Sierra El Research Articles

Seismically-induced mass movements and volumetric fluxes resulting from the 2010 Mw = 7.2 earthquake in the Sierra Cucapah, Mexico

The observation that sediment flux from mountain ranges struck by high magnitude earthquakes can be strongly influenced by coseismic mass movements brings into question the nature of coseismic deformation as a net contributor to mountain building. To better constrain the role of high-magnitude earthquakes in orogenesis, high-resolution data of earthquake induced mass wasting is required for areas of differing tectonic, morphological, and climatic settings. Here we compare the erosional flux to the tectonic flux associated with the 2010 Mw=7.2 Sierra El Mayor earthquake in Mexico and examine the landslide patterning of coseismic mass movements associated with this event. The ruptured fault system has a significant strike-slip component with subsidence along the eastern flank and uplift on the western flank of the range. Peak ground acceleration was highest along the steepest sections of the range such that the frequency of landslide occurrence was strongly correlated to slope gradient. Both vertical and horizontal coseismic displacement demonstrated a strong control over landslide initiation. This result suggests that strike-slip systems experience very different landslide patterning to thrust faults during earthquakes. Based on interferometric analysis of synthetic aperture radar images, the earthquake resulted in a total uplifted volume of 41.6×106m3 and a loss of 95.2×106m3 due to subsidence. This suggests a net tectonic volumetric flux of −53.6×106m3. Sediment mobilisation by coseismic landslides is estimated at −2.7×106m3 derived from a manually mapped inventory using SPOT 5 multispectral imagery. Thus, the net volume loss through coseismic subsidence of the mountain range generated a strongly negative mass flux, which was only marginally enhanced by mass wasting.

Age and depositional conditions of the marine vertebrate concentration Lagerstätte at Gomez Farías, southern Coahuila, Mexico

A 1.5 m thick coquinite discovered in the Upper Jurassic La Casita Formation of the Sierra El Jabalà near Gomez Farías, Coahuila, northeastern Mexico qualifies as a concentration Lagerstâtte owing to its richness in marine vertebrates. Ichthyosaurs, pliosaurs and crocodilians were described to some detail, but other taxa remained unstudied and the precise biostratigraphical age, as well as paleoecological conditions that led to the formation of the fossil deposit, are not known in detail. Here we describe ammonites, aptychi, bivalves and radiolarians, which allow for a stratigraphic assignation of the deposit to the uppermost Kimmeridgian Beckeri Zone. The unit under consideration accumulated in a hemipelagic mud bottom environment during a period of time characterized by low oxygen conditions, while a short term benthic colonization phase near the top of the coquinite corresponds to increased oxygen availability. A combination of upwelling, bottom currents, winnowing, offshore winds, storm events, circulatory nutrient traps, low oxygenated bottom waters, and a transgressional regime with reduced net sedimentation was crucial factors for the subsequent concentration of fossils, as well as for marine phosphate generation and phosphorus migration.

Agave doctorensis (Agavaceae), a new species from Sierra El Doctor, Querétaro, Mexico

Agave doctorensis, a new species from Sierra El Doctor, Queretaro, Mexico is described and illustrated. It is similar to Agave montium-sancticaroli but differs in having a dense, conical panicle; reflexed bracts; and filaments inserted at two levels. The new species occurs in rosette scrub and Juniperus forest.

Una especie nueva de Acidocroton Sección Ophellantha (Euphorbiaceae) de Tabasco, México

Se describe e ilustra a Acidocroton madrigalensis, una nueva especie del estado de Tabasco, México. Se conoce solamente de la Sierra El Madrigal y el Cerro La Campana, en donde es un componente abundante del estrato bajo en la selva alta perennifolia sobre afloramientos de roca caliza. Pertenece a la sección Ophellantha y se caracteriza por sus diminutas estípulas no espiniformes, hojas grandes y cortamente pecioladas y por tener flores estaminadas con 20 a 35 estambres. Se presenta un cuadro mostrando las diferencias principales entre A. madrigalensisy las otras dos especies del género conocidas de México, A. spinosus y A. steyermarkii.

