An integrated study of the calcareous nannofossil response to short-term paleoceanographic changes recorded in the Upper Jurassic Haynesville Formation of East Texas and North Louisiana, USA

The abundance distributions of Upper Jurassic calcareous nannofossils have been investigated in 76 core samples raised from three wells drilled into the Haynesville Formation in East Texas and North Louisiana, USA. Total and relative abundance of nannofossil assemblages is integrated with bulk carbonate carbon and oxygen isotope and sedimentological analyses. The goals of this study are to provide a detailed nannofossil biostratigraphy for the sections, understand the nature of variability among species of the Mesozoic coccolith Family Ellipsagelosphaeraceae and the influence of local paleoceanographic conditions on the assemblage composition and to attempt a palaeoceanographic reconstruction for Haynesville deposition.;Members of the Family Ellipsagelosphaeraceae dominate in all three sections. Generally, Watznaueria barnesae and W. fossacincta dominate the upper part of the sections, whereas Ellipsagelosphaera britannica and Cyclagelosphaera margerelii dominate the lower part. Stephanolithion bigotii, Bucanthus lusitanicus, Staurolithus sp., Polypodorhabdus escaigii, Cyclagelosphaera tubulata, Zeugrabdotus erectus, Calcivascularis cassidyi and Staurolithites lumina are present but rare in abundance throughout. The base of the sections are clearly younger than Oxfordian in age based on the highest occurrence of Bucanthus lusitanicus at the base of the sections and the presence of the Kimmeridgian taxon C. cassidyi which occurs throughout the interval. For the upper part of the sections, an age no younger than Kimmeridgian is indicated by the absence of Conusphaera mexicana mexicana, Conusphaera mexicana minor and Zeugrabdotus embergeri, whose first occurrences (FO) lie above this zone.;This study suggests that changing nutrient and temperature conditions are mainly responsible for the observed nannofossil species variations, and also that the changes in relative abundance variations among the Ellipsagelosphaeracea represent different environmental conditions rather than a preservational effect. Clearly, the different species had differing ecological preferences. The inference drawn from this study is that Watznaueria barnesae and W. fossacincta have been ecologically better adapted for less turbulent, nutrient/salinity-poor, high-oxygen, oligotrophic conditions. Conversely, Ellipsagelosphaera britannica and Cyclagelosphaera margerelii tend to be better adapted to eutrophic to high-mesotrophic environments where high nutrient/salinity flux, and low-oxygen levels prevail. Ellipsagelosphaera communis appears to be able to adapt well to non-extreme, mesotrophic environments.;The succession of calcareous nannofossils in the Haynesville Formation represent a trophic preference continuum from more eutrophic conditions (dominance of E. britannica and C. margerelii) at the base to more oligotrophic conditions (dominance of W. barnesae and W. fossacincta) at the top. The increased abundance of W. barnesae in the upper part of the section may indicate the influence of increasing ocean-surface temperatures.;The general trend of the Haynesville deposition from bottom to top reflects a rising sea-level and the influx of fresh water from terrestial sources and/or marine water from the open ocean.