Ana Luisa Carreño

Biostratigraphy of Lower Cretaceous microfossils from the Araripe basin, northeastern Brazil

Abstract A biostratigraphic study was carried out in the Lower Cretaceous Araripe basin, northeastern Brazil, allowing the recognition of several chronostratigraphic units: the Dom Joao (Jurassic?–Lower Cretaceous?), the Rio da Serra (Neocomian) and the Alagoas (Aptian/Albian) local stages. For the first time a large hiatus between the Rio da Serra and Alagoas local stages is carefully documented. The palynomorphs and the ostracode associations throughout the Jurassic?–Aptian/Albian sequence allow the interpretation of the paleoenvironmental evolution of the Araripe basin which otherwise confirms that a polycyclical sedimentation occurred in the basin, being one of the controlling factors on the distribution of ostracodes and palynomorphes.

Neogene sedimentary evolution of Baja California in relation to regional tectonics

Abstract During the Neogene, the tectonic and sedimentary evolution of the Baja California Peninsula followed four stages: (1) during the early Miocene (22 Ma), the initiation of transform motion between Pacific and North American plates, caused a rapid subsidence in the Continental Borderland Province and in some adjacent areas.This subsidence coincided in time with with a global rise in sea level. At this time, the eastern and southern parts of the peninsula did not show any evidence of subsidence. (2) During the middle Miocene (12 Ma), normal and strike slip faulting migrated eastward, causing subsidence in the northern part of the Gulf of California, where the oldest Tertiary marine sedimentary rocks were deposited. The areas in central Baja California Sur and the central part of the Gulf itself received abundant volcanic deposits related to continental extension. (3) During the late Miocene (8 Ma), the western margin of the Peninsula changed to a slightly compressive regime, while the northern part of the Gulf contained a marine basin with upper bathyal environments. The central area of the Gulf continued receiving abundant volcanic deposits, while the Los Cabos block received marine sedimentation, correlatable with sedimentary units reported from the continental margins in Nayarit, Jalisco and Michoacan. (4) Beginning in the early Pliocene (5 Ma), the present configuration of the Gulf of California developed through right-lateral strike slip and extension in the Gulf itself. Since Pliocene times, the Gulf presents widespread marine sedimentation with deep basins reaching lower bathyal depths.

A paleoenvironmental reconstruction of Laguna Babícora, Chihuahua, Mexico based on ostracode paleoecology and trace element shell chemistry

Paleoecology of Laguna Babícora, Chihuahua, Mexico was reconstructed using ostracode faunal assemblages and shell chemistry. The paleolimnological record is used to show the magnitude of paleoclimatic changes in the area from 25,000 years to the present.Faunal assemblages consist of four species of the genus Limnocythere: L. sappaensis, L. ceriotuberosa, L. bradburyi and L. platyforma, all associated with Candona caudata, Candona patzcuaro and Cypridopsis vidua. A paleosalinity index developed from these assemblages indicates that the lakes salinity fluctuated frequently from oligo- to meso-haline conditions during the last ∽ 25,000 years. This pattern and low salinity range are in good agreement with modern TDS (here used as an indicator of salinity) values recorded from 26 wells and one spring from the area (258–975 mg l−1). To estimate paleotemperature we examined the trace element content (Mg/Ca ratios) from individual valves of L. ceriotuberosa and L. platyforma, the two species most commonly recorded in Laguna Babícora.Shell Mg/Ca ratios of 204 specimens of these two species were used to estimate water temperature (Mg/Ca) by means of experimental standard coefficients. Our data show that paleowater temperature ranged from 5.6–21.3 °C (with 2σ values ranging from 0.2–4.8 °C), which suggest a close correlation with atmospheric temperatures around the lake. These results are in good agreement with a modern mean winter temperature (3.5 °C) and mean summer temperature (20 °C) recorded in the area between 1970 and 1980.

Late Cenozoic sea level changes evidenced by ostracodes in the Pelotas basin, southernmost Brazil

Abstract Ostracodes recovered from sediments in seven onshore boreholes within the Pelotas basin, State of Rio Grande do Sul, indicate several continuous nearshore environments and at least four brief transgressive-regressive cycles are present. This interpretation is based on the distribution throughout upper Neogene and Quaternary sequences of species that characterize one or more biofacies depending on the borehole position in the basin. Bradleya pelotensis, Krithe coimbrai, Henryhowella kempfi and H. rectangulata are considered shelf-upper bathyal species whereas the shelf species are represented by Ambostracon crucicostatum, Argenticytheretta variabilis, A. laevipunctata, Brasilicythere reticulispinosa, Caudites posdiagonalis, Costa riograndensis, Coquimba bertelsae, C. tenuireticulata, Cytheretta punctata, Quadracythere eichlerae and Protocytheretta sp. Species with lagoon-estuarine shelf affinities include Callistocythere litoralensis, C. marginalis and Perissocytheridea kroemmelbeini whereas Cyprideis maxipunctata, C. mostardensis, C. posteroinflata, C. salebrosa and C. sparsopunctata represent a lagoon or estuarine environment, and Cypris cassinensis and Limnocythere sp. are freshwater species. Correlation of the ostracode biozones places the four maximum regressive events in the Late Pliocene and Pleistocene (1250-485 ka, 185 ka and 15 ka), whereas the maximum transgressive events occur in the Late Miocene and Pleistocene (1.6 Ma, 423–400 ka and 120 ka), in a barrier-lagoonal depositional system controlled by glacio-eustatic sea level changes.

