Geo​logical Survey of Denmark and Greenland Bulletin 34 - 2015

Bulletin

The ammonites of the Middle Jurassic Cranocephalites beds of East Greenland

John H. Callomon, Peter Alsen & Finn Surlyk

The Mesozoic sedimentary succession of eastern Greenland is renowned for its Boreal Jurassic ammonite record. The marine Middle Jurassic deposits of the Jameson Land Basin in central East Greenland are particularly rich in ammonites and have been the subject of much detailed taxonomic and stratigraphic study since the first collections at the beginning of the 20th century. Over the last fifty years, much of the work on the Middle Jurassic ammonite faunas has been undertaken by John H. Callomon, working initially under Lauge Koch’s last expeditions and subsequently with the University of Copenhagen and the Geological Survey of Greenland (GGU, now GEUS).

This bulletin presents the culmination of these endeavours, published after John H. Callomon’s death. It focusses on the so-called Cranocephalites beds of the Ugleelv area of Jameson Land where a detailed ammonite stratigraphy comprising thirty-four faunal horizons is established for the Borealis, Indistinctus and Pompeckji Standard Zones. The Pompeckji Zone is subdivided into four new subzones and four new species are described. This detailed taxonomic and stratigraphic analysis confirms the status of the Jameson Land succession as the key Boreal reference section for this time interval, and furthermore allows a high-resolution study of the evolution of the ammonites which on this time-scale appears to be continuous.

Abstract

Thick successions of marine Middle Jurassic deposits rich in ammonites occur in the Jameson Land Basin in central East Greenland. The fauna of the so-called Cranocephalites beds of this basin, comprising the Borealis–Pompeckji Standard Zones, was until now largely represented by a single collection. This was made by T.M. Harris during a 1927 excursion up the valley of Ugleelv to Katedralen, the type area of Cranocephalites pompeckji, which is the oldest named species of this genus. Revisits to this area in 1994 and 1996 by JHC resulted in a large bed-by-bed collection of Cranocephalites. The number of faunal horizons that could be distinguished grew from the nine previously recognised to thirty-four today. The zonal stratigraphy of the Cranocephalites beds encompasses the Borealis, Indistinctus and Pompeckji Standard Zones. The Pompeckji Zone is subdivided into four new subzones, reflecting four successive basic morphologies of Cranocephalites that should be recognisable more widely and are thus useful for subzonal correlations. The detailed zonation that serves as the secondary standard zonation for the Boreal Province in the Middle Jurassic is thus highly improved. The biostratigraphic resolution obtained here is near the achievable limits. It allows a high-resolution study of the evolution of the ammonites which on this timescale appears to be continuous. Three new species are described: Cranocephalites carolae sp. nov., Cranocephalites intermissus sp. nov. and Cranocephalites episcopalis sp. nov.

An additional new species, Cranocephalites tvaerdalensis sp. nov., is described in the appendix by P. Alsen based on collections from Tværdal on Geographical Society Ø, North-East Greenland. This species is also recorded in Jameson Land.

The authors` addresses

J.H.C. (deceased)
University College London
Gower Street, WC1E 6BT, London, UK.


P.A. (corresponding author)
Geological Survey of Denmark and Greenland
Øster Voldgade 10, DK-1350 Copenhagen, Denmark
Mail: pal@geus.dk


F.S.
Department of Geosciences and Natural Resource Management
University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.