Lithostratigraphy of the Cromer Knoll Group of the Puffin Area, UK Central North Sea
This study is an evaluation of the lithostratigraphy of the Early Cretaceous Cromer Knoll Group in an area of the UK Central Graben, Central North Sea. These fine grained sediments were deposited in an open marine, clastic-dominated system. Background carbonate deposition became more dominant during periods of reduced clastic input. There were also periods of dysaerobic conditions with brief, but well defined, anoxic events, producing regionally correlateable markers. 13 wells and 650km of 2D and 3D seismic data have been used to construct detailed correlations and isochore maps. The factors governing the deposition of these sediments have been evaluated by the integration of detailed biostratigraphy, lithological and wireline log analysis and seismic interpretation. These factors are thought to have been an interaction between the thermally-sagging basin, the existing tectonic style, the movement of salt at depth and environmental controls such as the presence or absence of polar ice masses and fluctuations in hinterland precipitation. Limited available core information has been used as detailed lithological calibration. The tectonic style is governed by the thermally sagging basin, the boundaries of which were defined by the pre-existing Late Cimmerian and older fault patterns; there has been subsequent inversion within the study area during the Late Cretaceous and Tertiary. The problems of data resolution and scale are discussed, a striking example being the difference between the seismically-derived interpretation of simple onlap with the log-derived interpretation of condensed sequences over an intra-basinal high. Previously un-reported Barremian-aged ammonites and a new species of belemnite of Tethyan affinities have been recovered from the cores. These, together with the nannofossils of Tethyan origins, are presented as evidence for Tethyan incursions into the Boreal realm during the Barremian. The value of detailed biostratigraphy is clearly demonstrated as it constrains the log correlations.