Tectonic Structure and Relationship with Sinkhole Harzard in Bang Lung Area, Cho Don District, Bac Kan Province

By integration of remote sensing images analysis, geology, geomorphology, hydrogeology, geophysical method, and drilling data, the paper illustrates the structure tectonics, causes, and initial mechanism of a sinkhole forming in Bang Lung, Cho Don, Bac Kan province. The NE-SW normal slip faults are an essential fault system in the area, which created Bang Lung graben valley. This fault system also forms large fracture zones, creating advantage conditions for the groundwater runoff both vertically and horizontally to eroded and dissolved carbonate rock-forming underground karst caves. These are favorable natural conditions for forming a sinkhole. The sinkhole hazard in the Bang Lung area is initiated by some main factors such as tectonic activity, thickness, and characteristics of unconsolidated sediment layers, groundwater fluctuation, karst caves, and human activities. The most human impacts are mining exploitation and agricultural cultivation that promote sinkholes occurring faster and earlier. The horizontal and vertical movement of groundwater dragged the material on the ceiling karst caves into ground spaces. Thereby, weakening the cohesion of the unconsolidated sediment above caves leads to gravitational unbalance and creates a sinkhole. This study has also shown potential sinkhole areas in Bang Lung, which helps the authorities and local people in sinkhole prevention and mitigation mission.

Elevation of underlying basement rock, Ogdensburg Harbor, NY

Over six linear miles of shallow acoustic reflection geophysical data were collected in an 800 ft by 300 ft survey region at Ogdensburg Harbor, Ogdensburg, NY. To better accommodate modern commercial vessels and expand the harbor’s capacity, the current navigable depth of -19 ft Low Water Depth (LWD) needs to be increased to -28 ft LWD, and an accurate map of the nature of the riverbed material (e.g., unconsolidated sediment, partially indurated glacial till, or bedrock) is required to effectively plan for removal. A total of 28 boreholes were previously collected to map the stratigraphy, and the effort revealed significant spatial variability in unit thickness and elevation between adjacent boreholes. To accurately map this variable stratigraphy, chirp sub-bottom profiles were collected throughout the region, with an average line spacing of 13 ft. These sub-bottom data, validated and augmented by the borehole data, resulted in high-resolution spatial maps of stratigraphic elevation and thickness for the study area. The data will allow for more accurate assessment of the type and extent of different dredging efforts required to achieve a future uniform depth of -28 ft LWD for the navigable region.

Sedimentary and volcanic record of the nascent Izu-Bonin-Mariana arc from IODP Site U1438

The oldest known, intact sedimentary record of a nascent intraoceanic arc was recovered in a ∼100-m-thick unit (IV) above ca. 49 Ma basaltic basement at International Ocean Discovery Program Site U1438 in the Amami Sankaku Basin. During deposition of Unit IV the site was located ∼250 km from the plate edge, where Izu-Bonin-Mariana subduction initiated at 52 Ma. Basement basalts are overlain by a mudstone-dominated subunit (IVC) with a thin basal layer of dark brown metalliferous mudstone followed by mudstone with sparse, graded laminae of amphibole- and biotite-bearing tuffaceous sandstone and siltstone. Amphibole and zircon ages from these laminae suggest that the intermediate subduction-related magmatism that sourced them initiated at ca. 47 Ma soon after basement formation. Overlying volcaniclastic, sandy, gravity-flow deposits (subunit IVB) have a different provenance; shallow water fauna and tachylitic glass fragments indicate a source volcanic edifice that rose above the carbonate compensation depth and may have been emergent. Basaltic andesite intervals in upper subunit IVB have textures suggesting emplacement as intrusions into unconsolidated sediment on a volcanic center with geochemical and petrological characteristics of mafic, differentiated island arc magmatism. Distinctive Hf-Nd isotope characteristics similar to the least-radiogenic Izu-Bonin-Mariana boninites support a relatively old age for the basaltic andesites similar to detrital amphibole dated at 47 Ma. The absence of boninites at that time may have resulted from the position of Site U1438 at a greater distance from the plate edge. The upper interval of mudstone with tuffaceous beds (subunit IVA) progresses upsection into Unit III, part of a wedge of sediment fed by growing arc-axis volcanoes to the east. At Site U1438, in what was to become a reararc position, the succession of early extensional basaltic magmatism associated with spontaneous subduction initiation is followed by a rapid transition into potentially widespread subduction-related magmatism and sedimentation prior to the onset of focused magmatism and major arc building.

