Modern Deep Sea Research Articles

Graphoglyptid Burrows in Modern Deep-Sea Sediment

The complex, highly patterned, invertebrate burrow systems known as "graphoglyptids" in ancient sedimentary rocks have now been recovered in box cores of modern deep-sea sediment. Spiroraphe, Cosmoraphe, and Paleodictyon occur as grooves in the tops of washed cores, and they apparently were produced and maintained as horizontal tunnel systems just a few millimeters below the sediment surface. These burrows, which are important as indicators of deepwater sedimentary environments in ancient strata, have been predicted in the modern deep sea but have not been found there until now.

Ichnology and Depositional Environments of Upper Cretaceous Chalks, Southwestern Arkansas (Annona Formation; Chalk Member, Saratoga Formation): ABSTRACT

Both Annona and Saratoga chalks were deposited during transgression of middle to outer-continental-shelf environments. The basal contact of each chalk is a discontinuity surface underlain by marl. At these contacts chalk was piped across the discontinuity into marl through omission and postomission-suite burrows. Taxonomic composition of chalk-filled burrows below the Annona basal contact changes along a 60-km outcrop belt from dominance by Thalassinoides in the northeast to dominance by Zoophycos, Planolites, and Chondrites in the southwest. Such change indicates that this discontinuity surface was formed on a relatively steep slope which dipped southwest. Conversely, taxonomic composition of chalk-filled burrows below the Saratoga basal contact is unchanging (primarily ne species of Thalassinoides) over a 90-km outcrop belt, thus indicating that this discontinuity was formed on a relatively shallowly sloping surface. The basal 0.2 to 2.0 m of each chalk is a condensed bed rich in phosphate nodules and glauconite. Saratoga chalk was deposited at shallower depths, is generally much coarser grained, and contains more terrigenous detritus than Annona chalk. Saratoga quartz silt and sand may have been transported from a more shoreward area by poststorm seaward-directed currents. The trace-fossil assemblage within the Annona, consisting primarily of Planolites, Zoophycos, and Chondrites, is similar to that documented from modern deep-sea carbonate oozes. Bioturbation within the Saratoga chalk is preserved only as mottles. Thixotropic preservation and large size burrows in each unit indicate that during deposition these chalks had very soft substrates and that oxygen content was not limiting. End_of_Article - Last_Page 422------------

A reconnaissance of the sedimentology of lower Silurian mudstones, English Lake District

ABSTRACTPolished slabs and thin sections of lower Silurian graptolitic mudstones and interbedded barren mudstones show three main lithologies are present (a) unfossili‐ferous green mudstones, sometimes with indistinct silt laminae, (b) a similar black pyritic lithology, with rare graptolites, (c) striped mudstones, with prominent carbonaceous and silt laminae, and common graptolites. The abundance of silt laminae suggests that the striped mudstone facies represents the highest energy depositional conditions. Comparison of sedimentary structures with modern deep sea muds suggests this facies is distal turbidite. Graptolite preservation was favoured by rapid burial.

Sedimentation and Tectonics in Early Tertiary Continental Borderland of Central California: ABSTRACT

The most prominent Eocene paleogeographic elements of central California were the Sierran continental landmass on the east and the Salinian continental borderland on the west, both underlain by granitic crust and separated by a trough-shaped, deep-marine area underlain by oceanic crust. These linear elements trended approximately north-south, parallel with the edge of the continent. Erosion of the Sierran area yielded clastic detritus that was transported westward and deposited in successive fluvial, near-shore, shallow-marine, and deep-marine environments. The Salinian borderland, which probably had been detached from the Sierran landmass by pre-Eocene right-lateral faulting along its eastern edge, consisted of irregular island uplands interspersed with deep marine basin . Detritus eroded from these islands was deposited as submarine fans within the borderland basins and on the sea floor on the east and west. Microfossils indicate deposition in basins of bathyal to abyssal depths and unrestricted access to the ocean. The early Tertiary submarine-fan deposits are similar throughout the central Coast Ranges, and contain assemblages of proximal and distal turbidites, deep-sea conglomerates, grain-flow deposits, and hemipelagic shales. They differ from most modern deep-sea fan deposits and typical flysch sequences in that they are very thick, limited in areal extent, and consist mostly of coarse clastic detritus. The conglomerates contain coarse clasts, including boulders, and have very thick, irregular bedding. Proximal sandstones consist of Bouma ae to abc e sequences that are very thickly bedded and amalgamated, poorly graded to ungraded, commonly structureless, and contain large mudstone rip-up clasts and dish structures. Distal sandstones contain more complete Bouma sequences, a greater variety of sedimentary structures, and are graded. The borderland region was tectonically active during sedimentation, so that island source areas and depositional basins formed and changed abruptly. Submarine fan sedimentation was rapid, but the supply of sediment was probably not continuous for long periods of time. Paleocurrent directions are varied, reflecting the irregular distribution of source areas and depositional basins. End_of_Article - Last_Page 797------------

Patterns of Flysch Deposition of Lower Stanley Group (Mississippian), Ouachita Mountains, Oklahoma and Arkansas: ABSTRACT

A southern proximal and a northern distal flysch facies are recognized in lower Stanley strata over an area of 5,000 sq mi in the southern and central Ouachita Mountains. Four distinctive tuffs (25-120 ft thick) interbedded with marine graywackes and shales serve as key units for detailed correlation of 8 sections 500-1,500 ft thick. Sandstone geometry, lithology, sedimentary structures, and ratio of sandstone to shale in the proximal and distal facies are similar to modern deep sea fans and associated basin sediments off the southern California coast. Individual sandstones appear to be discontinuous finger- to fan-shaped bodies on isopach and paleocurrent maps. A gradational contact (10-100 ft) between the Arkansas Novaculite and the overlying lower Stanley strata over most of the Ouachitas records a gradual change from predominantly biological/chemical precipitation to clastic sedimentation. A local high, or highs, in the Ouachita trough is indicated by (1) novaculite conglomerate lenses, (2) an angular unconformity, and (3) thinning of the tuffs and strata between the tuffs. The lower 500 ft of the Stanley Group is predominantly a distal flysch facies of shales and thin (6-in.) siltstones over much of the Ouachitas. The distal facies was superseded by a prograding wedge of proximal flysch facies in the southern Ouachitas. The proximal facies changes laterally to a distal flysch facies in the central Ouachitas. The source area may have been a northeastern extension of the buried Luling overthrust front of Texas. End_of_Article - Last_Page 642------------