Tectonic controls of Phanerozoic sedimentary rock cycling

Abstract

The cyclic nature of the Phanerozoic sedimentary rock distribution, carbon-sulphur coupling, and material transfer among sedimentary reservoirs appears to be controlled by tectonic factors. The distribution of preserved sedimentary mass in terms of rock mass remaining U. geologic age shows a minimum c . 300-350 Ma ago, which separates two subcycles of erosion and deposition of sedimentary rocks. The older subcycle was interrupted because of the major continental collisions of the Devonian and late Carboniferous. These collisions resulted in a reduction of outcrop areas of rocks of the older cycle relative to their masses, leading to a decline in the probability of destruction and an increase in half-life of these older sediments. A strong correlation exists between the long-term cyclicity in the Phanerozoic global sea level curve and the distribution of carbon and sulphur among their major exogenic reservoirs. This correlation is related to two principal tectonic modes of the Phanerozoic: oscillatory and submergent. It is postulated that the submergent mode of active plate convergence, obduction and subduction of sediments, large ridge volume, and high sea level gave rise to low erosion and sedimentation rates, less restricted environments of carbonate deposition, and relatively high atmospheric CO 2 levels (high temperatures ?), resulting from an increased rate of production of CO 2 from diagenetic and metamorphic reactions at subduction zones. As sea level rose carbon was transferred from the sedimentary reservoir of reduced organic carbon to that of oxidized inorganic carbon in limestones, whereas sulphur moved from the oxidized sulphate reservoir to the reduced sulphide reservoir. As sea level fell, reservoir transfers were opposite to those above culminating in the oscillatory mode of generally elevated continental interiors. These reservoir transfers are consistent with secular changes observed in the distribution of δ 13 C and δ 34 S in Sedimentary materials during the Phanerozoic. Petrographic examination of Phanerozoic oolite formations shows that öoids with preserved calcitic relict textures are characteristic of pre-Carboniferous carbonate rocks, whereas öoids with relict textures indicative of initial aragonite mineralogy are dominant in rocks of younger age. These changes in öoid mineralogy may be interpreted as reflecting changes in CO 2 levels of the ocean-atmosphere system consistent with the above tectonic considerations. Atmospheric CO 2 levels were higher prior to Carboniferous time, favouring formation of calcitic öoids and skeletal parts; after the Carboniferous, CO 2 levels fell and aragonite and Mg-calcites of greater than 8 mol % Mg increased in abundance as precipitates.