Avistamientos recientes de águila real (Aquila chrysaetos) en la sierra El Mechudo, Baja California Sur, México

We report new sightings of Golden Eagle for 3 consecutive years (2007-2010) at 2 localities of the Sierra El Mechudo (El Saucito and Tarabillas), Baja California Sur. These sightings were recorded with camera traps and by direct observation. Most recent records from BCS are for the Sierra de la Laguna in 2000. At least 4 different individuals, both juveniles and adults, have been recorded. The importance of these new sightings is that for the first time we registered Golden Eagles infresh water wetlands.

Location of Aftershocks of the 4 April 2010 Mw 7.2 El Mayor-Cucapah Earthquake of Baja California, Mexico

A magnitude M w 7.2 earthquake occurred on 4 April 2010 at approximately 50 km southwest of the city of Mexicali, Baja California, Mexico. The regional networks Red Sismica del Noroeste de Mexico (RESNOM) that the Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE) operates and the Southern California Seismic Network (SCSN) located the main event between the sierras El Mayor and Cucapah at the southeastern end of the Pescadores fault. Twenty-four hours after the origin time of the main event, a temporal seismic network was installed around the faults that ruptured during this event. We used body-wave arrival times recorded by the first 14 stations installed to obtain precise hypocentral locations of this important sequence of earthquakes. Most of the aftershocks located on 6 and 7 April are distributed near the traces of the Pescadores and Cucapah faults, particularly at the southern end of these faults. The spatial distribution of the epicenters indicates that these faults and possibly others may extend southeast of Sierra Cucapah where the faults were buried by sediments and were activated during the seismic sequence. In this area, north of the Sierra El Mayor, where no faults were previously mapped, the epicenters align in the northwest–southeast direction. Most hypocenters have depths less than 15 km, suggesting that an important portion of the seismic slip was shallow. Based on the distribution of focal depths of the aftershocks, we infer that the seismogenic zone must be between 5 and 10 km.

LEAD ISOTOPIC ANALYSIS OF COPPER ORES FROM THE SIERRA EL ARAMO (ASTURIAS, SPAIN)*

This study presents the first ever lead isotope ratio data for copper ore samples (malachite and azurite) from the Sierra El Aramo (Asturias, northwestern Spain). The aim of this study was to supplement information in the existing reference database on the lead isotopic composition of Spanish ores. The Sierra El Aramo is particularly important because, while no data for the Asturian ores have been published so far, this northern province of Spain has played a significant role in terms of copper exploitation and metallurgy since the Copper Age.

Timing and controls on late Quaternary landscape development along the eastern Sierra El Mayor range front in northern Baja California, Mexico

