Abstract: Primary Bedding Rhythms in Pelagic Calcareous Sediments

M. A. Arthur, W. M. Roggenthen

Rhythmic bedding is common in pelagic limestone sequences of various ages now exposed on land. This bedding generally is expressed as an intercalation of two basic lithologies, shale or marl and limestone. Similar rhythms recently have been noted in pelagic sediments recovered in deep-sea cores. Detailed sedimentologic study of several land-exposed pelagic limestone sequences and integration with a comprehensive paleomagnetic stratigraphy has permitted estimation of the possible causes of the rhythmicity and the length of each rhythm.

Pelagic carbonate rocks of Albian through Paleocene age in the Scaglia basin of northern Italy and of Paleocene age in northern Spain show rhythmic shale or marl limestone couplets from 10 to 50 cm thick. The shale layers range from thin, fissile partings to 10 or 15-cm beds. These variations are related to changes in sedimentation rate, and thickness of each couplet is dependent directly on this parameter. Various features, including bioturbation relations, variations in organic-carbon contents and trace-element contents point to a primary depositional origin for these rhythms. Diagenetic enhancement of the rhythm is common, consisting of stylolitization along shale-carbonate contacts and differential compaction and cementation between calcite-rich shale or marl layers. Several lines of evidence suggest that shale layers represent a major part of the time in each couplet.

Detailed correlation of the magnetic reversal stratigraphy in these sediments with dated sea-floor anomalies permits an estimate of the time of each rhythm within each polarity epoch. These rhythms have an average duration of 80,000 years and probably are a result of a combination of variations in productivity of planktonic organisms and/or rhythmic fluctuations in the intensity of carbonate dissolution coupled with periodic variations of terrigenous influx.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA