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Intrastratal flow in the Cretaceous Gyeokpori Formation (SW South Korea)

100%

Byun U. H. , Van Loon A. J. T. , Kwon Y. K. , Ko K.

Geological Quarterly

2020

tom Vol. 64, No. 3

611--625

Intrastratal flow is a process that is still poorly understood, rarely described and difficult to interpret in ancient rocks. Sediments in the Cretaceous lacustrine Gyeokpori Formation of southwestern South Korea contain some chaotically deformed sandstone layers with deformed mudstone clasts that are ascribed to this process. The interpretation is based on the fact that these layers cannot be explained as a result of subaqueous debris flows or mass transport, whereas the sedimentary context, including the presence of other soft-sediment deformation structures, indicates that intrastratal flow must have been physically possible. The sedimentary setting was a lake in which mainly siliciclastic rocks were deposited, with some interbedded volcaniclastics. The nearby volcanic activity caused seismic shocks that affected the unstable lake margins resulting in the dominance of gravity-flow deposits, but also in a high sedimentation rate that facilitated soft-sediment deformation partly caused by intrastratal flow. This must have happened fairly frequently during a probably limited time-span, as several layers showing traces of intrastratal flow are present within a succession of only <1 m thick. The combined data on the geological setting and our findings regarding the origin of the various soft-sediment deformation structures may help to recognize the traces left by intrastratal flow elsewhere in the geological record.

Depositional environment of the Bagh-e-Vang Formation – the only occurrence of upper Lower Permian in Iran

88%

Yegane B. Y. , Van Loon A. J. T. , Fard S. A. , Farahpour M. M. , Sharahi S. Y.

Geologos

2021

tom Vol. 27, No. 1

1--14

A transgression of the Tethys Ocean occurred in east central Iran, like in other areas of the Tethys Ocean, around the Yakhtashian/Bolorian (regional chronostratigraphical units corresponding with the Artinskian/Kungurian of the Early Permian) transition. This led to the development of a carbonate platform that is represented in the Shirgesht area on the northern part of the Tabas Block by the Bagh-e-Vang Formation, which constitutes the only known sedimentary unit from the late Early Permian in Iran. Field data and thin-section analysis indicate deposition on a carbonate ramp with barriers separating a lagoonal area with intertidal mud flats from the open-marine environment. The overall transgres-sive development is indicated by the presence of open-marine sediments on top of the barrier and lagoonal sediments.

Slope failure on a Cambrian carbonate platform, mass-flow transitions and resulting complex deposit

75%

Wang Z. , Liu J. , Van Loon A. J. T. , Zhu D. , Qin P. , Han Z.

tom Vol. 64, No. 1

3--15

The quiet environment of the carbonate platform in the epeiric sea that existed during the Cambrian between present-day China and Korea was occasionally affected by processes that have hitherto not been described from such a setting. A conglomerate was found in the Middle Cambrian Gushan Formation near Chengouwan (Shandong Province, E China), eroded into the underlying sediments. The conglomerate is explained as a deposit consisting of material that was eroded up-slope when slope-failure took place, resulting in a slump that passed into a high-density debris flow with erosive power that passed, in turn, again into a slump. The slump came to rest when it lost its momentum on a less inclined part of the basin slope. Immediately after deposition, fluidization occurred in the lower part of the slump deposit, as proven by a funnel-shaped water-escape structure and a lateral injection of some metres long of brecciated material.