Trilobites of Black Cat Mountain

Paleogeographers have formulated a masterful reconstruction of the world 400 million years ago from knowledge of how sediment distributions depended upon latitudinal variations in temperature and other climatic effects. For instance, they found that carbonates usually precipitate in tropical to semi-tropical waters. Terrigenous clastics collect near the mouths of rivers. Pyroclastic dust and debris settle from volcano plumes. Evaporation salt deposits crust on the drying land surface in hot arid conditions. Black organic mud and shale digest and settle in enclosed, asphyxiated waters saturated with anaerobic bacteria. Heckel and Witzke used these kinds of relationships and the placements of these various deposits to reconstruct the world as it may have appeared in the Devonian. They achieved a view that is generally consistent with that derived from records of the Earth’s magnetic field frozen in surface exposures of igneous rock at different times in the planet’s history. These kinds of analyses yield inferences about the latitude and orientation of the North American continent at various times during the Paleozoic Era. From work such as this, come our general view of North American geography and weather conditions during the Devonian Period. The equator passed through Alaska, during the Lower Devonian. The North American continent was canted clockwise about forty-five degrees compared to today. This placed Oklahoma near the west to southwest margin, about latitude thirty-five degrees south of the equator. Carbonate depositional patterns on the continental surface through the Devonian record the changing conditions of inundation during the period. The Appalachian province spread across eastern states from Newfoundland to Alabama and then out across the mid continent to Oklahoma. The Cordilleran Province covered a similar sized stretch from the Canadian Northwest Territories down the Pacific coast into northern Mexico. Large evaporite deposits formed in several regions along the dry equator, possibly the largest in modern northwest Canada where shallow incursions of the sea were prevalent. These deposits came from evaporation of the water, concentrating its salts to the point of precipitation and ultimately dryness. The deposits migrated south and east as Devonian time passed, recording the northward drift of the continent, pushed by tectonic shear forces in the upper realm of the Earth’s mantle. In the Appalachian Province sedimentary deposits were predominantly carbonate, except directly along the northern Atlantic coast where the carbonate was mixed with terrigenous clastic sediments washing in from terrestrial highlands to the east. Interestingly, the carbonate sequences of the Appalachian Province were thinly blanketed by comparison with those found elsewhere. In the mid continent and eastward, they measure in hundreds of feet. In the Cordilleran Province, the Canadian Arctic Islands and in the Old World Realm of Eastern Europe and Russia, they reach hundreds to thousands of feet in thickness.