Recognising the Taghanic Crisis in the Devonian terrestrial environment and its implications for understanding land-sea interactions

摘要

The late Mid Devonian Eday Marl Formation from the Orcadian Basin, Scotland is a terrestrial equivalent of the marine Devonian Taghanic Crisis. The Eday Marl from Orkney contains a high-resolution archive of climatic change controlled by the relative strength of the seasonal insolation. This includes several distinct and discrete episodes of basin flooding as demonstrated by the deposition of lacustrine laminites, bedded evaporites, marginal sheet flood sands and marine influenced bioturbated sheet sands. These flooding events are intercalated with intense and sustained episodes of aridity indicating that insolation was relatively weak and the monsoon climate was ineffective at drawing in seasonal rainfall. Recognition of these basin flooding events has enabled the definition of three new units (the Wha Taing, Roeberry and Berstane members). The Eday Marl Formation can be correlated offshore and also recognised within marginal facies as an aeolian intercalation. An environmental, hydrological and climatic interpretation is given for the different events recognised in the Eday Marl. This sequence of events, when combined with the palynological age dates, enables a detailed correlation to be made with the marine Tully Formation of New York State. This suggests that regressions forced by climatic aridity are significant for understanding the deposition of the shallow water carbonates of the Tully Formation, in what is otherwise, a clastic depositional system. Comparison with the pelagic marine record shows that the latter, as yet, gives only limited resolution of the Taghanic Crisis. Recognising the Taghanic as a rapid alternation of cool arid and relatively hotter pluvial events provides a unifying explanation for the associated collapse in the terrestrial vegetation and the parallel faunal changes in the marine realm.