Harrison Sheng

Lower Continental Rise East of the Middle Atlantic States: Predominant Sediment Dispersal Perpendicular to Isobaths

The lower continental rise east of the Middle Atlantic United States consists of a series of megafans and well-defined levee-bounded giant valleys meandering across the surfaces of the fans. This region lies below the southwest-flowing Western Boundary Undercurrent, but morphologic features observed are best explained by dominant long-term, gravity-induced, downslope sediment transport processes. Sediment dispersal by contour currents flowing parallel to isobaths, recently described, clearly has not been a significant factor in shaping this sector of the lower rise.

Mineralogic Composition of Sand-Sized Sediment on the Outer Margin off the Mid-Atlantic States: Assessment of the Influence of the Ancestral Hudson and Other Fluvial Systems

The mineralogic composition of surface and core sand samples from the outer shelf, slope, and rise between Hudson and Norfolk Canyons off the mid-Atlantic states has been determined to evaluate the relative contributions of major drainage areas lying to the north and west. The observed mineralogic uniformity in space and time is largely attributable to the fact that the principal fluvial supply to this sector during the Pleistocene and Holocene Epochs was through river systems, all of which drain some part of a large and geologically variable glaciated region. Further complications in distinguishing specific sources probably result from the extensive migration of the Hudson River across the subaerially exposed continental shelf during various Pleistocene low sea-level stands, which produced considerable homogenizing of shelf sediment prior to their downslope transfer onto the continental rise. Trends of the major light and heavy mineral groups on this outer continental margin show that the ancestral Hudson probably extended at least to the head of Baltimore Canyon. The distribution of a distinctive variety of the mineral axinite suggests that the Hudson may have even exerted its influence as far south as the head of Washington Canyon, or some 140 km farther to the south than previously postulated. Vertical time-related changes in the mineral assemblages in slope and rise cores show a broadly similar pattern throughout the region. Vertical variability is particularly notable in cores on the slope off the Norfolk-Washington Canyon pair, where there appears to be a progressive increase in the influence of a western (Delaware-Susquehanna) supply in recent time. The marked compositional variation of sand with depth in cores from the lower rise is best explained by fluctuations in materials transported from adjacent shelf sources downslope to the southeast (lateral input and transport model) rather than by significant long-term dispersal parallel to isobaths (contour-current transport model).

Heavy minerals and provenance of late quaternary sands, eastern Nile Delta

Abstract Heavy minerals in radiocarbon-dated Nile Delta core sections indicate that, during the past 35,000 years, deposits in this major eastern Mediterranean depocenter have been derived primarily from the Ethiopian Plateau. Northward sediment transport has been largely by the Blue Nile and, to a lesser extent, by the Atbara River. The overall similarity of heavy mineral suites in the different late Quaternary sand layers of the northeastern Delta is primarily a function of mineralogical homogenization resulting from transport processes: 1. (a) masking of White Nile sediments by dominant Blue Nile and Atbara components recycled together north of Khartoum; and 2. (b) reworking of sediments back-and-forth between different fluvio-marine environments in the northern Delta. Study of specific heavy minerals such as the amphiboles which are derived from both central Africa and Ethiopian Plateau serves to more precisely interpret provenance and also to identify and map some distinct lithostratigraphic units. It is necessary to integrate heavy mineral data with other compositional and textural parameters to reliably correlate late Quaternary sand units between in the Nile Delta.

Current-influenced depositional provinces, continental margin off Cape Hatteras, identified by petrologic method

Abstract Analyses of DSRV “Alvin” core samples on the Cape Hatteras margin indicate major textural and compositional changes at depths of about 1000 and well below 2500 m. The distribution patterns of petrologic parameters correlate well with water mass flow and suspended-sediment plumes measured on this margin by other workers. Our study also shows: (a) vigorous erosion and sediment transport at depths of less than 400 m resulting from the NE-trending Gulf Stream flow; (b) deposition, largely planktonic-rich sediment released from the Gulf Stream, on the upper- to mid-slope, to depths of about 800–1200 m; (c) winnowing, resuspension and deposition induced by periodically intensified slope currents on the mid-slope to uppermost rise, between about 1000 and 2500 m; and (d) prevailing deposition on the upper rise proper (below 2500 m), from transport by the SW-trending Western Boundary Undercurrent. Sediments moved by bottom currents have altered the composition and distribution patterns of material transported downslope by offshelf spillover; this mixing of gravity-emplaced and bottom-current-transported sediment obscures depositional boundaries. Moreover, reworking of the seafloor by benthic organisms alters physical properties and changes erodability of surficial sediments by bottom currents. Measurement of current flow above the seafloor and direct observation of the bottom are insufficient to delineate surficial sediment boundaries. Detailed petrologic analyses are needed to recognize the long-term signature of processes and define depositional provinces.

Dispersal of Mediterranean and Suez Bay sediments in the Suez Canal

Abstract This study determines the extent to which sediments of Mediterranean, Suez Bay, and in-situ (authigenic, eroded channel) derivation have been displaced along the Suez Canal. Sediment transport is largely a response to hydrodynamics controlled by markedly different oceanographic conditions at both ends of the channeled byway. Petrology of sand, silt and clay fractions determine distributions of diagnostic tracer minerals. These are used to identify five sediment provinces in the canal which indicate long-term dispersal patterns. Sediments of Mediterranean origin (largely terrigenous from the Nile River) are transported southward to Ballah Bypass, while those of Suez Bay—southern Canal derivation (mixed carbonate and terrigenous) are transported northward to the northern edge of Great Bitter Lake. In-situ derived Bitter Lakes sediments, mostly carbonates, are not transported extensively from the lakes area. Similarly derived Lake Timsah sediment is transported into the short stretches of the canal to the north and south of the lake. The canal south of Little Bitter Lake is a zone of erosion, while the Bitter Lakes are sinks for sediments from several sources.

Trona in Nile Cone Late Quaternary sediments: Probable redepositional origin

Abstract Trona and thenardite, evaporites formed in arid terrestrial settings, are identified in Late Quaternary sediments cored on the outer Egyptian Shelf and the Rosetta Fan of the Nile Cone, eastern Mediterranean. The distinct spatial—temporal distribution of these minerals suggests emplacement by resedimentation rather than an authigenic or artifact origin. Trona probably was eroded from saline deposits on the subaerially exposed continental margin during the last major eustatic low stand when the Nile Delta migrated northward toward the outer shelf (from about 23,000 to 18,000 years B.P.); evaporites then were transported downslope and rapidly buried with fine-grained terrigenous sediment in deep marine environments. Trona in some younger sections (at about 5000 and 2500–3000 years B.P.) may record Late Holocene changes in climate or of paleoceanography, but more likely can be attributed to slumping and downslope displacement of some outershelf evaporite-bearing sections.

Palygorskite as a sediment dispersal tracer in the eastern Mediterranean

Palygorskite is a clay mineral widely distributed in late Quaternary deposits of the eastern Mediterranean, where, locally, it proves an ideal tracer of sediment dispersal. This study shows that the origin of this mineral is not limited to African sources. Cores containing late Pleistocene to recent sediment record the transport of palygorskite from the Plain of Helos on the Peloponnesus to specific deep basins along the Hellenic Arc. Palygorskite, commonly concentrated in the silt size fraction, may be overlooked in studies that focus only on the <2 μm fraction. In this region we find that the detrial palygorskite distribution is partially controlled by size-sorting effects related to gravity-induced transport processes. In consequence, comprehensive compositional analyses should be made both on the silt and clay fractions.