High resolution ammonite biostratigraphy and biochronology of the Aalenian and Bajocian stages (Middle Jurassic) of Dorset and Somerset, UK
Student thesis: Doctoral Thesis
The Inferior Oolite Formation of southern England is one of the foundations and testing grounds for the biostratigraphy of the Aalenian, Bajocian and lowermost Bathonian stages of the Middle Jurassic. The successions are highly discontinuous both vertically and laterally, comprising thin, lenticular deposits initially seemingly of limited use for standard chronostratigraphy. The beds are highly fossiliferous and are the source of many nominal genera and species of the classical literature introduced by S.S. Buckman. Publications herein represent a lifetime’s work and incorporate research that focuses on the re-exposure, collecting and recording of the stratigraphical succession of many historical locations. This thesis reexamines, refines and advances the understanding of the biostratigraphy of the Inferior Oolite ammonite assemblages of Dorset and Somerset and is an important resource for comparison with many areas worldwide and for which this author provides commentary. Critical assessment is made of new, large, in situ, collections of well-preserved specimens made by the author, supported by data and evaluation against types in several museums. New taxa are erected accordingly. Specimens have been prepared using state of the art methods that allow evaluation of the complete shells, stages of growth and sexual dimorphism. The precise levels from which almost all of Buckman’s Inferior Oolite specimens came is now known and this has enabled the variability of the fauna at discrete levels to be assessed, and has eliminated many ambiguities in the literature, to select those names that are useful and to decide which must be abandoned. Interpretation is made of patterns of ammonite migration, provincialism and Middle Jurassic biogeography. Distinguishable biohorizon lists present the biostratigraphy of the ammonites alongside correlation charts. Presently (2017), in the British Aalenian, there are 18. In the Lower Bajocian, there are 30 and the Upper Bajocian has 11 with 3 faunal horizons identified that require further study. Only the earliest Bathonian strata are considered and 3 horizons are recognised to date. It is therefore possible to discriminate 62 biohorizons in Dorset and Somerset for strata of the Aalenian-lowest Bathonian.