Seismic imaging and attribute analysis of Chicxulub Crater central sector, Yucatán Platform, Gulf of Mexico

Chicxulub Crater, formed ~66Ma ago by an asteroid impact on the southern Gulf of Mexico, is the best preserved of the three large multi-ring basins in the terrestrial record. The crater structure is characterized by a semi-circular concentric ring pattern, marking the crater basin, peak ring, terrace zone and basement uplift. Analysis of a grid of 19 seismic reflection profiles using seismic attributes, marker horizons, contour surfaces and 3-D views is used to investigate the stratigraphy of the central zone. We used interactive software and routine applications to map the impact breccias, breccia-carbonate contact and post-impact carbonates. Four horizons marked by high-amplitude reflectors representing high-impedance contrasts were identified and laterally correlated in the seismic  images. Complex trace attribute analysis was applied for petrophysical characterization. Surface contour maps of base and top of stratigraphic packages were constructed, which mapped the impactites and post- and pre-impact carbonate stratigraphy. Basin floor, marked by the contact between the impact breccias and overlying carbonates is shown by laterally discontinuous high-amplitude reflectors. Discontinuous scattered reflectors interpreted as the upper breccias beneath the crater floor, have an average thickness of ~300msm. The Paleogene sedimentary units are characterized by multiple reflectors with lateral continuity, which contrast with the seismic response of underlying breccias. The basal Paleocene sediments follow the basin floor relief. Upwards in the section, the carbonate strata are characterized by horizontal reflectors, which are interrupted by a regional unconformity. Onlap/downlap packages over the unconformity record a period of sea level change

Seismic imaging and attribute analysis of Chicxulub Crater central sector, Yucatán Platform, Gulf of Mexico

Chicxulub Crater, formed ~66Ma ago by an asteroid impact on the southern Gulf of Mexico, is the best preserved of the three large multi-ring basins in the terrestrial record. The crater structure is characterized by a semi-circular concentric ring pattern, marking the crater basin, peak ring, terrace zone and basement uplift. Analysis of a grid of 19 seismic reflection profiles using seismic attributes, marker horizons, contour surfaces and 3-D views is used to investigate the stratigraphy of the central zone. We used interactive software and routine applications to map the impact breccias, breccia-carbonate contact and post-impact carbonates. Four horizons marked by high-amplitude reflectors representing high-impedance contrasts were identified and laterally correlated in the seismic images. Complex trace attribute analysis was applied for petrophysical characterization. Surface contour maps of base and top of stratigraphic packages were constructed, which mapped the impactites and post- and pre-impact carbonate stratigraphy. Basin floor, marked by the contact between the impact breccias and overlying carbonates is shown by laterally discontinuous high-amplitude reflectors. Discontinuous scattered reflectors interpreted as the upper breccias beneath the crater floor, have an average thickness of ~300msm. The Paleogene sedimentary units are characterized by multiple reflectors with lateral continuity, which contrast with the seismic response of underlying breccias. The basal Paleocene sediments follow the basin floor relief. Upwards in the section, the carbonate strata are characterized by horizontal reflectors, which are interrupted by a regional unconformity. Onlap/downlap packages over the unconformity record a period of sea level change

A rock- and paleomagnetic study of a Holocene lava flow in Central Mexico

Magnetic measurements of the Tres Cruces lava flow (ca. 8500 years BP, Central Mexico) show the presence of two remanence carriers, a Ti-rich titanomagnetite with a Curie temperature between 350 and 400 °C and a Ti-poor magnetite with a Curie temperature close to 580°C. Magnetic changes after heating indicate that the titanomagnetite exsolves into magnetite w and ilmenite when the sample is heated to 580 °C. Paleointensity estimates with the Thellier and Thellier method [Thellier, E., Thellier, O., 1959. Sur l'intensité du champ magnetique terrestre dans le passe historique et geologique. Ann. Geophysique., 15, 285-376] were only successful up to temperatures of 350 to 400 °C. This temperature corresponds with the Curie temperature of the titanomagnetite, which is probably pseudo-single or multi-domain. Therefore, the paleointensities should be interpreted with caution. The magnetic composition changes after 580 °C heating may explain the large w variations in previous paleointensity determinations for the Tres Cruces rocks [Gonzalez, S., Sherwood, G., Bohnel, H., Schnepp, E., 1997. Palaeosecular variation in Central Mexico over the last 30,000 years: the record from lavas. Geophys. J. Int., 130, 201-219] using the [Shaw method Shaw, J., 1974. A new method of determining the magnitude of the palaeomagnetic field: application to five historic lavas and five archaeological samples. Geophys. J. R. Astr. Soc., 39, 133-141]

Evolution of ocean circulation and water masses in the Guaymas Basin (Gulf of California) during the last 31,000 years revealed by radiolarians and silicoflagellates in IODP expedition 385 sediment cores

The high-resolution analysis of radiolarians and silicoflagellates in sediments from Holes U1545A and U1549A drilled during IODP Expedition 385 in the Guaymas Basin, in the Gulf of California provides detailed insights into the evolution of ocean circulation and water masses, and its relation to Eastern Tropical Pacific Ocean climate conditions, over the past 31,000 cal years BP (based on AMS radiocarbon dates). In the pre-Last Glacial Maximum, the Guaymas Basin experienced alternating circulation patterns of California Current Water (CCW) and Gulf of California Water (GCW), with an extended presence of the Pacific Intermediate Water (PIW) owing to: amplified jet streams; southern movement of the California Current System (CCS) and the incursion of CCW into the gulf; and increased North Pacific Intermediate Water (NPIW) formation. The Last Glacial Maximum witnessed the incursion of CCW due to the stronger CCS. The dominance of the PIW indicates the expansion and formation of NPIW. The Heinrich-I event as manifested in the core record, displays two distinct patterns, one suggesting GCW-like dominance and the other, the occurrence of CCW. The Bølling-Ållerød interstadial featured the entry of Tropical Surface Water (TSW), GCW, and CCW, linked with the northward migration of the Intertropical Convergence Zone. In the Younger Dryas, CCW dominated, transitioning to GCW as colder climatic conditions and more intense CCS. The Holocene displayed alternating periods of TSW and GCW, with a modern monsoon regime from 7,600 to 1,000 cal years BP. From 1,000 cal years BP to the present the ITCZ shifted to the south

