Abstract

At Brora on the northwestern margin of the Inner Moray Firth Basin, an Upper Bathonian to Middle Oxfordian (Jurassic) sedimentary succession of marine shales, siltstones and sandstones is subdivided into four 4th order transgressive–regressive (T-R) cycles. The middle two of these cycles, B2 and B3, comprise the Callovian Brora Argillaceous Formation which is the subject of this study, integrating foraminiferal, sedimentological and stratigraphical data.

The principal factor acting on the foraminiferal distribution pattern both in cycle B2 and B3 is oxygen supply to the benthic environment. The oxygenation is controlled mainly by the changing water depth during transgressive and regressive episodes. In the studied faunal succession these changes are reflected by two main biofacies trends: 1) Diversity decrease, accompanied by increasing dominance and decreasing proportion of calcareous taxa. This faunal trend reflects increasing stagnation during transgression. 2) Diversity increase, coupled with reduction in dominance and expansion in proportion of calcareous taxa. In typical cases, this development reflects improving oxygenation during regression.

The intervals which include the maximum flooding surface, in both cycle B2 and B3, are typified by significantly increased organic carbon content and strongly reduced faunal diversity, developed in dark shales representing severe oxygen depletion. The transgressive phase of cycle B2 shows a faunal diversity decrease, while that of cycle B3 reveals a diversity expansion in its lower part. The regressive phase of both cycles is characterized by a diversity increase. In addition to these major faunal trends, the distribution of species and genera appears useful for “fingerprinting” individual components of T-R developments.

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