CARBONIFEROUS CARBON ISOTOPE STRATIGRAPHY OF WESTERN EURAMERICA

The Carboniferous carbon isotope record of marine carbonates in the western United States reveals two important features. The first is a transient +7‰ excursion in the late Kinderhookian and early Osagean which has now been traced through 10 different mountain ranges in the western United States (WY, UT, NV, ID, CA). This event is recognized across Euramerica into the Dinant Basin in Belgium, and Buggisch (2000) has presented documentation from the Canadian Rockies and western Europe. Evidence for enhanced organic carbon burial on a regional scale during the late Kinderhookian is based in part on the recognition of extremely high sedimentation rates in the proximal foreland of central Idaho (Drummond Mine Limestone), and the onset of phosphatic deposition associated with the termination of the excursion in parts of Utah, Idaho and Wyoming (mid-late Osagean Delle Phosphatic Member). Phosphogenesis likely represents the culmination of a sequence of events beginning with high organic matter burial in deep, poorly oxygenated portions of the Antler foreland and the production of a water mass enriched in phosphate that could be upwelled onto the shelf.

The second significant feature is a carbon isotope shift across the Mid-Carboniferous boundary that is much reduced in magnitude in comparison to European (paleo-Tethyan) sections. The two surface oceans, on opposite sides of the colliding Gondwanan and Euramerican land masses, were located in tropical to subtropical latitudes and likely differed in terms of nutrient contents rather than temperature and air-sea exchange factors. Because analogous gateway closure in the Pliocene does not appear to have produced a similar carbon isotope gradient in the surface oceans, it seems likely that increased water mass residence times in the Late Paleozoic epicontinental sea environment was a critical factor.