The seasonal environment of the Purbeck (Upper Jurassic) fossil forests

• Published in: Palaeogeography, palaeoclimatology, palaeoecology Vol. 48; no. 2-4; pp. 285 - 307
• Main Author: Francis, Jane E.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.12.1984; Elsevier Science
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Stratigraphy; British Isles, England, Dorset; Dorset; Mesozoic; Paleoclimatology; Upper Jurassic; Europe; United Kingdom; Histology; Dendrochronology; Tree rings; Seasonal variations; Seasonal variation; Paleoclimate; London Basin; Fossil wood; Gymnosperm flora; Purbeckian
• ISSN: 0031-0182; 1872-616X
• DOI: 10.1016/0031-0182(84)90049-X

In the basal Purbeck Formation of Dorset a paradoxical association of evaporites and fossil forest vegetation is found, representing well-developed gymnosperm forests which grew on the borders of the shallow, hypersaline Purbeck lagoon which covered southern England during the late Jurassic. The dominant tree was a cheirolepidiaceous conifer which appears from its morphology to have been adapted to growing in a semi-arid environment. The narrow and variable growth rings of the trees indicate that conditions were marginal for tree growth and highly irregular from year to year. Comparison with modern tree-ring data suggests that the Purbeck climate was of Mediterranean type, with warm wet winters when the trees were able to grow but with hot, arid summers suitable for the formation of evaporites. The seasonal nature of the climate is also reflected in adjacent sediments, including a clay containing both fresh-water fossils and evaporites, calcrete crusts in the palaeosols, the nature of the silicification and the presence of seasonal crustaceans in finely laminated shales. Analogous modern environments for both the forests and the seasonal lagoonal sediments can be found in the Mediterranean-type climatic regions of South Australia. This evidence shows that within the “equable”. Jurrasic climate marked seasonal variations affected the whole environment. The seasonal nature of this climate supports recent palaeoclimatic models which propose that such a climate prevailed along mid-latitude continental margins during the Mesozoic.