The Sierra El Mayor–Sierra Cucapá mountain range in northern Baja separates the Colorado River delta from the Laguna Salada Basin and is important in terms of understanding the structural transition from the northern Gulf Extensional Province to the San Andreas system. One of the principal Quaternary landscape features of the eastern Sierra El Mayor–Sierra Cucapá is a prominent surface that slopes ~ 1–2° eastward towards the Colorado River Delta. Along the eastern side of the Sierra El Mayor and facing the Colorado River delta, but not along other piedmont mountain fronts that face the delta, this surface is terminated by a series of stepped terrace surfaces. The terrace surfaces are cut into fluvial–deltaic deposits from the Colorado River delta system and are capped by ~ 1 m thick gravels. Optically stimulated luminescence ages from interbedded coarse sands in the capping gravels of the upper surface and two of the inset terraces range from ~ 17 to 31 ka (average 24 ± 3 ka) but lack a systematic age trend from upper to lower surface. The underlying fluvial–deltaic deposits have older optically stimulated luminescence ages of 31 to 39 ka and suggest Colorado River delta deposition during the relatively stable interstadial of the Middle Wisconsinan substage (Marine Isotope Stage 3) when sea level was ca − 60 to − 80 m. Samples from depth-profiles in the capping gravels and from the top of the upper surface have a mean 10Be terrestrial cosmogenic nuclide age of 59 ± 5 ka. When corrected for inheritance, an average age of ~ 21 ka is computed for exposure of the capping gravels on the upper surface and inset terraces, which is in the range of the optically stimulated luminescence burial ages. Both the upper surface and inset terraces formed 17–30 ka at a transition time from dry to relatively wet climate when increased aggradation caused alluvial fan development on the upper surface. Regionally, these surfaces correlate with the Q2c–Q3a age deposits of the southwestern North America alluvial fan and terrace deposits. Sea level was also falling rapidly at this time, which may have caused the Colorado River to become entrenched near the range front thus allowing rapid response and down-cutting to form the inset terraces on the east side of the Sierra El Mayor facing the delta. Assuming the 34 ka delta deposits graded to a base level that was − 60 to − 80 m during Marine Isotope Stage 3, the computed surface uplift rate for the eastern side of the Sierra El Mayor is ~ 2.1–2.7 mm/yr. This uplift may be accommodated on a fault or fault system that is a southward extension of the Mexicali spreading and seismic zone that is buried beneath the Colorado River delta east of the Sierra El Mayor. The uplift rates from this study are consistent with geologic and geodetic data for the Sierra El Mayor and Sierra Cucapá, and account for much of the missing extensional strain across the area.

Phylogenetic relationships of leopard frogs ( Rana pipiens complex) from an isolated coastal mountain range in southern Sonora, Mexico

Mitochondrial DNA sequence data from the control region and 12S rRNA in leopard frogs from the Sierra El Aguaje of southern Sonora, Mexico, together with GenBank sequences, were used to infer taxonomic identity and provide phylogenetic hypotheses for relationships with other members of the Rana pipiens complex. We show that frogs from the Sierra El Aguaje belong to the Rana berlandieri subgroup, or Scurrilirana clade, of the R. pipiens group, and are most closely related to Rana magnaocularis from Nayarit, Mexico. We also provide further evidence that Rana magnaocularis and R. yavapaiensis are close relatives.

Quaternary alluvial-fan development, climate and morphologic dating of fault scarps in Laguna Salada, Baja California, Mexico

Late Quaternary slip across the Cañada David detachment has produced an extensive array of Quaternary scarps cutting alluvial-fans along nearly the entire length (~ 60 km) of the range-bounding detachment. Eight regional alluvial-fan surfaces (Q 1 [youngest] to Q 8 [oldest]) are defined and mapped along the entire Sierra el Mayor range-front. Terrestrial cosmogenic nuclide 10Be concentrations from individual boulders on alluvial-fan surfaces Q 4 and Q 7 yield surface exposure ages of 15.5 ± 2.2 ka and 204 ± 11 ka, respectively. Formation of the fans is probably tectonic, but their evolution is strongly moderated by climate, with surfaces developing as the hydrological conditions have changed in response to climate change on Milankovitch timescales. Systematic mapping reveals that the fault scarp array along active range-bounding faults in Sierras Cucapa and El Mayor can be divided into individual rupture zones, based on cross-cutting relationships with alluvial-fans. Quantitative morphological ages of the Laguna Salada fault-scarps, derived from linear diffusive degradation modeling, are consistent with the age of the scarps based on cross-cutting relationships. The weighted means of the maximum mass diffusivity constant for all scarps with offsets < 4 m is 0.051 and 0.066 m 2/ka for the infinite and finite-slope solutions of the diffusion equation, respectively. This estimate is approximately an order of magnitude smaller than the lowest diffusivity constants documented in other regions and it probably reflects the extreme aridity and other microclimatic conditions that characterize the eastern margin of Laguna Salada.

