6533b862fe1ef96bd12c74ab

RESEARCH PRODUCT

Fossil wood in Middle–Upper Jurassic marine sedimentary cycles of France: relations with climate, sea-level dynamics, and carbonate-platform environments

subject

description

Abstract Vegetation, climate, and sea-level oscillations interact in complex ways. Intuition suggests that climatic variations are recorded in geological deposits by both fossil phytoclasts and sediments. Therefore investigation of palaeobotanical remains within a precise sedimentological framework may help to decipher this much-debated interaction. A database of 129 Jurassic (Bathonian–Oxfordian) fossil wood samples from France is used here as a case study. The palaeoenvironment (freshwater swamp, carbonate lagoon, shoreface, argillaceous offshore) and the second-order relative sea-level phases were determined for each datum. There is no correlation between the wood genera distribution and second-order relative sea-level cycles. However, 66% of the fossil wood samples were found within transgressive parts of cycles or at maximum flooding surfaces versus 34% in their regressive counterparts. By contrast, there is a close correlation between wood distribution and palaeoenvironment for Agathoxylon and Brachyoxylon, the two most common genera. Although both genera occur in the four main depositional systems, Agathoxylon is the more common in offshore facies, whereas Brachyoxylon predominates in protected lagoon environments. A model of wood deposition/preservation is proposed. It is shown that (1) the distribution of wood is not controlled by climate but by environmental processes related to relative sea-level changes; (2) the abundance of wood in transgressive 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 by low-diversity Brachyoxylon xylofloras, whereas offshore environments received a more diversified, probably multi-source flora from emergent land areas. Periods of Bathonian, Callovian and Oxfordian subaerial exposure of carbonate platforms may not have been conducive to the establishment of a diverse, stable climax xyloflora.