Date of Award:

1994

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

W. David Liddell

Abstract

Holocene sediments from intertidal and shallow subtidal zones in Bahia la Choya, Mexico exhibit significant differences based on their mineralogical content, constituent composition, textural parameters, and foraminiferal assemblages. The intertidal sediments are characterized by low calcium carbonate content, being dominated by quartz, and are moderately well sorted and coarse skewed with a fine mean grain size {Mz = 2.73 phi}. Total numbers {living and dead} of benthonic Foraminifera per ml of sediment are relatively low (12/ml) in the intertidal zones. In contrast, the subtidal sediments are characterized by high calcium carbonate content, being dominated by molluscan shell fragments, and are poorly sorted and nearly symmetrically skewed with a medium mean grain size {Mz = 1.41 phi}. Total numbers of benthonic Foraminifera per ml of sediment are relatively high (52/ml) in the subtidal zone.

The analysis of bioerosion intensity indicates that differences in susceptibility to bioerosion exist not only at the subordinal level but also at the generic level of Foraminifera. Overall rates of test destruction are rapid, apparently due to the combination of biological, chemical, and physical processes.

The experimental determination of test characteristics which correlate with settling or entrainment enables the delineation of foraminiferal morphotypes which are most likely to be transported. Test settling velocity is mostly affected by test size and weight. Movement threshold velocity is, also, mostly affected by test size, weight, and shape, in addition to the nature of the substrates and initial test orientation. Foraminifera from siliciclastic environments exhibit relatively low settling and movement threshold velocities. Thus, taxa from siliciclastic settings are more likely to be transported by currents than are those from carbonate environments, which show a wide range of settling and movement threshold velocities. Such information may be utilized to distinguish between autochthonous and allochthonous microfossil assemblages in the stratigraphic record.

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