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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 48 (1964)

Issue: 4. (April)

First Page: 515

Last Page: 515

Title: Snags in a Fresh-Water Paleoenvironment: ABSTRACT

Author(s): P. Allen

Article Type: Meeting abstract


The Lower Wealden cyclothems (sand^rarrclay) contain all the ingredients for the dangerous game of analogic reasoning. "Basal conglomerates," "seat-earths," etc., (e.g., Jour. Geology, 1962, p. 508, fig. 1) lie in wait for the stratigrapher who, stuffed with pedagogical cycles and formational names, remains blissfully unaware that scientific principles (derived from experiment and observation of modern sedimentation) are more important.

Interpretation of the cyclothems is primitive. We do not know for certain which members are transgressive and which regressive. "Offshore" and "onshore" are difficult directions to define because the same aquatic flora, fauna, and sedimentary facies lay out to sea as on the land. We are not sure whether the cycles are regressive^rarrtransgressive, regressive^rarrregressive, transgressive^rarrregressive, or transgressive^rarrtransgressive. Diachroneity can not be tested because a precise chronology is not available.

Paleoslope and paleosalinity data are inconclusive. Most soils, channels, and current-directional structures turn out to be environmentally ambiguous. Regional sedimentary drift can be determined only from large-scale structures, in the sandy, lower parts of the cyclothems. They include channels, Knight-type scoop-bedding, and textural and compositional "streaming." "Streaming" alone (after trend analysis) yields vectors. In the upper parts of the cyclothems, thin passage beds leading back (via aquatic soils) to clays suggest contradictory interpretations: (1) transgressive offshore sediments, (2) regressive point-bar and backswamp alluvia. Erosional surfaces (cutting ?subaerial soils) separate the upper and lower parts of the cyclothems. The aquatic soils were not eroded before depos tion of the overlying clays (cf. Pennsylvanian cyclothems--or are some of these misinterpreted?). Minor cyclothems, though resembling their hosts, may have a different significance.

Evidence from sedimentology, paleoecology, detrital provenance, basin structure, etc., lacks meaning until integrated. At present the best model is a southward-regressive sandy delta or bar-and-lagoon complex, blanketed rapidly beneath a northward-transgressive sheet of offshore clay.

Ultimate causes of the water-level movements are sought among the marine equivalents of the Wealden. Ephemeral barriers (depositional and/or diastrophic swells?) lay between the two environments. Correspondence of marine and non-marine movements suggests eustatic control.

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