HISTORICAL DISTRIBUTION OF DESERT BIGHORN SHEEP (OVIS CANADENSIS MEXICANA) IN COAHUILA, MEXICO

Abstract Historically, desert bighorn sheep occurred throughout Coahuila, Mexico, as far south as latitude 25°43′02″N. The subspecies Ovis canadensis mexicana probably was extirpated in Coahuila by 1970. We determined the historical range of desert bighorn sheep through a review of the available literature, interviews with long-term local residents, and a subjective habitat assessment. We found historical documentation of bighorn sheep in 14 mountain ranges (Sierra Alamitos, Sierra Maderas del Carmen, Sierra la Encantada, Sierra Hechiceros, Sierra del Pino, Sierra Mojada, Sierra el Rey, Sierra San Marcos y del Pino, Sierra Gavia, and Sierra la Paila), including 4 previously not recorded (Sierra el Fuste, Sierra el Almagre, Sierra de la Madera, and Sierra la Fragua). In addition, one archaeological site with remains of bighorn sheep was identified (La Candelaria Cave). The introduction of domestic livestock, particularly sheep and goats, and unregulated hunting probably were the major factors contributing ...

Major ignimbrites and volcanic centers of the Copper Canyon area: A view into the core of Mexico's Sierra Madre Occidental

Reconnaissance mapping along Copper Canyon highway has established ignimbrite stratigraphic relationships over a relatively large area in the central part of the Sierra Madre Occidental volcanic field in western Chihuahua, Mexico. The oldest ignimbrites are found in the central part of the area, and they include units previously mapped from north of the study area, in and around the Tomóchic volcanic complex. Copper Canyon, at the southern end of the study area, exposes younger units, including the intracaldera tuff of the Copper Canyon caldera and five overlying ignimbrites. Well-exposed calderas are found near San Juanito, in the central part of the map area, and at Sierra Manzanita, to the far north. Stratigraphic evidence for yet another caldera in the northern part of the area is found in the Sierra El Comanche. The stratigraphic and limited available isotopic age data suggest that volcanism was particularly active ∼30 m.y. ago. This reconnaissance survey also documented lava-flow lithologies consistent with previous observations from Tomóchic that intermediate lavas have erupted throughout that area's volcanic history and that basaltic andesite became particularly abundant as felsic volcanism waned. The combined Copper Canyon–Tomóchic area gives the first view into the core of the giant Sierra Madre Occidental volcanic field, expanding that offered by earlier reports, mostly from peripheral regions. The emerging picture is one of a dramatically thickened and stratigraphically complex volcanic section related to numerous overlapping caldera complexes, much like that documented for the core of the San Juan Mountains volcanic field, Colorado.

Quercus tuitensis (Fagaceae, Quercus sect. Lobatae), a new deciduous oak from western Jalisco, Mexico

Abstract Quercus tuitensis (Fagaceae, Quercus sect. Lobatae), a new deciduous oak from western Jalisco, Mexico. Brittonia 55: 42–48. 2003.—Quercus tuitensis, a deciduous red oak known only from the Sierra El Tuito on the Pacific slopes of western Jalisco, Mexico, is described, illustrated, and compared to Q. praineana Trel. In addition, Q. aequivenulosa and Q. coffeaecolor are reaffirmed as synonyms of Q. praineana.

Habitat use by desert bighorn sheep in sonora, México

The use of habitat components by desert bighorn sheep ( Ovis canadensis ) was examined to explain sexual segregation of sheep in Sierra el Viejo, Sonora, Mexico. We evaluated 265 plots used by bighorns and 278 random plots from April 1997 to December 1998. Groups of segregated males and females preferred the elephant tree ( Bursera microphylla )-salvia ( Salvia mellifera )-limber bush ( Jatropha cuneata ) association (ESL) and avoided the foothill palo verde ( Cercidium microphyllum )-desert ironwood ( Olneya tesota ) association (FDD. Segregated females selected the ocotillo ( Fouquieria splendens )-desert agave (Agave spp.yhop bush ( Dodonoea viscosa ) (OAH) vegetation association, formed larger groups, were closer to escape terrain, and occupied more rugged areas during autumn and spring than males. Segregated females selected areas that provided more opportunities to evade predation than did males.

