Kenneth J. McMillen

Distribution of living polycystine radiolarians in the Gulf of Mexico and Caribbean Sea, and comparison with the sedimentary record

Abstract Plankton tows and bottom samples from the Gulf of Mexico and Caribbean Sea were examined to study the distribution, ecology, and preservation of modern radiolarians. Abundances of radiolarians from this region, an area of low primary productivity, are low. Diversities are variable in the surface waters; diversity indices decrease with increasing water depth, indicating that fewer niches are occupied by radiolarians in deep water. Living radiolarians are confined to specific water masses, some species ranging between two masses, and can be used as indicators of these water masses in ecologic or paleo-oceanographic studies. The record of radiolarian tests present in the sediment is dissimilar to the record of living radiolarians in the water column, but is similar to the record of empty tests falling through the water column. This indicates that a considerable length of time is involved in the settling of radiolarian tests. It also confirms the validity of ecologic interpretations based on a study of sediments only.

Seismic Facies of Shelf, Slope, and Submarine Fan Environments of the Lewis Shale, Upper Cretaceous, Wyoming

The Maestrichtian Lewis Shale of south-central Wyoming was deposited on the west side of the Sevier foreland basin. Well-log, core, and outcrop data document a vertical succession consisting of a condensed shelf shale overlain by bioturbated, probable turbidite sheet sandstone, deeper water sandy turbidites, and a progradational sandstone-shale interval of turbidite and deltaic deposits. Basin deepening and turbidite deposition were controlled by tectonic subsidence during the Sevier orogeny or early stages of the Laramide orogeny, and was possibly affected by eustatic sea-level rise.

An Electro-Osmotic Knife for Slicing Large Box Cores

ABSTRACT Large box cores can be sliced vertically or horizontally on board ship with a knife that has an electrified metal blade. The knife operates on the principle of electro-osmosis, which reduces drag along the blade during slicing and minimizes distortion of the sediment. The slices are useful for subsampling the core and for studying physical and biogenic sedimentary structures. Making slices helps solve the problems of handling, storing, and shipping the large volumes of sediment recovered in box cores.

Transverse Extension of Guatemala Active Margin: Implications for Accretion: ABSTRACT

The inner trench wall of the Middle America Trench off Guatemala does not show evidence of accretion, based on DSDP Legs 67 and 84. The presence of normal faults on the lower inner trench slope has resulted in various subsidence models for the margin. Fault traces would be expected to trend parallel to the margin (northwest-southeast) if margin subsidence had occurred. Instead, the faults trend north-south and occur in groups of faults downdropped to the east and to the west. The faults do not seem to be related to margin subsidence but rather to previously proposed Cenozoic Central American rifting. The rifting resulted from crustal bending as the Central American block rounded southern Mexico during differential North American/Caribbean plate motion. The rifts, which extend nearly to the trench axis, underlie the San Jose submarine canyon and align with the Guatemala City graben. Possible east-west accommodation zones, between fault zones that dip in opposite directions, exist near the shelf edge. These accommodation zones may have formed along lines of weakness where oceanic crust was previously emplaced into the margin during or prior to the Eocene. These rifts show that compressional and extension features can occur simultaneously with underthrusting on active margins.

Miocene Submarine Fans in Deep Western Gulf of Mexico as Interpreted from Seismic Reflection Profiles: ABSTRACT

Sequences of regularly spaced, seaward-dipping reflections seen on east-west seismic profiles along the lower slope of the western Gulf of Mexico are interpreted to be prograding clinoforms in the distal parts of large submarine-fan complexes. North-south lines define the lobate nature of the fans, some of which are over 50 km across. Some fans appear to be composites of overlapping, individual fan lobes. DSDP hole 90 recovered upper Miocene laminated, silty mud at the top and bottom of one of the prograding sequences, suggesting that any sand in the system must have been deposited farther upslope in the more proximal parts of the fan. Deposition of these sequences may have occurred during a major late Miocene lowering of sea level. The late Miocene fans appear to be part of an overall fining-upward sequence along the lower slope beginning with the middle Miocene sandy turbidites and continuing through deposition of late Miocene silty clay laminites into Pleistocene deposition of hemipelagic and pelagic sediments. The sandy middle Miocene turbidites were derived from the Mexican mainland on the west and correspond to a zone on the seismic profiles characterized by chaotic and discontinuous reflections. These reflection patterns suggest a fan environment more proximal than the younger fans above--perhaps a suprafan environment with fan channels. The overall fining-upward sequence and gradual cessation of turbidite deposition probably were due to the late Tertiary growth of the Mexican Ridges foldbelt, which apparently trapped terrigenous sediment and prevented turbidity currents from reaching the deep sea. End_of_Article - Last_Page 426------------

