Palynology and chemostratigraphy of the Cenomanian to Lower Campanian chalk of southern and eastern England
A high-resolution (sub-metre, ~ 100 kyr) study of the palynology (dominantly dinoflagellate cyst) and host-rock (major-element and stable isotope) geochemistry is presented for three Upper Cretaceous Chalk sections from onshore UK. A total of 411 samples from the Banterwick Barn borehole (Berkshire, Cenomanian - Coniacian) and Gun Gardens outcrop (East Sussex, Cenomanian - Turonian) in the Anglo-Paris Basin, and the Trunch borehole (North Norfolk, Cenomanian - Campanian) in the southern North Sea Basin, contained diverse assemblages, including 212 dinocyst and acritarch species. One questionably new genus Cordosphaeridium?, two new species C? swanjartochii and Chatangiella spinosum, one new subspecies Senoniasphaera rotundata alveolata and two morphotypes of Exochosphaeridium bifidum are recorded. Odontochitina diversus is a new name proposed for Odontochitina sp. A of Costa & Davey (1992). The first fully quantitative absolute abundance data for dinocysts from Upper Cretaceous sequences have been produced, and these indicate that stratigraphically significant spatial and temporal trends occur. Palynological data have been calibrated chronostratigraphically by macrofossils and provide the most detailed record to date of dinocyst biostratigraphy for the Upper Cretaceous of NW Europe. Bioevents identified in the Banterwick Barn borehole, and confirmed using additional data from southern English sites, are graphically correlated to comparable events in the Trunch borehole to develop a robust bioevent stratigraphy for onshore UK. This is integrated with dinocyst data from NW Europe and a new dinocyst zonation scheme, consisting of five zones and nine subzones (the latter providing ~ 1 Myr resolution), from the uppermost Cenomanian to lower Campanian is proposed: Litosphaeridium siphoniphorum Zone (? - uppermost Cenomanian); Cyclonephelium membraniphorum Zone (uppermost Cenomanian - Turonian); Senoniasphaera rotundata alveolata Subzone (lower - mid-Turonian); Raetiaedinium truncigerum Subzone (upper Turonian); Oligosphaeridium pulcherrimum Zone (Coniacian); Senoniasphaera rotundata rotundata Subzone (lower Coniacian); Cyclonephelium filoreticulatum Subzone (upper Coniacian); Spinidinium echinoideum Zone (Santonian); Trithyrodinium sp. A Subzone (lower - mid-Santonian); Chatangiella spinosum Subzone (mid- upper Santonian); Rhynchodintopsis aptiana Zone (upper Santonian - lower Campanian); Senoniasphaera protrusa Subzone (upper Santonian - lower Campanian); Thalassiphora? spinosa Subzone and ChatangielIa manumii Subzone (lower Campanian), A new palaeoecological model based on dinocyst assemblage composition, absolute abundance, diversity and environmental proxy data (Gonyaulacacean and peridinioid - gonyaulacoid ratios, stability/predictability ecological concepts, stable-isotopes) is developed. The primary control on dinocyst assemblages are suggested to be sea-level, relative-water depth and sea-surface temperature. Dinocyst assemblages have been compared to sequence stratigraphic interpretations. Salinity variations were probably negligible in the Anglo-Paris and North Sea basins during the Late Cretaceous and variations in lithofacies are not considered to have significantly effected the overall assemblage abundance. Cluster analysis of geochemical and absolute abundance data suggests that the variation in the clastic / carbonate flux had an effect on the abundance of individual species. Species are grouped into three main clusters that correspond to clastic-associated, and carbonate-associated elements, and a facies independent group. The palaeoecological interpretation of these groupings is unclear, but they suggest that major differences in the ecology of cyst-producing dinoflagellates occurred in the Late Cretaceous. Dinocyst and geochemical data from the Cenomanian / Turonian boundary (CTB) at Gun Gardens, indicate that the palaeoenvironmental conditions giving rise to a major change in the flora during the CTB event were a response to normal oceanographic conditions operating during the Late Cretaceous transgression. The migration of taxa toward shallow-water refugia is suggested to be the cause of the apparent extinction, and was not a result of an expanding oxygen-minimum zone, as proposed by previous authors. Major similarities between the major-element geochemical profiles from southern and eastern England are demonstrated, with consistent trends observed for Sr and the molar element / Al ratios of constituents associated with the clastic fraction (i.e. Si, Ti). This chemostratigraphic framework is calibrated by high-resolution dinocyst data, and by stage and macrofossil zone boundaries. Individual elements have limited correlation value, but when all elements are considered together they provide an excellent chemostratigraphic tool. A comparison with published data from the Danish Subbasin, suggests that the chemostratigraphic framework can be applied over distances of> 650 km. Detailed ([delta]sup]13 C and [delta][sup]18 O stable-isotope profiles from the Trunch borehole demonstrate that Northern Province chalk show greater similarities to Gennan rather than southern English sections.