Regional stratigraphy of the Zagros fold-thrust belt of Iran and its proforeland evolution

Abstract

The latest Neoproterozoic through Phanerozoic stratigraphy of the Zagros fold-thrust belt of Iran has been revised in the light of recent investigations. The revised stratigraphy consists of four groups of rocks, each composed of a number of unconformity-bounded megasequences representing various tectonosedimentary settings. In the lowest group, ranging in age from latest Precambrian to Devonian(?), the uppermost Neoproterozoic to middle Cambrian rocks constitute a megasequence of evaporites, siliciclastic deposits, and interlayered carbonates, which were deposited in pull-apart basins that developed by the Najd strike-slip fault system. This megasequence is overlain by a second one, Middle to Late Cambrian in age, which consists of shallow, marine siliciclastic and carbonate rocks representing deposition in an epicontinental platform. The overlying shales, siltstones, and partly volcanogenic sandstones of Ordovician, Silurian, and Devonian(?) age are local remnants of stratigraphic units that were extensively eroded during development of several major unconformities. The second group consists of two megasequences, one Permian and the other Triassic, composed of widespread, transgressive basal siliciclastic rocks and overlying evaporitic carbonates of an equatorial, epi-Pangean, very shallow platformal sea. The third group is composed of four megasequences formed of shallow- and deep-water carbonates with some siliciclastic and evaporite deposits, which accumulated on a Neo-Tethyan continental shelf during earliest Jurassic through late Turonian time. The fourth group comprises siliciclastic and carbonate deposits of a largely underfilled, NW- to SE-trending, forward and backward migrating, late Cretaceous to Recent proforeland basin, which has evolved as an integral part of the Zagros orogen. This last group consists of three megasequences (IX, X, and XI) with distinctive lateral and vertical facies variations, which reflect specific tectonic events. Megasequence IX comprises uppermost Turonian to middle Maastrichtian prograding and retrograding siliciclastic and carbonate deposits, whose accumulations reflect emplacement (“obduction”) of ophiolite slivers and subsequent collisional events in the Zagros orogen. Megasequence X consists of uppermost Maastrichtian to upper Eocene siliciclastic and carbonate rocks, which deposited first progradationally in front of the Zagros orogenic wedge with reduced contractional tectonic activity, and then retrogradationally due to intensified thrust stacking in the interior parts of the orogen. Megasequence XI consists of Oligocene and lower Miocene carbonate strata deposited retrogradationally shortly after a period of intensified late Eocene thrust faulting in the deformational wedge, and an overlying succession of upward-coarsening, northeasterly-derived siliciclastic deposits of lower Miocene to Recent age which are composed of erosional byproducts of the southwest-vergent Zagros thrust sheets.