Evolution, paleoecology and sequence architecture of an Eocene carbonate ramp, southeast Zagros Basin, Iran

Abstract

We document and discuss the results of detailed fieldwork, facies analysis and the subsequent integration of paleoecological evidence from the Middle Eocene carbonate ramp succession in the southeast Zagros Basin (Jahrum Formation). A combination of a sea-level fall and tectonic and diapiric basement uplift favored the initiation of the Jahrum carbonate platform. The lower portions are affected by pervasive, probably early diagenetic dolomitization, whilst the upper Jahrum consists mainly of limestone. Here, the focus is on the limestone portions of the Jahrum Formation. Based on the abundance, diversity and rapid evolutionary turnover of the alveolinids and nummulitids, the limestone intervals of the Jahrum Formation are interpreted for the upper Middle Eocene (Bartonian). The Jahrum Formation is capped by a major unconformity and overlain by the Lower Oligocene mixed clastic/carbonate Razak Formation. Based on data from field sections, eight facies associations and a series of sub-types have been established, which correspond to inner-, middle-and outer-ramp depositional environments. In their overall context, these data show a southward-dipping inner-ramp-to-basin transect. Towards the Coastal Fars (e.g. Hulur-01 Well) the Jahrum grades laterally into deep-marine Pabdeh foredeep shale units. Based on facies analysis and paleoecological evidence from larger benthic foraminifera, a major transgressive-regressive pattern is recognized in all outcrop sections of the Jahrum. The lowermost stratigraphic units of the formation are here interpreted as a distally steepened ramp. Evidence comes from abundant allochthonous shallow-water facies in the distal, deeper-ramp setting. Shallow-water carbonate clasts were exported towards the basin, a feature that is probably linked to relative sea-level fall control. Furthermore, local to regional basement instabilities by salt diapir-related basement reorganization was arguably of significance. Upsection, evidence is found that the ramp system evolved from a distally steepened to a homoclinal geometry with an overall very gentle slope geometry during the Late Bartonian. The data shown here are significant for those concerned with the Paleogene evolution of the southeast Zagros Basin and provide a well-exposed case example of a Middle Eocene carbonate ramp factory.

Biostratigraphyand Paleoenvironments of Benthic Foraminifera From Lower Part of the Damlouk Member, Western Desert, Iraq

“…Based on that, the examined samples from the studied section were grouped into two facies, each including a different assemblage with different environmental settings. These assemblages and their environmental setting were correlated with other Paleogene Zagros carbonate ramps [17][18][19][20][21]. Similarities were recognized and thus support the interpretation of paleoecological distribution.…”

Paleoclimate and Paleoenvironment Reconstructions from Middle Eocene Successions at Beni-Suef, Egypt: Foraminiferal Assemblages and Geochemical Approaches

“…This conclusion was supported by the dominance of large benthic foraminifera (Nummulites spp., Operculina spp., which need light for living with their symbionts), macrofauna (bivalves and gastropods), the high abundance of the epifaunal calcareous foraminifera (0-9% arenaceous/calcareous ratio), and the occurrence of trace fossils (Thalassinoides spp. ), which prefer inner shelf settings for their biological activities where high oxygen levels, clear and warm conditions are predominant [18][19][20]23,32,38,[99][100][101][102][103] and suggest low organic flux. The occurrence of large benthic foraminifera (Nummulites spp.…”