Conodont colour alteration patterns in Devonian rocks of the eastern Anti-atlas, Morocco

Abstract

The thermal and burial history of the eastern Anti-Atlas during Late Palaeozoic time is investigated using conodont colour alteration data and the electron spin resonance technique (ESR). Both the conodont colour alteration index (CAI) patterns and the results of ESR investigations indicate that the Devonian rocks of the eastern Anti-Atlas attained their present-day maturation level during Carboniferous time prior to Variscan uplift. The observed CAI values from 3 to 5 reflect heating to temperatures of 120°C and 310°C respectively. The conodont CAI pattern observed in the Devonian rocks is a product of burial and loading by thick Carboniferous cover but the maturation level is too high to be accounted for by burial beneath the known stratigraphical overburden under normal heat flow conditions. The analysis of palaeotemperatures inferred from conodont CAI values shows variation in heat flow during Carboniferous time, with an average geothermal gradient of 30°C/km in the Béchar Basin and higher gradients in the region of the mader and Tafilalt. The style of the observed CAI patterns suggests that depositional trends in the Carboniferous cover, now largely removed by erosion, followed the Devonian subsidence patten.