Late Neogene stratigraphy and tectonic control on facies evolution in the Laguna Salada Basin, northern Baja California, Mexico

Abstract The Laguna Salada Basin (LSB) in northeastern Baja California records late-Neogene marine incursions in the Salton Trough and progradation of the Colorado River delta. Early subsidence and subsequent tectonic erosion are related to evolution of the Sierra El Mayor detachment fault during late Miocene time (

Cretaceous and Tertiary sedimentary, magmatic, and tectonic evolution of north-central Sonora (Arizpe and Bacanuchi Quadrangles), northwest Mexico

The Arizpe and Bacanuchi Quadrangles provide a geologic history representative of the north-central part of Sonora, where lithologies are dominated by late Mesozoic and Cenozoic igneous rocks. In this study, new geologic mapping, 40Ar/39Ar dating, and geochemical analyses have been combined to provide a stratigraphic framework for this area. Ten lithostratigraphic units and several igneous and tectonic events can be recognized. The oldest outcropping rocks are Lower Cretaceous strata of the Bisbee Group, which along with the Picacho conglomerate record a middle Cretaceous compressive tectonic event and associated sedimentation. Laramide igneous activity is widespread and represented by (1) highly altered andesitic flows and volcaniclastic rocks (Arroyo Alcaparros andesitic rocks) of late Campanian to Maastrichtian age, (2) less altered andesitic and dacitic flows (Cerro Las Jarillas volcanic rocks) of late Paleocene age, and the intrusive bodies of (3) Sierra El Manzanal granodiorite and (4) Rancho Vaqueria quartz monzonite. The Sierra El Manzanal granodiorite was emplaced at ca. 68 Ma on the basis of a 40Ar/39Ar biotite age (67.97 ± 0.19 Ma) and cooled relatively rapidly according to less precise 40Ar/39Ar hornblende and K-feldspar ages from the same sample (64.8 ± 1.0 Ma and 62.8 ± 0.3 Ma, respectively). The Cerro Las Jarillas volcanic rocks are slightly younger (40Ar/39Ar biotite age of 58.67 ± 0.17 Ma). The Rancho Vaqueria quartz monzonite was emplaced at ca. 57 Ma (40Ar/39Ar biotite age of 56.73 ± 0.14 Ma and a less precise 40Ar/39Ar hornblende age of 55.0 ± 0.7 Ma); a protracted cooling history of this pluton is indicated by the age spectrum of K-feldspar from the same sample. A probable magmatic lull and denudation seem to have occurred between middle and late Eocene time and probably until the early Oligocene. Subsequently, rhyolitic to mafic volcanism began close to late Oligocene time and lasted until the early Miocene. Felsic volcanism is represented by the Cerro Cebadehuachi volcanic rocks, from which 40Ar/39Ar hornblende ages of 27.25 ± 0.09 and 27.32 ± 0.06 Ma and a biotite age of 26.97 ± 0.06 Ma were obtained at three different localities. The Mesa Pedregosa volcanic rocks represent the transition to younger, mafic volcanic activity that occurred during the late Oligocene, as indicated by a sanidine 40Ar/39Ar age of 25.48 ± 0.05 Ma. This late Oligocene and early Miocene magmatism was paired by two episodes of extensional deformation. The first phase is characterized by northwest-striking normal faults and folds, which expose the deepest structural levels of the area, and by the related basin fill, the Bacanuchi conglomerate. The second phase is represented by north-striking normal faults and by the syntectonic basin fill, the Arizpe conglomerate. Basaltic andesite volcanic flows at the base of the Arizpe conglomerate yielded 40Ar/39Ar (whole-rock) ages of 23.52 ± 0.17 and 21 ± 0.20 Ma. The extensional deformation (27 to 23 Ma) in the study area is coeval with the development of metamorphic core complexes in neighboring areas of Sonora and with the onset of extension in southern Sonora. The mafic volcanic rocks and clastic sedimentary units associated with this extension resemble the basin fills that in other parts of Sonora are assigned to the Baucarit Formation. Geochemical information from samples representing each of the igneous events displayed high-K calc-alkalic and mostly metaluminous compositions. The older units including the Arroyo Alcaparros andesitic rocks, the Cerro Las Jarillas volcanic rocks, the Sierra El Manzanal granodiorite, and the Rancho Vaqueria quartz monzonite are characterized by steep chondrite-normalized REE (rare earth element) slopes and generally well-developed negative Eu anomalies, suggesting garnet and plagioclase removal in the source. The younger igneous events including the Cerro Cebadehuachi and Mesa Pedregosa volcanic rocks, and the basaltic flows associated with the Arizpe conglomerate, showed basin-shaped REE slopes with no Eu anomalies, suggesting clinopyroxene or amphibole fractionation.

