The Upper Jurassic Garedu Red Bed Formation of the northern Tabas Block: elucidating Late Cimmerian tectonics in east-Central Iran

Kazem Seyed-Emami,Franz Theodor Fürsich,Mahmoud Reza Majidifard,Markus Wilmsen

doi:10.1007/s00531-021-01988-z

Abstract

The Garedu Red Bed Formation (GRBF) of the northern Tabas Block (Central-East Iranian Microcontinent, CEIM) is a lithologically variable, up to 500-m-thick, predominantly continental unit. It rests gradually or unconformably on marine limestones of the Esfandiar Subgroup (Callovian–Oxfordian) and is assigned to the Kimmeridgian–Tithonian. In the lower part, it consists of pebble- to boulder-sized conglomerates/breccias composed of limestone clasts intercalated with calcareous sandstones, litho-/bioclastic rudstones and lacustrine carbonates. Up-section, sharp-based pebbly sandstones and red silt-/fine-grained sandstones of braided river origin predominate. Palaeocurrent data suggest a principal sediment transport from west to east and a lateral interfingering of the GRBF with marine greenish marls of the Korond Formation at the eastern margin of the Tabas Block. Westwards, the GRBF grades into the playa deposits of the Magu Gypsum Formation. Red colours and common calcretes suggest arid to semi-arid climatic conditions. The onset of Garedu Red Bed deposition indicates a major geodynamic change with the onset of compressive tectonics of the Late Cimmerian Tectonic Event (LCTE), being strongest at the eastern margin of the northern Tabas Block. When traced southwards, the same tectonic event is expressed by extension, indicating a shift in tectonic style along the boundary fault between the Tabas and Lut blocks. The complex Upper Jurassic facies distribution as well as the spatio-temporal changes in tectonic regime along the block-bounding faults are explained by the onset of counterclockwise vertical-axis rotation of the CEIM in the Kimmeridgian. The block boundaries accommodated the rotation by right-lateral strike slip, transpressional in today’s northern and transtensional in today’s southern segments of the block-bounding faults. Rotation occurred within bracketing transcurrent faults and continued into the Early Cretaceous, finally resulting in the opening of narrow oceanic basins encircling the CEIM. Palaeogeographically, the GRBF is part of a suite of red bed formations not only present on the CEIM, but also along the Sanandaj-Sirjan Zone (NW Iran), in northeastern Iran and beyond, indicating inter-regional tectonic instability, uplift and erosion under (semi-)arid climatic conditions across the Jurassic–Cretaceous boundary. Thus, even if our geodynamic model successfully explains Late Jurassic tectonic rotations, fault motions and facies distribution for the CEIM, the basic cause of the LCTE still remains enigmatic.

Highlights

Mesozoic, especially Jurassic rocks are very widespread in east-Central Iran and superbly exposed
Our section corresponds to section B of Ruttner et al (1968), who already noted a pronounced facies and thickness asymmetry in the syncline displayed by the Garedu Red Bed Formation (GRBF): the western limb is more than three-times thicker and characterized by thick conglomerate units in the lower 100–150 m that are missing in the eastern limb
The Garedu Red Bed Formation (GRBF) of the northern Tabas Block represents a lithologically variable, up to 500-m-thick unit combining a complex suite of predominantly continental facies types

Summary

Introduction

Especially Jurassic rocks are very widespread in east-Central Iran and superbly exposed. The area is part of the Iran Plate and consists of the Central Iranian blocks, i.e. the Yazd, Tabas, and Lut blocks (Fig. 1a). The thick Upper Triassic to Jurassic succession of this hitherto geologically largely unknown area was subdivided into several formations that were subsequently merged into two groups, the Upper Triassic to lower Middle Jurassic Shemshak Group and the Middle to Upper Jurassic Magu Group (northern Tabas Block) or Bidou Group (southern Tabas Block), respectively (cf Aghanabati 1977, 1998; Seyed-Emami 1999). On the northern Tabas Block, the LCTE is reflected by the demise and tectonic dissection of the large-scale carbonate system of the Esfandiar Subgroup (Fürsich et al 2003a; Wilmsen et al 2009a, 2010), and the deposition of syntectonic sediments of the Garedu Red Bed Formation (GRBF; Ruttner et al 1968) and its lateral equivalents, the Magu Gypsum Formation (Wilmsen et al 2003). An integrated stratigraphic–sedimentological study of the GRBF of the northern Tabas Block has been carried out and the results are evaluated in the framework of new geodynamic findings

Methods

Results

Discussion

Conclusion