The stratigraphy and petrology of Middle Jurassic (Ravenscar Group) sediments in cored boreholes from the north Yorkshire coast

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Abstract
SUMMARY: The Scalby Ness [TA 0363 9108] and Castle Head [TA 0520 8892] boreholes drilled on the Yorkshire coast have provided a first opportunity to study the petrography of Ravenscar Group (Aalenian - Bathonian) sediments without the problems associated with the weathering that can occur in the sequences exposed in cliff sections. The cored sequence is a lithologically varied succession dominated by the paralic, siliciclastic sediments of the Cloughton and Scalby formations. This period of paralic sedimentation in the Cleveland Basin was interrupted by three marine incursions, two of which, represented by the limestone-dominated Millepore Bed and Scarborough Formation successions, were cored in the boreholes. The diagenesis of the Ravenscar Group succession is complex, as would be expected in a lithologically heterogeneous sedimentary sequence, with a high proportion of disseminated organic detritus, that has undergone a long burial history. Contrasting original depositional environments appear to have exerted little control on the early diagenesis of the sandstones with, for example, pyrite and siderite present in both the marine and non-marine sequences. Although such early diagenetic cements are common in the succession, many of the authigenic cementation phases present probably began to develop during early diagenesis and continued into the late diagenetic, burial regime; for example, ferroan calcite, calcic ferroan dolomite, kaolinite and quartz cements. The clay mineralogy (particularly the absence of a significant smectite component), moderate depths of burial and comparatively modest geothermal gradients achieved in the basin, suggest that clay mineral transformation reactions played only a partial role in supplying

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