The use of LA-ICP-MS in a pilot study for determining the concentration of selected trace elements in rudist shells

ABSTRACT A protocol for the determination of the concentration of selected elements in two rudist shells was developed using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Element analysis in rudists has never been performed beyond including major and some minor elements, and thus this work presents the first results in this field. The concentrations of 35 isotopes were analyzed. Low iron and manganese concentrations reflect the absence of a diagenetical imprint on both shells. There is a negligible amount of crust-associated elements, which reveal little evidence of terrigenous input. Phosphorus amount is also low; however, it is high enough to mask reliable Rare Earth + Yttrium (REY) pattern measurements. Strontium and magnesium concentrations in the inner layers of a shell of Titanosarcolites show relicts of an original aragonitic composition. On the contrary, concentrations of these elements in a shell of Biradiolites rudissimus reveal an original low Mg-calcite mineralogy in its outer layers. The two shells were selected, considering their apparently a priori good preservation, based on the petrological examination of thin sections. No significant concentrations of elements such as chromium, cobalt, nickel, copper, zinc, molybdenum or antimony could be found. The concentrations of vanadium and barium may reflect evidence of biological activity, since they apparently do not correlate significantly with silicon, aluminium or sulphur. The low cost and precision of this method in comparison to others such as the use of electron microprobe, or X-ray fluorescence, makes it an optimum alternative for geochemical analyses of fossils, thus opening a new line of research. Keywords: Cretaceous, geochemistry, LA-ICP-MS, rudists, trace elements. El uso de LA-ICP-MS RESUMEN EXTENDIDO EN CASTELLANO Introducción y Metodología Los Rudistas son moluscos bivalvos sésiles extintos que dominaron los ambientes arrecifales del Jurásico Superior hasta el Cretácico Superior En todos los casos, la exactitud de los resultados ha sido atribuida a la escasa diagénesis como producto de la mineralogía de las conchas (calcita baja en magnesio). En investigaciones más recientes, Hennhöfer y colaboradores (2012) llevaron a cabo análisis de isótopos de oxígeno en conchas seleccionadas de rudistas dentro de un pequeño bouquet de la especie . Muy pocos son los trabajos que abordan aspectos geoquímicos mediante LA-ICP-MS en fósiles provenientes de facies carbonatadas (Burla et al., 2009; Evans and Müller, 2013), pero ninguno lo ha hecho con rudistas. Esta es la razón por la cual decidimos proponer este protocolo. Dos secciones pulidas provenientes de valvas inferiores de conchas de rudistas fueron analizadas. Estas conchas pertenecen a un ejemplar de "Titanosarcolites sp.nov. 1 pendiente de denominación" Oviedo-García del Maastrichtiense de Chiapas, México (Figura 1), y a un ejemplar de Biradiolites rudissimus Trechmann del Campaniense?-Maastrichtiense, también de Chiapas, México (Figura 2). La selección de las conchas fue llevada a cabo a través del análisis petrológico de láminas delgadas, entre cinco conchas del género Titanosarcolites y tres del género Biradiolites La ablación con láser se realizó utilizando un sistema Excimer de 193 nm COMPEX 110 (Lambda Physik, Göttingen, Alemania) Sánchez-Beristain, F. et al., 2015. The use of LA-ICP-MS in a pilot study for determining... Boletín Geológico y Minero, 160 Sánchez-Beristain, F. et al., 2015. The use of LA-ICP-MS in a pilot study for determining... Boletín Geológico y Minero, 126 Resultados y Discusión Los resultados observados para ambas conchas son similares en cierta medid

Late Quaternary evolution of alluvial fans in the Playa, El Fresnal region, northern Chihuahua desert, Mexico: Palaeoclimatic implications

The Playa El Fresnal area is a tilted terrane characteristic of an extensional basin. It is a half graben/tilted-block system with a playa-lake on the basin floor flanked by piedmonts covered by alluvial fans. Structural heterogeneities within normal fault zones influenced the geomorphic expression of the uplifted footwall blocks of associated volcanism, and the downdropped hanging wall. The footwall area is the main sediment source, but the hanging wall-derived sediments are more extensive. The ancient alluvial fans are in the distal part, whereas the hanging-wall sediments are located in the apex area. A geomorphic analysis of the relative topographic position of the alluvial fans, degree of dissection of the original surfaces, general sedimentology (facies description), and stream channel network type, highlights the importance of climatic change in interpreting alluvial-fan surfaces. Three generations of alluvial fans were identified on the footwall and hanging wall slopes. They were formed during the late Quaternary climatic shift, consistent with the main climatic changes recorded in the paleolake stratig-raphy of northern Mexico and the American Southwest. These alluvial fans consist mainly of debris-flow deposits from flash floods, probably triggered by a change from relatively moist to arid conditions. They contrast with the typically lower-flow-regime of thick-bedded, cross-bedded, and lenticular channel facies, and associated floodplain sequences of rivers