Variety of shell concentrations in Aitamir Formation and their distribution in sequence stratigraphic framework in Amirabad anticline-Northeast Mashhad

IntoductionDescriptive analysis of shell beds is a useful tool within the context of the sequence stratigraphic model (Holland, 2000, 2001; Parras and Casadío, 2005; Cantalamessa et al., 2005; Di Celma et al., 2005). The formation and distribution of shell concentrations in the stratigraphic section are directly controlled by biogenic production and rate of background sedimentation (Kidwell, 1991a,b). Mid-Cretaceous shallow marine of the Albian-Cenomanian Aitamir Formation from the Kopet-Dagh basin of northeast Iran are rich in shell concentration dominated by bivalves especially Oyster, with minor admixture of gastropod, brachiopod and ammonite. The present paper focuses on Mid-Cretaceous shallow marine Aitamir Formation from the Kopet-Dagh basin with a twofold firstly, to reconstruct the sequence stratigraphic framework of this formation; secondly, to discuss the results in terms of the interpretation of the paleoecology and taphonomy of the shell beds occurring within this formation. The Aitamir Formation was studied bed-by-bed, both sedimentologically and taphonomically. Physical sedimentological studies attributes (thickness, lateral extent, geometry, stratigraphic contacts, association with significant surface, and position within depositional sequence) as well as vertical trends and stacking patterns, were all integrated to define and interpret facies and facies associations. Taphonomical attributes involves taxonomic composition, life habits, orientation, fragmentation, abrasion, disarticulation, encrustation, bioerosion, and microarchitecture (Kidwell, 1991a,b; Fürsich, 1995; Fürsich and Oschmann, 1993).
DiscussionA stratigraphic and sedimentologic study of the entire sedimentary succession indicated that sedimentary facies recur in consistent deepening–shallowing third-order transgressive–regressive patterns that represent a depositional sequence in which forced regressive (FRST) and lowstand system tract (LST) deposits are not present. Based on taphonomic and sedimentological observations, these shell concentrations belong to six assemblages, including; (1) fair weather wave concentration; (2) storm wave concentration; (3) distal tempestite or distal storm flow concentration; (4( primary biogenic or polyspecific concentrations; (5) winnowed concentration; (6) transgressive lag concentration. They represent a range of environments but are mainly shallow water settings. Together with sedimentologic characteristics, these fossiliferous levels are important indicators of facies architecture and, therefore, are useful tools for interpreting sequence stratigraphy. These shell concentrations are found in distinct positions within the depositional sequence: as a transgressive lags at the base of the transgressive systems tract (TST), in the maximum flooding zone (MFZ), and at or close to the top of the highstand systems tract (HST). Characteristic features of concentrations at the base of the TSTs are distinct basal erosional surface (ravinement surface), moderate time-averaging, sorting, a preferred convex-up orientation, fragmented and abraded with a chaotic orientation, and nearly total disarticulation of shells. They are suggestive of an environment (shoreface) in which reworking and local transports were frequent events. These concentrations, which form just seaward of the area of active wave ravinement, are condensed deposits resulting from general conditions of low terrigenous sediment input within high-energy settings and record stratal convergence at the costal onlap (Parras and Casadío, 2005; Cantalamessa et al., 2005; Di Celma et al., 2005). Similar features are shown by concentrations near the tops of the HSTs, except that they do not show ravinement surface and their shells were largely concentrated in lenses and in pavements rather than in beds as in the transgressive lags. Associated sedimentary structures indicate deposition above fair weather wave base in a high-energy environment. Concentrations occurring in the MFZ, in contrast, are autochthonous and highly time-averaged, having accumulated during times of low rates of sedimentation below storm wave base. Therefore, they are useful tools for defining depositional sequences.
ResultsThe present study documents shell concentrations from the Albian-Cenomanian Aitamir Formation (from the Kopet-Dagh basin of northeast Iran) in the background of overall sediment deposition and highlights taphonomic attributes of the shell concentrations and examines their mode of occurrence in sequence stratigraphic framework of the succession. Based on the results of this study, the complexities of shell concentrations can be resolved with the help of sedimentologic and taphonomic attributes of individual skeletal elements and their mode of occurrence in sequence stratigraphic framework. Based on taphonomic and sedimentological observations, shell concentrations are classified as lag, event, composite/multi-event and hiatal types. Overall, the shell concentrations occupy the basal part of the sequence i.e. upper part of the transgressive systems tract (TST), the middle part of the sequence or the maximum flooding zone (MFZ), and close to the top of the highstand systems tract (HST). Lag concentrations are found in the lower part of the sequence as a transgressive lags at the base of the transgressive systems tract (type 6) while winnowed concentrations (type 5), the major contributors in the sequence, occur in upper part of the highstand systems tract and in the middle part of the transgressive systems tract. The hiatal concentrations (type 4; primary biogenic concentrations) are associated with maximum flooding surface while the position of event concentrations (type 1, 2 and 3) is independent of sequence stratigraphic framework.