sediment recycling, from the Colorado Plateau Mesozoic basins and related fluvial transport systems. Appalachian- and Ouachita-like detrital zircon age distributions are characteristic of Jurassic Cordilleran forearc basins from northeast Oregon to west-central Baja California, indicating deposition within the same continent-fringing west-facing arc system.

The Mesozoic Peninsular Ranges batholith, part of a long-lived Cordilleran subduction orogen, is located at a critical juncture at the southwest corner of cratonal North America. The batholith is divided into northern and southern segments that differ in their evolution. In this paper, we focus on the more poorly understood southern Peninsular Ranges batholith, south of the Agua Blanca fault at ~31.5°N latitude, and we compare its evolution with the better-known northern Peninsular Ranges batholith. Adding our new insights to previous work, our present understanding of the geologic history of the Peninsular Ranges consists of the following: (1) stronger connections between the Paleozoic passive-margin rocks in the eastern Peninsular Ranges batholith and similar assemblages in Sonora, Mexico, to the east and the Sierra Nevada batholith to the north that were originally proposed by earlier workers; (2) continuity of the Triassic–Jurassic accretionary prism and forearc basin assemblage from the northern Peninsular Ranges batholith through the southern Peninsular Ranges batholith; (3) possible synchronous subduction of an ocean ridge or ridge transform along the Peninsular Ranges batholith in late Middle Jurassic time; (4) continuity of the Early Cretaceous Santiago Peak continental arc from the northern Peninsular Ranges batholith along the entire margin, including the southern Peninsular Ranges batholith; (5) development of the Alisitos oceanic arc in Jurassic and possibly Triassic time, much earlier than originally thought; and (6) removal of part of the Santiago Peak assemblage in the southern Peninsular Ranges batholith during collision of the Alisitos terrane in latest Early Cretaceous time.

Abstract Preliminary exploration to evaluate the La Primavera (Jalisco) geothermal prospect began in the 1960s with photogeological interpretation and geochemical surveys of the surface shows. In the late 1960s the (then) Office of Geothermal Exploration of the Comisión Federal de Electricidad (CFE) drew up a wideranging geological project to explore the central portion of Jalisco State. Results indicated La Primavera to be the most favorable of several geothermal prospects by reason of its geologic structural conditions and the ages and compositions of its rocks. Detailed geological, geochemical, geophysical, and geohydrological surveys were carried out to select exploratory well locations. Five geothermal wells (PR-1, PR-2, PR-4, PR-5, and RC-1) were drilled in a first phase between January 1980 and August 1982. Subsequently, PR-1 was deepened, and drilling began on PR-8 in 1984. The Sierra La Primavera lies 15 km west of the city of Guadalajara (Fig. 1) between longitudes 103°28′ and 103°38′W and latitudes 20°32′ and 20°43′N, in a volcanic area of fumaroles and hot springs. The region forms part of a series of valleys, basins, and block mountains of mainly mid-Tertiary to Pliocene- Quaternary volcanic rocks. Drainage is parallel and dendritic; in the rainy season, intermittent streams carry water to the valleys and basins draining toward the Río Grande de Santiago in the north and the Río Ameca in the west, and thence to the Pacific Ocean.