Pre-Miocene palaeogeography of the Los Cabos Block, Baja California Sur: geochronological and palaeomagnetic constraints

The Los Cabos Block (LCB) is located at the southern end of the Baja California peninsula and is composed mainly of intrusive rocks that were emplaced into pre-Cretaceous heterogeneous metasediments and are partly covered by Miocene volcanic and volcaniclastic rocks. Field observations provide evidence that the undeformed, homogeneous gabbronorite of the Sierra El Novillo in the NE part of the LCB is the oldest crystalline unit, which is intruded by undeformed to slightly deformed tonalitic-quartz-dioritic granitoids. For these rocks an Rb–Sr intrusion age of 129±15 Ma and a biotite whole-rock cooling age of 116±2 Ma were obtained. Low initial 87Sr/ 86Sr and high ε-Nd values of about 0.7036 and +5 to +6 respectively characterise the intrusives as primitive, mantle-derived granites. Further to the southeast and south, granitoids are more silicic, with abundant deformed sequences — orthogneisses, diatexites and migmatites, among others. From the southern part of the LCB an Rb–Sr intrusion age for the undeformed granites of 115±4 Ma and a biotite cooling age of 90±2 Ma were determined. Initial 87Sr/ 86Sr was determined to be 0.7054 and ε-Nd values fall between −2 and 0. The isotopic data do not clearly allow us to characterise the deformed granitoids of the LCB as being intruded by the undeformed unit. Deformation could have been partly due to syn-intrusive tectonics. Palaeomagnetic data suggest minor, if any, northward displacement of the LCB with respect to continental Mexico, corresponding to the rifting in the Gulf of California since the late Miocene, and significant 35–45° clockwise rotations. Possible tilting effects, which may have occurred given the dip of Tertiary volcanics on top of the intrusives, would modify these values for rotation and to a lesser degree of northward displacement. Geochemical, isotopic and palaeomagnetic results and the cooling history of the LCB are similar to those from the Puerto Vallarta Batholith, whereas other Mexican continental margin plutons further to the SE display lower intrusion ages, more rapid cooling rates and generally more primitive magma compositions. In the light of the data presented, a common magmatic evolution of the LCB and the Puerto Vallarta Batholith is strongly suggested.

Range-front fault scarps of the Sierra El Mayor, Baja California: Formed above an active low-angle normal fault?