Radiolarian Ratios and Pleistocene-Holocene Boundary: ABSTRACT

End_Page 1607------------------------------The ratios among general types of radiolarians are useful tools in the correlation of late Quaternary sediments in piston cores from the continental margin off the west coast of Guatemala. Cores from the outer slope and lower inner slope contain practically no calcareous microfossils because of solution, so that methods of using planktonic forams to locate the Pleistocene-Holocene boundary are useless. Upper-slope cores, however, show a down-core increase in the abundance of planktonic forams. Previous workers have correlated a similar increase in foram abundance offshore Oregon with the Pleistocene-Holocene boundary. The ratio of nassellarian (cone-shaped) radiolarians versus spumellarian (spherical) radiolarians decreases abruptly down core to a distinct minimum in cores from the up er slope as well as in cores from deeper water. This radiolarian minimum occurs just below the increase in planktonic foram abundance and makes a good marker for identifying the Pleistocene-Holocene boundary in deeper water cores which contain no forams. The radiolarian ratio minimum (which shows a dominance of spumellarian radiolarians in the late Pleistocene) might be a response to a lowering of sea level, to changes in climate or circulation, or to the core sites being closer to shoreline during times of lowered sea level. The last explanation is supported by a study of radiolarian populations from the south Texas outer continental shelf of the Gulf of Mexico, which shows an increase in spumellarian abundance inshore. This radiolarian ratio is useful not only in locating the Pleistocene-Holocene boundary in noncalcareous sediments, but also might be a tool for qualitatively indicating proximity to shoreline in older samples. End_of_Article - Last_Page 1608------------

Radiolaria--Present and Past Indicators of Distance from Shore, Water Depths, Currents, Water Masses, Upwelling, Eutrophy, and Tectonism: ABSTRACT

Certain living radiolarian species and taxonomic groups identified in plankton tows from North Atlantic, Gulf of Mexico, Caribbean, Pacific, and Antarctic waters are useful biologic indicators of physical oceanographic parameters. Changes in dominance and diversity of these radiolarians may signal: (1) distance from shore or position on shelf; (2) relative depths (e.g., above and below seasonal and permanent thermoclines); (3) direction, strength, and provenance of currents; End_Page 531------------------------------ (4) specific water masses; (5) conditions of preservation; (6) presence and strength of upwelling onto shelves and under boundary currents; (7) eutrophic conditions; and (8) tectonic events (isolating radiolarian populations). These biologic indicators and their related forms are useful in determining similar parameters in the fossil record. Studies of radiolarians from Neogene Deep Sea Drilling Project and onshore samples indicate: (1) relative distance from continents; (2) relative paleodepths; (3) strengths and directions of paleocurrents; (4) presence and degree of paleo-upwellings; (5) origin, development, and presence of paleowater masses; (6) paleo-eutrophic conditions; and (7) paleotectonic activity. These biologic indicators may provide a framework for paleo-oceanographic interpretations of Mesozoic and perhaps even Paleozoic radiolarian-bearing sediments. End_of_Article - Last_Page 532------------

Geometry and Morphology of Crustacean Burrows in Torrey Pines and Bodega Estuaries, California: ABSTRACT

Resin casts were used to document the geometry and morphology of crustacean burrows from two California estuaries. Burrows studies include those of two ghost shrimps (Callianassa californiensis and C. longimana), a fiddler crab (Uca crenulata), and two grapsoid crabs (Pachygrapsus crassipes and Hemigrapsus oregonensis). Further documentation is under way with the use of direct observations and radiographs of ghost shrimp burrowing through layered sediment in aquaria. The Callianassa burrows are in muddy to clean sand found in the lower parts of tidal creeks and on sand flats. Their burrows have a main shaft up to 1 m long with constant diameter (up to 2 cm) except for narrowing produced by excurrent activity either at the surface or between burrow systems, and except for enlarged turn-around nodes commonly present at branches or direction changes. Up to 5 openings were observed per system; they are connected by twos and threes in horizontal to inclined Ys with the junction of the Y up to 15 cm below the surface. The geometry of the main shaft is dependent on species, intertidal position, sediment size, and layering. The burrows have a smooth internal and external morphology. The grapsoid burrows are on and above the banks of tidal creeks in slightly silty clay to slightly muddy sand. They vary from complex shapes, with several layers and entrances in a box-type framework, to a simple U-shape depending on topography, tidal level, and the number of organisms and species per system. Commonly two H. oregonensis, one P. crassipes, and one or more U. crenulata are found using parts of the same burrow system. The numerous entrances allow only lateral passage but internal enlargements permit turning around and passage of individuals. In cross section the burrows are lenticular, and the morphology of the walls is very knobby. Burrow entrances of Uca crenulata are at, or near, higher high water and extend either into a grapsoid system or a simple J-shape (up to 20 m), both of which may have a Y-shaped entrance. The entrance and extremity chamber are about twice the diameter of the knobby, cylindrical shaft which normally has a diameter less than 1 cm. End_of_Article - Last_Page 794------------