FOSSIL WOOD IN MIDDLE-UPPER JURASSIC MARINE SEDIMENTARY CYCLES OF FRANCE - RELATIONS WITH CLIMATE, SEA-LEVEL DYNAMICS, AND CARBONATE-PLATFORM ENVIRONMENTS

Vegetation, climate, and sea-level oscillations interact in complex wa ys. Intuition suggests that climatic variations are recorded in geolog ical deposits by both fossil phytoclasts and sediments. Therefore inve stigation of palaeobotanical remains within a precise sedimentological framework may help to decipher this much-debated interaction. A datab ase of 129 Jurassic (Bathonian-Oxfordian) fossil wood samples from Fra nce is used here as a case study. The palaeoenvironment (freshwater sw amp, carbonate lagoon, shoreface, argillaceous offshore) and the secon d-order relative sea-level phases were determined for each datum. Ther e is no correlation between the wood genera distribution and second-or der relative sea-level cycles. However, 66% of the fossil wood samples were found within transgressive parts of cycles or at maximum floodin g surfaces versus 34% in their regressive counterparts. By contrast, t here is a close correlation between wood distribution and palaeoenviro nment for Agathoxylon and Brachyoxylon, the two most common genera. Al though both genera occur in the four main depositional systems, Agatho xylon is the more common in offshore facies, whereas Brachyoxylon pred ominates in protected lagoon environments. A model of wood deposition/ preservation is proposed. It is shown that (1) the distribution of woo d is not controlled by climate but by environmental processes related to relative sea-level changes; (2) the abundance of wood in transgress ive deposits may be related to ravinement of previously emerged areas and/or to the high preservational potential associated with increased accommodation space; (3) carbonate platforms were probably inhabited b y low-diversity Brachyoxylon xylofloras, whereas offshore environments received a more diversified, probably multi-source flora from emergen t land areas. Periods of Bathonian, Callovian and Oxfordian subaerial exposure of carbonate platforms may not have been conducive to the est ablishment of a diverse, stable climax xyloflora. (C) 1998 Elsevier Sc ience B.V. All rights reserved.