Jay E. Leonard

Evaluation of cold-water carbonates as a possible paleoclimatic indicator

Abstract The common belief currently shared by many geoscientists concerning the climatic interpretation of limestones is that a warm-water environment is essential. This concept is not necessarily true because the rate and extent of terrigenous sediment dilution, rather than water temperature, is the primary factor determining whether or not a limestone forms at nearshore or continental shelf depths. Because carbonate productivity is lowest in cold climates, however, CaCO3 abundance and the thickness of carbonate accumulations tend to be least at high latitudes. In this regard present-day continental shelves and beaches offer a poor model for comparing cold-water and warm-water carbonates because of the generally emergent continental tectonic framework, recent eustatic sea-level changes, and the presence of ice caps at the modern poles. Typically, the influence of climate on non-reef continental shelf and beach environments cannot be clearly distinguished by the presence or absence of major taxonomic groups. Faunal diversity and equitability are more sensitive in this regard. The absence of shelf-depth inorganic carbonate precipitates, micrite envelopes, and peloids may also point to the cold-water origin of a rock. Skeletal mineralogy and oxygen isotopes of certain unrecrystallized carbonates may be good paleoclimatic indicators; however, trace elements and physical-textural attributes of the carbonate fraction are probably temperature insensitive. Previous studies of high-latitude continental shelves have concentrated merely on the abundance of calcareous material and there is seemingly a disproportionate amount of information with respect to low-latitude carbonate studies. Further research on cold-water carbonates may open up new avenues for alternative paleoenvironmental and paleoclimatic interpretations.

STORM INDUCED PERIODICITIES OF SUSPENDED SAND MOVEMENT

The equations describing conservation of mass, momentum and energy in a turbulent free surface flow are derived for a controle volume extending over the whole depth. The effect of the turbulent surface oscillations are discussed but neglected in the following analysis, where the equations are applied to the energy balance in a surf zone wave motion. This leads to results for the wave height variation and the velocity of propagation. The results cannot be reconciled completely with measurements and the concluding discussion is aimed at revealing how the model can be improved.A three-dimensional morphodynamic model of sequential beach changes Is presented. The model Is based on variations in breaker wave power generating a predictable sequence of beach conditions. The spectrum of beach conditions from fully eroded-dissipatlve to fully accreted reflective is characterised by ten beach-stages. Using the breaker wave power to beach-stage relationship the model Is applied to explain temporal, spatial and global variations In beach morphodynamlcs.The agents of initial damage to the dunes are water, which undermines them, and animals (including man) which damage the protective vegetation by grazing or trampling. Of these, man has recently assumed predominant local importance because of the popularity of sea-side holidays and of the land-falls of certain marine engineering works such as oil and gas pipelines and sewage outfalls. The need is therefore increasing for active dune management programmes to ensure that under these accentuated pressures, the coast retain an equilibrium comparable with that delicately balanced equilibrium which obtains naturally at a particular location.

Lithostratigraphy and Source Potential of Keweenawan Rocks in Mid-Continent Rift: ABSTRACT

In the aftermath of Amocos deep test in the Mid-Continent rift, exploration moves slowly forward in this potentially vast hydrocarbon province. Important questions are being asked about the source potential of the 1 billion year-old lacustrine shale known only from its presence in the Lake Superior district. Specifically, is there enough source rock distributed in the rift to generate significant reserves Did maturation occur so long ago that subsequent tectonism and erosion preclude its present existence Did thermal activity associated with rifting destroy the source potential in the rift If not, can the authors determine what portions of the rift are most prospective Although few data are available to answer such questions, petrographic studies coupled with stratigraphic and burial history modeling of the Keweenawan source from Michigan offer important information to the explorationists. Results indicate that significant amounts of source rock are likely available throughout the Mid-Continent rift. Additionally, the rift may be partitioned into oil-prone and gas-prone regions.

Megascale Variability of Sediments on United States Atlantic Continental Shelf and Slope--Multivariate Study: ABSTRACT

The Atlantic continental shelf of the United States provides an excellent laboratory for application of statistical techniques designed to augment the megascale distribution of spatially oriented geologic data. Multivariate methods have been applied to data from over 2,000 surface-sediment grab samples in a baseline study conceived both to discern regional trends of textural and mineralogic parameters and to evaluate the relative usefulness of these statistical strategies. The data include textural information on mean grain size, sorting, skewness, and various mineralogic constituents, whereas the statistical techniques employed include cluster, ordination, and trend surface analysis. Distinct sedimentary facies were first determined by Q-mode cluster analysis, based on the similarity among individual samples utilizing Sorensons coefficient as the similarity index. To interpret the relations among sedimentary facies, the clusters were plotted in n-dimensional space by Q-mode ordination, employing a highest dissimilarity criterion. The method suggests, for example, that the sediments from the shelf south of Cape Hatteras can be classified on the basis of water depth and calcium carbonate. The sedimentary facies include: a low CaCO3 inner-shelf facies, an outer-shelf facies with variable CaCO3 content, a shelf-break facies, an biogenic upper-slope facies, and a lower-slope facies. Plotting the spatial distribution of these clusters shows the tr nd toward low CaCO3 content in sediments on the shelf off Georgia, thereby emphasizing the inverse relation between clastic influx and CaCO3 content. As would be expected, trend surface analysis ignores local variations in sedimentary parameters in favor of extracting the regional gradient. The trend from fine sand nearshore to coarser sand on the shelf below wave End_Page 481------------------------------ base is consistent. Strong regional trends are also present for the calcium carbonate fraction and the aragonite/calcite ratio on the shelf south of Cape Hatteras. The existence of a nearshore zone of present sediment reworking, longshore clastic transport, and skeletal comminution is implied by the low CaCO3 values along the coastline. Generally, the combination of cluster, ordination, and trend surface analysis proves to be an excellent strategy for the extraction of sedimentary trends, particularly where local variations obscure the regional gradient. End_of_Article - Last_Page 482------------