Bituminous shales and oolitic ironstones as indicators of transgressions and regressions

Abstract

Bituminous shales can often be demonstrated to be relatively deep water deposits and tend to occur near the base of transgressive sequences, implying rather rapid rises of sea level, but they probably formed at a variety of depths controlled primarily by regional topographic restrictions on bottom water circulation. Oolitic ironstones, in contrast, frequently mark the late stages of regional regressions, with terrigenous input to the depositional area diminishing in many places as the sea shallowed. Many such regional changes in water depth can be related to eustatic rises and falls of sea level, not only for the NW European Jurassic, from which most examples are taken, but for rocks ranging widely through the Phanerozoic. The evidence from bituminous shales and ironstones supports a Jurassic eustatic model characterised by moderately rapid rises and falls of sea level, separated by longer phases of stillstand.