30 years in the life of an active submarine volcano: A time-lapse bathymetry study of the Kick-‘em-Jenny Volcano, Lesser Antilles

Effective monitoring is an essential part of identifying and mitigating volcanic hazards. In the submarine environment this is more difficult than onshore because observations are typically limited to land-based seismic networks and infrequent shipboard surveys. Since the first recorded eruption in 1939, the Kick-‘em-Jenny (KeJ) volcano, located 8km off northern Grenada, has been the source of 13 episodes of T-phase signals. These distinctive seismic signals, often coincident with heightened body-wave seismicity, are interpreted as extrusive eruptions. They have occurred with a recurrence interval of around a decade, yet direct confirmation of volcanism has been rare. By conducting new bathymetric surveys in 2016 and 2017 and reprocessing 4 legacy datasets spanning 30 years we present a clearer picture of the development of KeJ through time. Processed grids with a cell size of 5m and vertical precision on the order of 1-4m allow us to correlate T-phase episodes with morphological changes at the volcano's edifice. In the time-period of observation 7.09x106 m3 of material has been added through constructive volcanism – yet 5 times this amount has been lost through landslides. Limited recent magma production suggests that KeJ may be susceptible to larger eruptions with longer repeat times than have occurred during the study interval, behavior more similar to sub-aerial volcanism in the arc than previously thought. T-phase signals at KeJ have a varied origin and are unlikely to be solely the result of extrusive submarine eruptions. Our results confirm the value of repeat swath bathymetry surveys in assessing submarine volcanic hazards

The impact of high-density spatial sampling versus antenna orientation on 3D GPR fracture imaging

Three-dimensional Ground Penetrating Radar (3D GPR) surveys are necessary to reconstruct complex fracture geometries in the subsurface. Two of the most important factors controlling image quality are antenna orientation relative to fractures and density of acquisition grids. This study, conducted in the Madonna della Mazza quarry (Italy), compares two acquisition methods with the goal of optimizing the imaging of fractures and related 3D fracture networks. We acquired two very dense, orthogonal 3D GPR surveys with a 250 MHz antenna and 5 cm trace spacing on the same day, covering the same area of 20 x 20 m. By decimation of the original raw datasets, reduced survey densities of 10 cm and 20 cm spacing are simulated. The results show differences in the imaging quality of the two methods to depths of 75 cm, while for a depth of 130 cm and deeper, image quality is similar. At the same trace density/m2, a single, unidirectional survey with a densely sampled grid is the preferred method rather than two surveys with orthogonal antenna orientations but larger profile spacing. The extra effort of conducting surveys with an acquisition grid of an eighth of wavelength of the antenna centre frequency guarantees that we can properly sample the high-frequency content of the GPR signal spectrum and results in optimum image quality regardless of fracture orientation. The simple survey design principle found in this study is universally applicable to any field condition, target geometry, and antenna frequency. Such high-density 3D GPR survey design enables the high-resolution characterization of 3D fracture networks on subsurface timeslices in near photographic quality

Ramp reef depositional facies model for the Mid-Pliocene Golden Gates Reef Member of the Tamiami Formation, South Florida

The depositional setting for northern most Atlantic coral reef development during the Mid-Pliocene Warm Period is a gentle sloping mixed carbonate–siliciclastic ramp. Five core transects document the distribution of the reef complex in an area approximately 65 by 12 km, and nine repetitive depositional facies are identified. Paleoecological and sedimentological evidence documents facies development along a water depth–energy gradient. The mid-Pliocene Tamiami Formation is characterized by coral boundstone developed over level skeletal rudstone depositional units dominated by mollusks. Hyotissa haitensis (Sowerby), one of the last Gryphaeid oysters, is the dominant fossil found in the most continuous skeletal facies and overlies deeper water green clay facies, the only facies with pelagic foraminifera. Ground penetrating radar documents reef depositional topography, onlapping stratigraphy and two episodes of reef growth. Two cycles of deposition are recognized, separated by subaerial exposure. The coral boundstone and the skeletal rudstone exhibit both high primary and secondary porosity and overlie the impermeable clay facies. The upper surface of the coral boundstone lies at ~ 4 m in elevation whereas contemporaneous estuarine deposits are found to the north at elevations of 20–25 m. High porosity bank reef complexes along a shallow dipping ramp provide an alternative to the standard model of reef and porosity development along the outer shelf margin. Understanding the differences in associated facies between these two depositional environments permits better interpretation of observed heterogeneities in subsurface geobodies associated with inner shelf and platform settings

Margin collapse and slope failure along southwestern Great Bahama Bank

Steep convex-bankward embayments into carbonate platform margins, often called “scalloped margins”, have been observed in ancient examples and along modern platform of the Gulf of Mexico and the Caribbean region and are interpreted as erosional features produced by large-scale margin failures. Multibeam bathymetry data from the southwestern corner of Great Bahama Bank (GBB) image four margin failures and their associated erosional products. The bankward-convex embayments range in diameter from 3 to 23km. The largest and southernmost collapse produces a scalloped margin while the other three are not changing the generally linear platform margin. The typical slope angle of the upper slope in this portion of GBB ranges from 20 to 40° with the margin break at ~60 to 65m water depth. But in the four areas of platform margin collapse the slope angle increases and the margin break is shallower. The largest collapse eroded more than 350m of the bank margin with an estimated ~15km3 of platform margin materials shed onto the adjacent slope. These margin collapses shed large debris blocks to the toe-of-slope and basin floor some tens of kilometers from the platform margin. In the southernmost segment margin collapse is followed by slope failures that produce mass transport complexes (MTC) that litter the lower slope and basin floor. The largest block in one MTC is 2000×800m in dimension and is displaced by 1.2km. The margin collapses are more common along the southwestern GBB than the northern portions of GBB where large-scale slope failures are more common. This lateral distribution is attributed to the tectonic activity in the vicinity of the Cuban fold and thrust belt. Faults breaking the modern seafloor and Holocene growth strata on the Santaren Anticline document neo-tectonic activity within the belt. Thus, tectonic activity and associated seismic shock might be the primary trigger for the margin collapse and occurrence of the scalloped margin along the Old Bahama Channel