Hydrodynamics of bivalve shell entrainment and transport, and their applications to paleoenvironmental reconstruction
Abstract
Shell accumulations result from a complex interaction among biological, sedimentary, and eustatic processes. Among the sedimentary processes, hydrodynamic processes have a great influence in shell bed formation, especially in shallow coastal areas. Understanding of the hydrodynamic information contained in shell deposits can help reveal the influence of tides, currents, and waves in ancient deposits, contributing to reconstruction of past depositional environments. This work presents an extensive laboratory study of single bivalve shells' hydrodynamic behavior, complemented with a field verification in Recent environments. This analysis permitted the development of methods useful for interpreting the hydrodynamic information contained in shell beds. The laboratory studies include flow visualization and surface pressure measurements over single bivalve shells of various morphologies. Flow visualization provides kinematic information, showing the characteristics of flow structures around shells. Pressure measurements provide quantitative dynamic information, used to calculate drag and lift forces for shells. The experimental information leads to qualitative and quantitative relationships between shape and hydrodynamic characteristics. A predictive method to characterize any shell in terms of hydrodynamic behavior results from these relationships. Methods were developed to: (1) estimate velocity necessary for shell entrainment; (2) qualitatively evaluate the degree of transportability of shells. Shell attributes which have a positive impact on transportability are: size, convexity, elongation, triangular plan shape, surface roughness (at high Reynolds numbers), and asymmetry. Attributes which have a negative impact on transportability are: weight, and roughness (at low Reynolds numbers). Entrainment velocities may be applied to the study of ancient environments as a method to estimate paleocurrent velocities. The assessment of transportability degree of shells is useful in distinguishing transported from untransported shell deposits, and in explaining fossil distributions in the fossil record. Field experiments in a tidal channel at Bahia La Choya (Sonora, Mexico) were used to verify the application of results to more complex, natural environments. Field observations indicate that transport is dominated by weight, plan shape, and convexity, strongly supporting the laboratory results. This study is unique in using an engineering methodology to solve a geologic problem. Additionally, it constitutes a contribution to fluid dynamic studies over irregular bodies resting on a bottom.
Degree
Ph.D.
Advisors
Wood, Purdue University.
Subject Area
Paleontology|Mechanics|Hydrology
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