Solution pans and linear sand bedforms on the bare-rock limestone shelf of the Campeche Bank, Yucatán Peninsula, Mexico

A high-resolution, near-surface geophysical survey was conducted in 2013 on the Campeche Bank, a carbonate platform offshore of Yucatán, Mexico, to provide a hazard assessment for future scientific drilling into the Chicxulub impact crater. It also provided an opportunity to obtain detailed information on the seafloor morphology and shallow stratigraphy of this understudied region. The seafloor exhibited two morphologies: (1) small-scale (<2 m) bare-rock karstic features, and (2) thin (<1 m) linear sand accumulations overlying the bedrock. Solution pans, circular to oblong depressions featured flat bottoms and steep sides, were the dominant karstic features; they are known to form subaerially by the pooling of rainwater and dissolution of carbonate. Observed pans were 10–50 cm deep and generally 1–8 m wide, but occasionally reach 15 m, significantly larger than any solution pan observed on land (maximum 6 m). These features likely grew over the course of many 10's of thousands of years in an arid environment while subaerially exposed during lowered sea levels. Surface sands are organized into linear bedforms oriented NE-SW, 10's to 100's meters wide, and kilometers long. These features are identified as sand ribbons (longitudinal bedforms), and contained asymmetric secondary transverse bedforms that indicate NE-directed flow. This orientation is incompatible with the prevalent westward current direction; we hypothesize that these features are storm-generated

Mantle deformation beneath the Chicxulub impact crater

Accepted versio

Seismic Stratigraphy of the Sabrina Coast Shelf, East Antarctica: Early History of Dynamic Meltwater-rich Glaciations

High-resolution seismic data from the Sabrina Coast continental shelf, East Antarctica, elucidate the Cenozoic evolution of the East Antarctic Ice Sheet. Detailed seismic stratigraphic and facies analysis reveal the Paleogene to earliest Pliocene glacial evolution of the Aurora Basin catchment, including at least 12 glacial expansions across the shelf indicated by erosional surfaces and chaotic acoustic character of strata. Differences in facies composition and seismic architecture reveal several periods of ice-free conditions succeeded by glacial expansions across the shelf. A deep (∼100 m), undulating erosional surface suggests the initial appearance of grounded ice on the shelf. Following the initial ice expansion, the region experienced an interval of open-marine to ice-distal conditions, marked by an up to 200-m-thick sequence of stratified sediments. At least three stacked erosional surfaces reveal major cross-shelf glacial expansions of regional glaciers characterized by deep (up to ∼120 m) channel systems associated with extensive subglacial meltwater. The seismic character of the sediments below the latest Miocene to earliest Pliocene regional unconformity indicates intervals of glacial retreat interrupted by advances of temperate, meltwater-rich glacial ice from the Aurora Basin catchment. Our results document the Paleogene to late Miocene glacial history of this climatically sensitive region of East Antarctica and provide an important paleoenvironmental context for future scientific drilling to constrain the regional climate and timing of Cenozoic glacial variability

Seismic images of the Transition fault and the unstable Yakutat-Pacific-North American triple junction

In southern Alaska, the Pacific plate and Yakutat terrane subduct beneath the North American plate along the Aleutian Trench and Pamplona zone, respectively, and are sliding past each other at minimal rates along the Transition fault. As the deformation front of the Pamplona zone stepped eastward during the Pliocene–Pleistocene, the Pacific–North American–Yakutat triple junction became unstable. Four recent seismic images reveal that the Transition fault changes from a single strike-slip boundary east of the deformation front to three strands that step increasingly seaward between the deformation front and the Aleutian Trench. The southern two strands deform the Pacific crust, and the outermost of these became increasingly convergent sometime since 1 Ma, as demonstrated by young growth strata. We propose that this internal deformation of the Pacific plate is an attempt to reattain stability, which can only be reached by creating a tectonic boundary collinear with the Pamplona zone. The plate reorganization will result in initiation of subduction such that a portion of former Pacific crust will become accreted to the North American plate. Such accretion events caused by triple-junction instability may be an important mechanism for transferring oceanic crust to continental margins

Seabed characterization on the New Jersey middle and outer shelf: correlatability and spatial variability of seafloor sediment properties

Nearly 100 collocated grab samples and in situ 65 kHz acoustic measurements were collected on the New Jersey middle and outer shelf within an area that had previously been mapped with multibeam backscatter and bathymetry data, and more recently with chirp seismic reflection profiling. Eighteen short cores were also collected and probed for resistivity-based porosity measurements. The combined data set provides a basis for empirically exploring the relationship among the remotely sensed data, such as backscatter and reflection coefficients, and directly measured seabed properties such as grain size distribution, velocity, attenuation and porosity. We also investigate the spatial variability of these properties through semi-variogram analysis to facilitate acoustic modeling of natural environmental variability. Grain size distributions on the New Jersey shelf are commonly multi-modal, leading us to separately characterize coarse % (\u3e4 mm), fine % ( \u3c 63 Am) and mean sand grain size to quantify the distribution. We find that the backscatter is dominated by the coarse component (expressed as weight %), typically shell hash and occasionally terrigenous gravel. In sediment types where coarse material is not significant, backscatter correlates with velocity and fine weight %. Mean sand grain size and fine % are partially correlated with each other, and combined represent the primary control on velocity. The fine %, rather than mean grain size as a whole, appears to be the primary control on attenuation, although coarse % may increase attenuation marginally through scattering. Vertical-incidence seismic reflection coefficients, carefully culled of unreliable values, exhibit a strong correlation with the in situ velocity measurements, suggesting that such data may prove more reliable than backscatter at deriving sediment physical properties from remote sensing data. The velocity and mean sand grain size semi-variograms can be fitted with a von Ka´rma´n statistical model with horizontal scale f12.6 km, which provides a basis for generating synthetic realizations. The backscatter and coarse % semi-variograms exhibit two horizontal scales: one f8 km and the other too small to quantify with available data