Different pockmark systems and their potential importance for the hydrological and biogeochemical balance of a peri-alpine lake

<p>Pockmarks are crater-like depressions on the floor of oceans and lakes formed by the upward transport of fluids through the unconsolidated sediment column. The fluid flow through marine pockmarks is considered to enhance hydrological and biogeochemical exchanges between the sediments and the water body. While a similar relevance can be expected in lakes, the importance of lacustrine pockmarks in this regard is virtually unexplored.</p><p>Lake Thun (48.3 km<sup>2</sup> surface area), Switzerland, is an excellent system to study lacustrine pockmarks as it exhibits several sites with different geological and biogeochemical settings. One of the pockmark sites is characterized by evident methane (CH<sub>4</sub>) ebullition and high CH<sub>4</sub> concentrations from ~2.4 to 8.9 mM within the sediments beneath. A large pockmark with a diameter of 110 m is located adjacent to the rock wall of a karst system and might thus be associated with groundwater discharge into the lake. Finally, spikes in electrical conductivity detected during a survey with a remotely operated vehicle (ROV) at a third pockmark site suggest a hydrogeological connection with the groundwater system in the underlying Triassic bedrock.</p><p>This third pockmark site we are studying more closely. We observed that the sediments inside the pockmark were clearly more liquified as compared to those at a reference site (outside the pockmark), providing further evidence for groundwater discharge that might presently be active. The porewater chemistry was similar at the two sites, except for the total dissolved Fe concentration which was about 2 to 5-fold lower inside the pockmark than at a reference site. Further chemical analysis of porewaters and the water column above the pockmark as well as a molecular investigation (e.g. 16S rRNA) of the sediments will be performed at two different seasons of the year (in fall and spring during the snowmelt season). All together, these results should help us to better assess the influence of groundwater discharge via this pockmark site on the hydrological balance and on the biogeochemistry of the lake, as well as to expand our limited knowledge on the mechanism of lacustrine pockmarks in general.</p>

The Geochemistry of 1 ky Old Euxinic Sediments of the Western Black Sea

To get more data on the geochemistry of Black Sea euxinic sediments, a 50-cm core was collected at a depth of 600 m on a Western Black Sea Continental Platform slope. The core contained unconsolidated sediments rich in cocoolithic ooze and mud. Epithermal Neutron and Prompt Gamma Activation Analysis were used to determine the content of nine major (Na, Mg, Al, Si, K, Ca, Ti, Mn, and Fe as oxides) and 32 trace elements (Cl, Sc, V, Cr, Co, Ni, Zn, As, Se, Br, Rb, Sr, Zr, Mo, Sn, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Yb, Hf, Ta, W, Th, and U) with a precision varying between 3 and 9%. The core contained unconsolidated sediment rich in coccolithic ooze and mud. Previous 210 Pb geochronology suggests an age of ∼1 ky of considered sediments. Major components distribution showed that, except for Cl and Ca, the contents of all other elements are similar to Upper Continental Crust (UCC) and North American Shale Composite (NASC). The distribution of the 32 trace elements showed similarities to the UCC, except for redox-sensitive metals Fe, Se, Mo, and U, of which the significantly higher content reflects the presence of euxinic conditions during deposition. A chondrite normalized plot of nine rare earth elements indicated a similarity to UCC and NASC, suggesting a continental origin of sedimentary material.