Richard A. Laws

BENTHIC DIATOMS FROM THE NORTH CAROLINA CONTINENTAL SHELF: INNER AND MID SHELF1

Sediment samples collected by SCUBA divers at two times and three sites on inner and middle continental shelf of North Carolina a well‐developed benthic diatom flora. The most abundant species included the pennate forms Cocconeis disculoides, Diploneis papula, Naticula pullus, Delphineis surrirella, Amphora sublaevis, and A. tenerrima. Biomass of this distinct benthic microflora ranged from 16 to 97 mg chl a + m−2 and is comparable to sediment chlorophyll a values reported in previous studies.

Viable diatoms and chlorophylla in continental slope sediments off Cape Hatteras, North Carolina

Abstract Continental slope sediments off Cape Hatteras, North Carolina, were sampled by ☐ coring in late summer, 1992. The chlorophylla concentrations measured in sediments from 16 sites at depths ranging from 530 to 2003 m averaged 19.9 mg chla m−2, a concentration much higher than observed elsewhere on the eastern U.S. continental slope, indicating high depositional rates for microalgal material. The variability in the chlorophylla values suggests strong environmental heterogeneity at both small and large spatial scales in this slope habitat, probably a consequence of both topography and bioturbation. Viable diatoms were found in sediment samples across the range of depths sampled, and up to 14 cm deep in sediments, indicating high rates of deposition and bioturbation. Bulk sediment samples contained planktonic, tychopelagic and benthic diatoms, indicating that both phytoplankton and benthic microalgae from the continental shelf may be sources of organic matter for these slope sediments.

Late Triassic depositional environments and molluscan associations from west-central Nevada

Abstract The sequence of lithofacies and faunal associations described from the Upper Norian Gabbs Formation in Nevada formed in response to the general Late Triassic regression. This fauna, because of its position at the Triassic—Jurassic boundary, should reflect the Late Triassic mass extinction event. However, the within-habitat species diversity of these five bivalve—ammonite associations is comparable to that of ecologically similar Mesozoic deposit- and suspension-feeding associations. The peculiar absence of gastropods is correlated with an abundance of small heteromorph ammonites suggesting a possible causal relationship.

Paleogene stratigraphy and sea-level history of the North Carolina Coastal Plain: global coastal onlap and tectonics

Abstract Coastal onlap of Paleogene depositional sequences, mapped by stage, and compared to curves of global coastal onlap and eustasy facilitate differentiation of the effects of local uplift and subsidence from eustasy on the Onslow and Albemarle Blocks about the Neuse Hinge in North Carolina. Differential uplift and subsidence of these blocks has controlled the stratal geometries and patterns of relative coastal onlap on each block. Potential mechanisms to produce the uplift and subsidence include episodic flexural deformation resulting from sediment loading in the Salisbury Embayment, and horizontal compressional deformation resulting from plate motion. Danian sea-level rise produced the initial Paleogene siliciclastic sequences (TA1.2 and TA1.3) on both blocks. After deposition of the Danian sequences and prior to Thanetian inundation, the Onslow Block was elevated relative to the Albemarle Block. The Ypresian (TA2.5–2.9, one) is generally restricted to the downdip Albemarle Block, has a depositional updip limit, and represents a major basinward shift in local coastal onlap at a time when global coastal onlap and long-term eustatic curves predict a major landward shift. This suggests that both blocks were uplifted during the Ypresian. Lutetian-Bartonian sequences (TA3.3–TA3.5/3.6) represent the most extensive landward shift in coastal onlap during the Paleogene in North Carolina and show erosional updip limits on both blocks. Differences in the degree of overstepping on the two blocks reflects post-middle Eocene uplift, but greater relative uplift of the Albemarle Block. The Priabonian (TA4.1 and either the TA4.2 or TA4.3) has erosional updip limits on both blocks and represents a significant basinward shift in local coastal onlap. This shift occurs during a time when global coastal onlap and long-term eustasy indicate a landward shift as great as that during the Lutetian-Bartonian. Therefore, both blocks were uplifted, but the distribution of the Priabonian on the Albemarle Block indicates greater relative uplift. In North Carolina, the Rupelian (TA4.4) oversteps the Priabonian on both blocks but not as far as predicted by global coastal onlap and eustasy. Based on the offlapping relationship of the erosional updip limit on both blocks, both areas were uplifted prior to Rupelian deposition. Chattian sediments (TB1.1–1.4, lower part) reflect a downward shift in coastal onlap which is consistent with global coastal onlap and long-term eustatic fall.

Origin and Diagenesis of Middle Eocene Diatomite, Tallahatta Formation, Southwest Alabama

ABSTRACT Scanning electron microscopy/energy dispersive X-ray analysis, X-ray diffractometry, and petrographic study of siliceous mudstone samples from the Tallahatta Formation in Clarke County, Alabama, reveal abundant diatom molds and casts along with original valve fragments. Molds and casts are opal-CT lepispheres, whereas rare valve fragments consist of biogenic opal-A. These are set in a matrix of opal-CT lepispheres and minor authigenic minerals including heulandite, clinoptilolite, pyrite, microquartz, and chalcedony. Minor detrital clay, quartz, feldspar, muscovite, biotite, glauconite, heavy minerals, and calcitic skeletal debris are also present. Whole-rock chemical analyses average: SiO2, 81.9%; Al2O3, 4.9%; CaO, 2.5%; MgO, 0.6%; Na2O, 0.1%; K2O, 0.8%; Fe2O3, 1.6%; MnO, 0.01%; TiO2, 0.2%; P2O5, 0.05%; and LOI, 6.5%. Scanning electron photomicrographs show a high density of whole, large diatom valves. Possible taxa include the open-water forms Coscinodiscus radiatus, Nitzschia, Stephanopyxis, Triceratium, and rare neritic chain-formers such as Paralia and Melosira or Aulacosira. Diatom abundance and paucity of terrigenous mud indicate that the original sediment was a diatom ooze, which probably accumulated in high productivity areas of the outer shelf and slope. Absence of relict volcanic textures, pyrogenic minerals, or diagnostic alteration products supports Wise and Weavers 1973 interpretation of a primary biogenic origin for these deposits. Our studies indicate a progressive diagenetic change from original diatom ooze to cristobalitic mudstone and chert. We recognize the following sequence: 1. blades of opal-CT formed through direct replacement of diatom opal-A along micron-scale solution films; 2. opal-CT blades coalesced to form incipient lepispheres that replaced diatom frustules while maintaining original areolae pattern; 3. detachment and continued growth of lepispheres in intraskeletal pores filled areolae and obliterated the original lattice pattern; and 4. metastable opal-CT recrystallized to microcrystalline and chalcedonic quartz.

A general mathematical model for balanced global isostasy

A general mathematical model of balanced global isostasy is presented that describes the geometrical relationships among atmospheric, oceanic, lithospheric, and asthenospheric components relative to a fixed, external frame of reference in terms of 19 parameters of mass, density, area, and thickness in five basic equations and various corollaries. The model is applicable and necessary for evaluating lithospheric processes that operate on a global scale or that affect globally oriented parameters such as sea level, freeboard, and ocean basin depth. It also provides a means of testing the internal consistency of a given set of mass, density, volume, and area parameters relative to present crustal geometry by showing how well they combine to predict present continental free-board. With modification, the basic model can be used to examine more complex questions involving glacially induced sea level fluctuations and long-term crustal evolution resulting from differential energy flux to Earth, short-term modulation of heat flux from the asthenosphere, and long-term monotonic cooling.