CLIMATE CHANGE, SEA LEVELS, AND EPEIRIC SEA-CONTINENTAL SLOPE OXYGENATION IN THE CAMBRIAN

The potential to reconstruct Cambrian global climate change is suggested by a pronounced latitudinal temperature gradient reflected by shallow-marine lithofacies and biotas on Cambrian continents. For example, Avalon was persistently high latitude, with its platform dominated by shales with thin limestones that primarily appear near the strand line as fossil hash beds or small carbonate mud mounds. Western Gondwana, by comparison, featured an early low-latitude carbonate platform (with evaporites, oolites, thrombolites, and archaeocyathans) that was transformed into a siliciclastic shelf with rapid southerly movement in the late Early Cambrian. Global climate changes are best recorded in successions marginal to continents that did not undergo significant latitudinal translation. Preliminary evidence for Cambrian and later Early Paleozoic global climate change is provided by macroscale changes in color and organic content of slope and, locally, platform siliciclastic mudstone. Mudstone (now often slate) colors in the Taconian allochthons, eastern Laurentia, are a proxy for Early Paleozoic climate, sea level, and oxygenation of the mid-water mass on the continental slope. Black, organic-rich, trace fossil-poor mudstone, deposited under a more intense, thicker dysaerobic slope-water mass with climate amelioration (greenhouse conditions) and reduced oceanic circulation, commonly has bedded limestones that reflect off-shelf transport of active carbonate platforms. Green (and purple and red) mudstones with abundant traces and limited limestones reflect improved mid-water oxygenation, climate mimimum (icehouse conditions), and improved deep-water circulation. Slope–platform correlations equate slope black mudstones with carbonate platform expansion and sea-level rise that sometimes brought dysaerobic water onto the shelf (e.g., late Early Cambrian Sinsk event, terminal Early Cambrian Parker Slate onlap, Middle–Late Cambrian gray and “alum” shales of the Avalon and Baltic continents) and green mudstones with eustatic fall (Hawke Bay event) and shelf sandstone. Persistently and particularly high Middle Cambrian–earliest Ordovician sea levels seem to have led to continued organic-rich mud deposition on the slope, and macroscale black–green mudstone alternations reappear only in the Lower–Middle Ordovician