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This study documents a Liassic example of the long-ranging effects of mass extinction on carbonate systems. Biohistoric constraints inherent in the Liassic carbonate depositional system are deciphered from normal-marine, sub-tidal deposits of the central High Atlas rift basin (Morocco) through ?Hettangian/ Sinemurian to Early Toarcian times. The integration of results from the analysis of lithofacies, depositional geometries, microfacies, macrobenthos, carbonate build-ups, carbon and oxygen stable isotopes, and rare earth element + yttrium distribution patterns allows the intrinsic (or biohistoric) control on the central High Atlas deposits to be separated from extrinsic factors, such as basin development and palaeoclimate. The survival interval in the aftermath of the end-Triassic mass extinction persisted until the Early Sinemurian indicated by a severely depleted carbonate system impoverished in skeletal organisms. A tectonic pulse at the Early to Late Sinemurian boundary interval caused a basin widening with immigration of a marine fauna. However, until the latest Sinemurian (macdonelli Subzone of the raricostatum Zone) the deposits were dominated by filter-feeding benthic heterotrophs (sponges, brachiopods, polychaetes and crinoids). During this stage, primary production within the enlarged basin must have been largely planktonic and there was a net-flux of organic matter to the sea floor (oxygen minimum zone). A regional radiation of organic-walled phytoplankton is inferred to explain the selective success of the filter-feeding community and the occurrence of sponge mounds. Thus, significant effects of the end-Triassic mass extinction are still present during the Late Sinemurian. Through almost the entire Pliensbachian a highly productive, shoal-rimmed carbonate platform existed; it developed subsequent to tectonic reorganization and a marine recirculation event (radiolarian facies, Dd 13 C )1AE1, strongly negative Ce-anomaly). Photosymbiotic sediment producers (mainly large bivalves) now state the success of specialists and environmental equilibrium conditions. In the latest Pliensbachian the climax stage was reached with the development of a coralgal reef-rimmed carbonate platform. The Liassic carbonate depositional system experienced a terminal, multicausal Early Toarcian drowning event during which most of the large bivalves became extinct.

From the Permian onwards, the Gondwana‐derived Iran Plate drifted northward to collide with Eurasia in the Late Triassic, thereby closing the Palaeotethys. This Eo‐Cimmerian Orogeny formed the Cimmeride fold‐and‐thrust belt. The Upper Triassic–Middle Jurassic Shemshak Group of northern Iran is commonly regarded as the Cimmerian foreland molasse. However, our tectono‐stratigraphic analysis of the Shemshak Group resulted in a revised and precisely dated model for the Triassic–Jurassic geodynamic evolution of the Iran Plate: initial Cimmerian collision started in the Carnian with subsequent Late Triassic synorogenic peripheral foreland deposition (flysch, lower Shemshak Group). Subduction shifted south in the Norian (onset of Neotethys subduction below Iran) and slab break‐off around the Triassic–Jurassic boundary caused rapid uplift of the Cimmerides followed by Liassic post‐orogenic molasse (middle Shemshak Group). During the Toarcian–Aalenian (upper Shemshak Group), Neotethys back‐arc rifting formed a deep‐marine basin, which developed into the oceanic South Caspian Basin during the Late Bajocian–Late Jurassic.

The Tabas Block of east-central Iran shows very thick and well-exposed Upper Triassic-Jurassic sequences, which are crucial for the understanding of the Mesozoic evolution of the Iran Plate. The succession is subdivided into major tectonostratigraphic units based on widespread unconformities related to the Cimmerian tectonic events. As elsewhere in Iran, there is a dramatic change from Middle Triassic platform carbonates (Shotori Formation) to the siliciclastic rocks of the Shemshak Group (Norian-Bajocian), reflecting the onset of Eo-Cimmerian deformation in northern Iran. Following the marine sedimentation of the Norian-Rhaetian Nayband Formation, the change to non-marine, coal-bearing siliciclastic rocks (Ab-e-Haji Formation) around the Triassic-Jurassic boundary is related to the main uplift phase of the Cimmerian orogeny. Condensed limestones of the Toarcian-Aalenian Badamu Formation indicate widespread transgression, followed by rapid lateral facies and thickness variations in the succeeding Lower Bajocian Hojedk Formation. This tectonic instability culminated in the middle Bajocian compressionalextensional Mid-Cimmerian event. The resulting Mid-Cimmerian unconformity separates the Shemshak Group from the Upper Bajocian-Upper Jurassic Magu (or Bidou) Group. The succeeding Late Bajocian-Bathonian onlap of the Parvadeh and Baghamshah formations (Baghamshah Subgroup) was caused by increased subsidence of the Tabas Block rather than a eustatic sealevel rise, followed by the development of a large-scale platform-basin carbonate system (Callovian-Kimmeridgian Esfandiar Subgroup). Block faulting starting in the Kimmeridgian (Late Cimmerian event) resulted in the destruction of the carbonate system, which was covered by Kimmeridgian-Tithonian limestone conglomerates, red beds and evaporites (Garedu Subgroup or Ravar Formation). Virtually the same pattern of relative sea-level change, facies development and succession of geodynamic events is recorded from the Late Triassic-Jurassic of northern Iran (Alborz Mountains), suggesting that the Iran Plate behaved as a single structural unit at that time.

