Sequence stratigraphy of coastal alluvial plain Westphalian B Coal Measures in Northumberland and the southern North Sea

Abstract

Westphalian B coastal alluvial plain Coal Measures of Northumberland and the southern North Sea are susceptible to stratigraphic base-level fluctuations of various magnitudes which create a hierarchy of flooding surfaces: (1) high magnitude third order maximum flooding surfaces corresponding to the Anthracoceratites vanderbeckei and Anthracoceras aegiranum marine bands defining the top and bottom of the succession, when vast areas of the Westphalian foreland basin were flooded by marine incursions; (2) high frequency fluctuations promoting small-scale fourth order flooding of the entire or large areas (100's km 2) of the alluvial plain resulting in deposition of non-marine bivalve beds or marine bands depending on their position on the third order base level curve; and (3) small-scale localised base level rises due to differential compaction and autocyclic switching of minor crevasse-splays and distributary channels. A regionally extensive third order tectonically induced sequence boundary is also recognised in the southern North Sea, slightly above the maximum stand in stratigraphic base level recorded by the vanderbeckei marine band. This sequence boundary, which represents the lowest point on the background eustatic base level curve between the two maximum flooding surfaces, is overlain by the multilateral, multistorey Caister Sandstone. The lack of significant erosional relief beneath this channel sandstone suggests that accommodation space at this time was being created at a very low rate. The succession above the Caister Sandstone sequence boundary, up to the aegiranum marine band maximum flooding surface, records an increasing rate of stratigraphic base level rise. This equates to a third order transgressive systems tract or sequence set, comprising a number of fourth order sequences which change upwards in response to the background low frequency rise. The increasing rate of base-level rise and increased accommodation space is accompanied by a change in fluvial style to more isolated channels enclosed within floodplain deposits.