Ostracodes (crustacea) from Cananéia-Iguape estuarine/lagoon system and geographical distribution of the mixohaline assemblages in southern and southeastern Brazil

The ostracode assemblages from Cananeia-Iguape estuarine/lagoon system (southernmost State of Sao Paulo) are here discussed in detail for the first time. Thirty-four sites, approximately 1 km equidistant, were sampled along the system, including the Cananeia Sea, Pequeno Sea, Cubatao Sea, Ribeira de Iguape River and Itapitangui River. The ostracodes throughout this area have poor assemblages, with a total of 662 specimens of dead and living organisms. The majority of the ostracode fauna is composed of euryhaline species, as follows: Cyprideis multidentata Hartmann, 1955 (174 specimens), Minicythere heinii Ornellas, 1974 (54 specimens), Tanella gracilis Kingma, 1948 (96 specimens) and Whatleyella sanguinettiae Coimbra, Carreno & Ferron, 1994 (226 specimens). Although there are few studies on the Brazilian mixohaline ostracode faunas, including the euryhaline marginal marine taxa, the published data show that the group is best known in the south and southeast regions. Based on this review and with the new data presented in this paper, the geographical distribution of eight mixohaline key species in southern and southeastern Brazil is also discussed.

The San Nicolás Formation: A Proto-Gulf Extensional-Related New Lithostratigraphic Unit at Bahía San Nicolás, Baja California Sur, Mexico

Abstract A detailed study and mapping of the Neogene volcanosedimentary rocks cropping at the Bahía San Nicolás, Baja California Sur, was carried out, allowing reconnaissance and formal description of the San Nicolás Formation (a lithostratigraphic unit constituted from older to younger by the lower Toba San Antonio, Los Volcanes, Arroyo Amarillo, and the upper El Saucito members). These units reflect syn-to-postextensional episodes associated to the Gulf of California Province that show initial high subsidence onto dry land associated with low rates of sedimentation followed by shallow marine water deposition dominated by storm and macro tidal processes. The maximum age assigned to the units is 3.3 ± 0.5 Ma.

Biogenic coastal deposits: Isla del Carmen, Gulf of California, Mexico

Coastal sedimentary deposits including beach and dunes within the south-central region of the Gulf of California show a high percentage of biogenic components, up to 84%, derived from the fragmentation of organisms that, among other oceanographic factors, reflect wave energy and effect of seasonal winds. Our report shows that due to high-energy waves, coastal deposits on points or islands with a northern exposure contain massive biocalcarenite deposits derived from mollusk shells, while areas of lower water energy located on coastal areas facing east or west commonly accumulate the crushed debris of coralline red algae, both as beach and dune deposits. Sheltered areas along any point or island oriented to the south will most likely attract colonization by coral reefs. Isla del Carmen clearly displays all these different types of biogenic deposits, making it an ideal model for the development of coastal carbonates throughout the western Gulf of California. Equally important, the general model has been persistent since early Pliocene times for the whole region. In particular for Isla del Carmen, this scenario precludes any tectonic block rotation, postulated in previous reports.

Abundance and distribution of benthic foraminifera as indicators of the quality of the sedimentary environment in a subtropical lagoon, Gulf of California

Abundance and spatial distribution of benthic foraminifera were used to evaluate the impacts of anthropogenic activities on sediment quality in a coastal lagoon in the Gulf of California. In 1985 and 2013, 27 samples of superficial sediments were collected. The foraminifera genera Ammonia, Cribroelphidium, Quinqueloculina and Peneroplis were dominant in both years. The abundance of Ammonia increased from 41% to 60%, while Peneroplis abundance decreased from a maximum of 50% in 1985 to 7% in 2013. The greater abundance of Ammonia and the greater spatial coverage of Ammonia and Cribroelphidium suggest a marked environmental deterioration in the quality of the sedimentary environment, which contrasts with studies of trace elements in the sediment of this lagoon. The Foram Stress Index indicates that sediment quality has deteriorated over time, likely due to the effects of anthropogenic activities around the lagoon.