Abstract Sedimentary structures in carbonate sands of portions of the Great Bahama Bank are described and interpreted using data from air photographs, visual inspection of the bottom, echo-sounding traverses, and 50 box-cores impregnated with epoxy resin. Surface forms described include: avalanche ripples, with angle-of-repose lee slopes; accretion ripples, with lee slopes less than the angle of repose; flat, swash surfaces; large-scale current lineations on the order of 1000 feet long; bars; and irregular shoals. Strata are classed as avalanche deposits if they are cross-strata dipping at the angle of repose with coarser particles concentrated down their dip; accretion deposits if they are cross-strata less than the angle of repose without down-dip assortment; and sheet deposits if they are flat or gently dipping strata approximately parallel to underlying truncation surfaces From hydrodynamic theory and field observations the conclusion is reached that avalanche deposits form where the current separates from the bed on the lee side of avalanche ripples and other migrating embankments; that accretion deposits form where a traction carpet is moving down the lee side of an accretion ripple or other embankment with a significant component of tangential fluid flow; and that sheet deposits are formed in the upper flow regime on beaches, and on level-bottom areas by rare, high velocity currents. Meta-ripples and cross-ripples are described. Biogenic structures, including biolaminites and bioturbites, dominate deposits in intertidal and sheltered subtidal areas of the shelf-lagoon where bottom traction is small compared to the rate of organic activity.

Abstract An optimum empirical classification is defined as one in which there is exactly one category for each group of samples separated from other groups by discontinuities in the ranges of their observed properties. A statistical scheme for identifying discrete sample groupings (if they exist) in a set of data is developed which gives equal weight to any number of properties, and treats nonhomogeneous properties simultaneously. Essential features of the scheme are (1) representation of each sample as a vector in an n -coordinate system, where n is the number of attributes considered, (2) use of the angle of separation between sample vectors as an inverse measure of similarity, (3) application of factor analysis to determine the minimum number of coordinates necessary to express main features of the data efficiently, and (4) inspection of the resulting vector array for discrete vector clusters. Applied to 200 samples of modern Bahamian carbonate sediments studied by Purdy, this scheme identifies five discrete sample groups (oolite, oolitic, grapestone, coralgal, and lime mud facies) whose discrete character is not apparent if only a small number of attributes are considered simultaneously. The same 200 samples are treated according to Folk’s limestone classification, and the results compared with the optimum classification scheme. Application of Folk’s criteria yields nine named categories belonging to Type I (sparry allochemical limestone) and Type II (microcrystalline allochemical limestone), although it is difficult to make the I–II distinction in many samples. In general, correspondence between the two classifications is good in Type I, and poor in Type II.

Many lines of geological and biological investigation depend upon inferences drawn from a study of biofacies (the total biological characteristics of a sedimentary deposit). Three steps can be recognized in the process of drawing such inferences: analysis, synthesis, and interpretation. Of these, analysis must be considered as fundamental because it involves the primary observations on which synthesis and interpretation depend. The scope and accuracy of our interpretations, therefore, are basically limited by the potentialities of the original observational data. If new dimensions of evolutionary, ecological, and geochronological understanding are to be attained, our observational techniques must be sharpened to extract a maximum of information. At least seven basically different methods of biofacies analysis can be employed: (1) taxonomic analysis, based on enumeration of the kinds of organisms represented in a sample; (2) morphologic analysis, in which observations on organic size and form, not explicit in formal taxonomy, are recorded; (3) frequency-distribution analysis, involving observations on the shape of frequency distributions of biological variates; (4) abundance analysis, in which estimates of absolute or relative individual abundances are obtained; (5) incomplete-specimen analysis, in which consideration is given to broken or disarticulated specimens; (6) compositional analysis, in which information on the mineralogical, chemical, or isotopic composition of skeletal material is utilized; (7) textural analysis, in which field and laboratory observations are made on the arrangement of organisms in a deposit. Some of these techniques (especially those dealing with taxonomic and morphologic analyses) have been widely used, but most of them have not been systematically exploited. Synthesis of this primary information is usually accomplished by bringing together data on available collections and considering the biofacies pattern thus formed in relation to the known ecologic or stratigraphic framework. A variety of synthetic techniques are in widespread use. It is judged that future work will emphasize more strongly (1) comparisons of modern and fossil data, and (2) simultaneous study of lithofacies and biofacies.