Barry Kohl

Influence of relative sea level changes on the construction of the Mississippi Fan

A conceptual sea-level-driven depositional model for individual fanlobes (channel-overbank systems) of the Mississippi Fan does not permit direct application of the sequence stratigraphic principles of Vail and colleagues. Deep Sea Drilling Project Leg 96 results suggest that, during initial relative lowering of sea level, the canyon and upper fan channel were formed; excavated fine-grained slope sediments may have formed a debris flow deposit base for the fanlobe. Continued lowering produced constructional channel-levee-overbank deposits. Rising relative sea level inhibited input of coarse clastics, and channel depressions filled with muds. A blanket of (hemi)pelagics represents relative high sea level stand.

Fossil Foraminifera from four active mud volcanoes in the Gulf of Mexico

Samples were collected for foraminiferal studies by the Johnson Sea-Link I and II manned submersibles on the Louisiana continental slope. This paper documents that the mud, extruded onto the sea floor from depth by four mud volcanoes, ranges in age from Miocene to Pleistocene based on studies of the planktonic foraminiferal fauna. The vents are in water depths ranging from 300 to 690 m located in Garden Banks Block 382, Green Canyon Blocks 143 and 272, and Mississippi Canyon Block 929. Two mud volcanoes in GB 382 and MC 929 also have rich fossil foraminiferal microfaunas. We suggest that the extrusion of fossil sediments onto the sea floor during the Quaternary is a reasonable explanation for frequent occurrences of displaced fossil microfaunas encountered at depth in wells drilling on the flanks of salt diapirs in the slope environment. Results of this study have important implications for age dating subsurface sediments in bathyal locations.

Latest Quaternary deposition on the outer shelf, northern Gulf of Mexico: Facies and sequence stratigraphy from Main Pass Block 303 shallow core

Sedimentologic, biostratigraphic, and isotopic geochemical data from a nearly continuous 91.4 m core and high-resolution seismic-reflection data from the middle to outer shelf east of the Mississippi River delta document deposition during changes in latest Quaternary sea levels. The data suggest that the shelf edge of the northern Gulf of Mexico is constructed from deltas deposited during falling sea level and lowstands, from sediment deposited in valleys during rising sea level, and from highstand clay. The Main Pass Block 303 core records sedimentation from the late middle Pleistocene (before ca. 135 ka) to the present. A lower delta-front interval was sampled at the core base (78.5–91.4 m). Delta sediment is truncated by a sequence boundary probably eroded during maximum glaciation corresponding to oxygen isotope stage 6 (late Illinoian?) and then modified during the following transgression. Above the sequence boundary is a transgressive shelf sand (77.2–78.5 m), overlain by burrowed, hemipelagic clay (∼63.4–77.2 m). The sand and clay were deposited during postglacial sea-level rise and a highstand. The rise and highstand correspond to oxygen isotope stage 5 and Ericson zone X and part of Ericson zone Y Sangamonian-“Eowisconsinan”). The clay interval from ∼51.8 to 63.4 m depth, in turn, was likely deposited during oxygen isotope stages 3 and 4 during the early and middle Wisconsinan. Overlying, with a transitional contact, is a relatively thick mud and sand (16 to ∼51.8 m) deposited during the subsequent sea-level fall to the maximum late Wisconsinan lowstand (oxygen isotope stage 2). The mud and sand interval correspond to steeply dipping clinoforms on seismic-reflection records. An interpreted sequence boundary, formed during fluvial incision of the shelf during the maximum late Wisconsinan lowstand, separates deltaic sediment from overlying fluvial, bay, and marsh deposits, which fill an incised valley. Thin, regressive delta and shelf facies overlie the valley fill. Sediment above the sequence boundary (above 16 m) was deposited during the rise in sea level from the maximum late Wisconsinan lowstand to the present. Late Wisconsinan deltaic and overlying incised-valley-fill sediment was derived from an ancestral river system that drained the southeastern United States.

Biostratigraphy and Sedimentation Rates of the Mississippi Fan

Paleontologic studies of DSDP Leg 96 cores support several mechanisms for deposition of Quaternary sediments on the Mississippi Fan: 1) density flow, 2) debris flow, and 3) suspension load. The constituents of the faunal assemblages suggest that the sediments were derived from: 1)upper Mississippi Valley—reworked Cretaceous foraminifers and radiolarians, 2)Louisiana continental shelf—displaced shallow water benthic foraminifers, and3Florida continental shelf—displaced carbonate bank assemblage.

Mud Volcanoes in the Gulf of Mexico: A Mechanism for Mixing Sediments of Different Ages in Slope Environments

ABSTRACT The extrusion of older, unconsolidated sediments onto the seafloor during the Quaternary is one explanation for frequent occurrences of displaced fossil microfaunas encountered at depth in wells drilled on the flanks of salt diapirs in the continental slope environment. Samples from mud volcanoes on the Louisiana continental slope were collected in 1992 and 1993 for foraminiferal studies by using the Johnson Sea-Link I & II manned submersibles. Age dating by use of planktonic foraminifera has shown that sediments ranging in age from middle Miocene to Pleistocene are being deposited on the modern seafloor by active mud vents/volcanoes. These sediments, when buried by modern pelagic or more rapidly deposited terrigenous clastics, would be anomalous when encountered in coreholes or exploratory wells. Vents, as shown by seismic sections, are associated with faults which provide the connection to older subsurface fine-grained sediments that are: 1) mobilized by gas and fluids, 2) carried to the surface, and 3) ejected onto the seafloor to form mud volcanoes and fine-grained sediment flows. The mud volcanoes are in water depths ranging from 300 to 690 meters located in Garden Banks Block 382, Green Canyon Blocks 143 and 272, and Mississippi Canyon Block 929. This study is part of an ongoing investigation of the geology around cold vents which support chemosynthetic faunal communities in the Gulf of Mexico. These results have important implications for age dating subsurface sediments encountered in wells and coreholes on the continental slope.