Characteristics and Genesis of reef-bank complexes in deep shelf Facies: A Case Study of Middle–Late Jurassic in the Northern Amu Darya Basin, Central Asia

A large-rimmed carbonate platform was developed in the Amu Darya Basin during the middle–late Jurassic Callovian–Oxfordian period. What distinguishes it from typical carbonate platforms is that a series of reef-bank complexes were extensively developed in the deep shelf sedimentary zone of the basin. However, only a few studies have reported on the classification, characteristics, and genesis of these reef-bank complexes in relatively deep water, greatly limiting the development of deep-water carbonate sedimentology. To address this issue, the types and the genesis of reef-bank complexes in the deep shelf environment have been clarified based on the systematic petrography, seismic sedimentology, and geomorphology study of Callovian–Oxfordian carbonate rocks in the northern Amu Darya Basin during the middle–late Jurassic period. The results show that the reef-bank complexes are widely distributed in the deep shelf environment in the study area with laminar, reticulated, and zonal distributions. The reef-bank complexes include barrier-bonding reef-bank complexes, lime-mud mounds, and granular shoals. The deep shelf environment can be further divided into inner shelf, shelf margin, and shelf slope. The inner shelf and shelf margin have relatively shallow water body and a strong sedimentary hydrodynamic force, mainly developing reticulated reef-bank complexes and laminar granular shoals, whereas the shelf slope mostly developing zonal lime-mud mound deposits in strips. The scale of the reef-bank complexes is mainly controlled by basement paleogeomorphology and water energy. Relatively high-energy reef-bank complex bodies are developed on the seaward side of the paleo-uplift limb with relatively turbulent hydrodynamic conditions, while low-energy lime-mud mounds are mostly developed on the high position of paleo-uplift and landward side. The obtained findings can deepen our understanding of relatively deep-water carbonate sedimentation and enrich the carbonate sedimentation theory.