Research Article| March 01, 1999 Range-front fault scarps of the Sierra El Mayor, Baja California: Formed above an active low-angle normal fault? Gary J. Axen; Gary J. Axen 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA Search for other works by this author on: GSW Google Scholar John M. Fletcher; John M. Fletcher 2Departamento de Geología, Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California, Mexico Search for other works by this author on: GSW Google Scholar Eric Cowgill; Eric Cowgill 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA Search for other works by this author on: GSW Google Scholar Michael Murphy; Michael Murphy 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA Search for other works by this author on: GSW Google Scholar Paul Kapp; Paul Kapp 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA Search for other works by this author on: GSW Google Scholar Ian MacMillan; Ian MacMillan 1Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA Search for other works by this author on: GSW Google Scholar Ernesto Ramos-Velázquez; Ernesto Ramos-Velázquez 2Departamento de Geología, Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California, Mexico Search for other works by this author on: GSW Google Scholar Jorge Aranda-Gómez Jorge Aranda-Gómez 3Unidad de Ciencias de la Tierra, Instituto de Geología, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Querétaro, México Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (3): 247–250. https://doi.org/10.1130/0091-7613(1999)027<0247:RFFSOT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gary J. Axen, John M. Fletcher, Eric Cowgill, Michael Murphy, Paul Kapp, Ian MacMillan, Ernesto Ramos-Velázquez, Jorge Aranda-Gómez; Range-front fault scarps of the Sierra El Mayor, Baja California: Formed above an active low-angle normal fault?. Geology 1999;; 27 (3): 247–250. doi: https://doi.org/10.1130/0091-7613(1999)027<0247:RFFSOT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Cañada David detachment, a west-dipping low-angle normal fault, juxtaposes Pliocene and Pleistocene(?) sedimentary units over sheared bedrock in the Sierra El Mayor, Baja California. Along the west-central range front, the detachment is overlapped by Quaternary alluvial fans that are, in turn, cut by fault scarps up to 7 m high. Map relationships suggest that scarp-forming faults may sole into the detachment at very shallow depth (<200–300 m). Palinspastic restoration of a topographic profile across 15 north-striking scarps suggests that the ratio of net heave to net throw along scarp-forming faults is ∼2, consistent with them soling into a low-angle (∼30°) fault. These 15 scarps may have all formed in one earthquake, with rupture propagation from depth to the near-surface aided by clay gouge along the detachment. The detachment is abandoned in the footwall east of the scarps. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Hydrocarbons Related to Early Cretaceous Source Rocks, Reservoirs and Seals, Trapped In Northeastern Neuqun Basin, Argentina: ABSTRACT

The Jurassic-Cretaceous backarc Neuqun Basin, located in the west central part of Argentina, is currently the most prolific oil basin of the country. The primary objective of this study is to evaluate an Early Cretaceous to Tertiary petroleum system in the northeastern portion of the basin, where oil and gas occurrences (e.g., Puesto Hernandez, Chihuido de la Sierra Negra, El Trapial and Filo Morado oil fields, among others) provide 82 MMBO/yr comprising 67% of the basin oil production and 31% of Argentina. The source rocks are represented by two thick sections of basinal kerogen type I and II organic-rich shales, deposited during transgressive peaks (Agrio Formation), with TOC content up to 5.1%. Lowstand sandstones bodies, 10 to 100 m thick, are composed of eolian and fluvial facies with good reservoir conditions (Avil and Troncoso Sandstones). The seals are provided by the organic-rich shales resting sharply upon the Avil Sandstone and a widespread Aptian-Albian evaporitic event (Huitrin Formation) on top of the Troncoso reservoir. Tertiary structural traps (duplex anticlines) are developed in the outer foothills, whereas structural, combined and stratigraphic traps are present in the adjacent stable structural platform. Oil-to-source rock and oil-to-oil correlation by chromatographic and biomarker fingerprints, carbon isotopicmore » composition and the geological evidences support the proposed oil system.« less

Diachronous uplift and recycling of sedimentary basins during Cenozoic tectonic transpression, northeastern Caribbean plate margin

