LATE DEVONIAN EVENTS AS CONSEQUENCES OF GLACIATION: A REVIEW

As has been well demonstrated during the Carboniferous, the best evidence for exact timings of glacial onsets is in lower latitude, continuous successions. Similarly, it is now apparent that Late Devonian carbonate and siliciclastic stacking patterns, lacunae and resedimentation events in Euramerica and Asia reflect short-term sealevel fluctuations within a general framework of second order global transgression. As shown by others in this session, as well as previously published work, there is a profound change in Late Devonian climate, most notably at the end of the Famennian. There are others, and their timings are still debated. However, the end Devonian glacial event is well-documented from a broad paleolatitudinal area (Laurentia and Gondwana, including much of South America and into Africa). Some have argued that its duration was quite short (L. lepidophyta palynozone). Nonetheless, the event was a response to climatic shift, as a response to a significant drop in atmospheric carbon dioxide. Sealevel drops are recorded in several places. In the western USA Cordillera are lacunae, evaporites, and resedimented conodonts within the latest Devonian Three Forks Formation. Subaerial exposure appears to have affected older Devonian units, causing brecciation (because of phreatic zone weathering). In eastern USA are forced regression sands (Berea Sandstone) that were deposited in a mostly euxinic shallow sea, producing black shales (Bedford Shale and correlatives). These units show high TOC, which indicates high primary production as a response to increased dissolved nutrients from tectonic highs. We will have discussions of equivalent black shales and their genesis in this session. In the Czech Republic, atoll carbonates shed breccias (Køtiny Formation). Iron oolites and drowned forests are reported from Libya. And the Yunghsien Formation (and equivalents) at Guilin, China shows short term sealevel fluctuations. Timings of all these events need more precise dating.