The Upper Triassic–lower Middle Jurassic Shemshak Group is a siliciclastic unit, up to 4000 m in thickness, which is widespread across the Iran Plate of northern and central Iran. The group is sandwiched between two major unconformities: the contact with the underlying platform carbonates of the Elikah and Shotori formations is characterized by karstification and bauxite–laterite deposits; the top represents a sharp change from siliciclastic rocks to rocks of a Middle–Upper Jurassic carbonate platform–basin system. In the Alborz Mountains, the group consists of a Triassic and a Jurassic unit, separated by an unconformity, which is in part angular in the northern part of the mountain range and less conspicuous towards the south. Published lithostratigraphic schemes are based on insufficient biostratigraphic and lithological information. Here we present a new lithostratigraphic scheme for the central and eastern Alborz Mountains modified and enlarged from an unpublished report produced in 1976. Two major facies belts, a northern and a southern belt running more or less parallel to the strike of the mountain chain, can be distinguished. In the north, the Triassic part of the group is composed of the comparatively deep-marine Ekrasar Formation with the Galanderud Member (new name) at the base followed by the Laleband Formation, which represents prodelta–delta front environments. Up-section, the latter is replaced by the fluvial–lacustrine, coal-bearing Kalariz Formation. The equivalent Triassic lithostratigraphic unit in the south is the Shahmirzad Formation, redefined here, with the Parvar Member at the base. The formation represents fluvial, coastal plain and shallow- to marginal-marine environments. In the north, the Jurassic part of the group consists exclusively of the Javaherdeh Formation, coarse conglomerates of alluvial fan–braided river origin, which towards the south grades into the Alasht Formation, rocks of fluvial–lacustrine origin with coal. Further south, the Alasht Formation represents intertonguing marginal-marine–flood-plain environments and is followed by the Shirindasht Formation, sandstones and siltstones, indicative of the storm-dominated shelf, and the Fillzamin Formation (new), which is characterized by comparatively deep-marine shales. In the south, the group ends with the Dansirit Formation of deltaic–coastal-plain origin. This lithostratigraphic scheme reflects the tectono-sedimentary evolution of the Shemshak Foreland Basin of the Alborz Mountains where, during the Late Triassic, a relict marine basin in the north became gradually infilled, whereas in the south non-sedimentation and subaerial erosion prevailed and sediments record largely non-marine–marginal-marine conditions. During the early Lias, the basin was filled with erosional debris of the rising Cimmerian Mountain Chain, deposited largely in non-marine environments. During the early Middle Jurassic, in contrast, rapid subsidence in the south resulted in the deepening and subsequent infilling of a marine basin.

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The Mid-Cimmerian tectonic event of Bajocian age can be documented all across the Iran Plate (Alborz Mountains of northern Iran, NE Iran, east-central Iran) and the southern Koppeh Dagh (northeastern Iran). In the Alborz area, the tectonic event consisted of two main pulses. A distinct unconformity (near the Lower–Upper Bajocian boundary) at or near the base of the Dansirit Formation is the sedimentary expression of rapid basin shallowing due to uplift and erosion. Another unconformity is developed in the early Upper Bajocian, close to or at the top of the Dansirit Formation. Locally, it is expressed as an angular unconformity due to block rotation and is overlain by a thin transgressive conglomerate followed by silty marls of the deep-marine Upper Bajocian–Callovian Dalichai Formation. This upper unconformity signals a rapid subsidence pulse. On the Tabas Block of east-central Iran, a single unconformity can be documented that is time-equivalent to those bounding the Dansirit Formation (i.e. ‘mid-Bajocian’). Local folding gives direct evidence of compressional tectonics, and conglomerates indicate subaerial denudation of older Mesozoic or Palaeozoic strata. After a stratigraphic gap, transgressive sediments of ?Late Bajocian–Bathonian age follow, suggesting a fusion of the lower and upper Mid-Cimmerian unconformities in east-central Iran. Along the southern margin of the Koppeh Dagh Mountains (NE Iran), a Late Bajocian subsidence pulse initiated the opening of the strongly subsiding Kashafrud Basin, an eastwards extension of the South Caspian Basin. In all of these areas, one phase of uplift and erosion took place followed by a pronounced pulse of subsidence running counter to trends of the eustatic sea-level curve. Thus, what is generally understood as the Mid-Cimmerian tectonic event is now thought to consist of a tectonic phase, confined to the Bajocian. This phase is explained as the expression of the onset of sea-floor spreading within the South Caspian Basin situated to the north of the present-day Alborz Mountains. This strongly subsiding basin developed close to the Palaeotethys suture during the Toarcian–Aalenian and went through a change from the rifting- to the spreading-stage during the Bajocian. The Mid-Cimmerian event therefore reflects the break-up unconformity of the South Caspian Basin.