Four marine sedimentary sequences of Late Cretaceous to Pleistocene age crop out in the 320 km long, 1 to 30 km wide Peralta-Rio Ocoa belt of Hispaniola (Haiti and Dominican Republic). The four sedimentary sequences are dominated by marine turbiditic rocks ranging up to 11.5 km in apparent thickness and exhibit mainly northwest to southeast, belt-parallel paleocurrents. The three younger sequences are generally in fault or unconformable contact with the underlying sequence. From northwest to southeast, the four belts become progressively wider, younger, less metamorphosed, less folded and faulted, and lower in topographic elevation. We interpret the three younger sequences as a syn-tectonic stratigraphic record of diachronous, northwest to southeast transpressional closure of a Coniacian-Danian back-arc basin represented by the oldest exposed sedimentary sequence at the northwest end of the belt and inferred at depth beneath the three younger sequences. Using the stratigraphic record, we infer the following three stages in the closure of the back-arc basin and the overlying younger basins. Stage One: latest Cretaceous to early Late Eocene closure. A minimum of 11 km of Paleocene turbidites and limestone in the Padre las Casas area and a minimum of 11.5 km of Early Eocene to early Late Eocene pelagic limestone, mudstone, sandstone, and siltstone in the Peralta and Sierra El Numero areas was deposited in an elongate basin derived from the first stage of latest Cretaceous-early Late Eocene closure and erosion of the Coniacian-Danian back-arc basin to the northwest. Paleocene-Eocene turbidites in both areas contain large amounts of reworked Campanian-Paleocene microfauna and exhibit northwest to southeast belt-parallel paleocurrents. Syn-deformational features in Eocene sedimentary rocks in the Peralta and Sierra El Numero area indicate that convergent deformation accompanied sedimentation (Witschard and Dolan, 1990). Stage Two: Middle Eocene to Early Miocene closure. Up to 8.6 km of Middle Eocene to Early Miocene turbidites, debris flows, and olistostromes of the Rio Ocoa Group of the Dominican Republic were deposited in an elongate basin derived from Middle Eocene to Early Miocene closure, uplift, and erosion of the Peralta Group to the west and northwest. Turbidites contain reworked Eocene faunas and lithologies similar to those in the underlying Peralta Group and exhibit northwest to southeast, belt-parallel paleocurrents. Erosion of Late Cretaceous island arc and overlying Paleogene carbonate rocks of the Cordillera Central provided additional belt-perpendicular sources of coarser-grained conglomerate and olistostromes. Following deposition of the Rio Ocoa Group, both the Peralta and the Rio Ocoa Group were deformed in a southwest-verging fold-and-thrust belt during the Early Miocene. Stage Three: Middle Miocene to Recent closure. Up to 1.5 km of Middle Miocene to Pleistocene sandstone, conglomerate, and reefal limestone of the Ingenio Caei Group of the Dominican Republic were deposited in an elongate basin above a pronounced angular unconformity developed on the older, deformed rocks of the Rio Ocoa Group. Sediments of the Ingenio Caei Group were derived from Middle Miocene to Recent closure, uplift, and erosion of the Rio Ocoa and Peralta Groups to the northwest. Onshore exposures of the Ingenio Caei Group can be correlated with submerged strata seen on seismic reflection profiles across the offshore San Pedro basin. Recent submarine clastic sedimentation in the San Pedro basin is dominated by northwest to southeast influx of clastic turbidites derived from active erosion of onshore exposures of the Rio Ocoa and Ingenio Caei Groups. Middle Miocene (?) to Recent sediments of the San Pedro basin are being actively shortened by northward to northeastward-directed underthrusting along the Muertos trench. Underthrusting at the Muertos trench is suggested to represent ongoing closure of the back-arc basin thought to be deeply buried beneath the San Pedro basin. Tectonic transpression along the closing back-arc basin marks one edge of an actively deforming microplate within the North America-Caribbean strike-slip plate boundary zone.