The Organic Reef Developed In The Lower Jiwozhai Formation Of The Middle Devonian In Dushan, Southern Guizhou, China

In southern Guizhou, the middle Devonian Buzhai Reef at Buzhai Village, southern Dushan and the Devonian scetions near Dushan County are typical geological sections in China. However, there is still litter research on the correlations between the shelf margin reef at Buzhai and the platform reefs near Dushan County. A series of clastics, which is found in the middle part of the Buzhai Reef, can be correlated with the Songjiaqiao Formation, a clastic satrata near Dushan County and divide the Buzhai Reef into the Jipao Reef and Jiwozhai Reef, from the bottom up. Unconformably overlaid on the Songjiaqiao Foramtion, The Reef Member of the lower Jiwozhai Formation with abound Stringocephalus is dated as the middle-late Polygnathus varcus zone of the Devonian Givetian. The organic reefs in the lower Jiwozhai Formation can be divided into 7 microfacies: reef base, bafflestone, baffle-boundstone, bedded framestone, agglomerated framestones, collapse breccias, and backreef limestone. The Buzhai Reef, a completely developed shelf margin reef, is composed of bafflestone, baffle-boundstone, bedded framestone, and thick agglomerated framestones. Its base is mainly shell or bioclastic limestone; and the cycle formed by micrite limestone and sparite limestone overlaid on the reef. The platform reefs near Dushan County are incompletely developed and show bedded or lentoid small-reefs with the relatively simple structure. Their bases were mainly deposited in the relatively reducing environment; and they were covered by the sedimentary cycles that are composed of the collapse breccias deposited in fore-reef and the limestones deposited in restricted platform. The Buzhai Reef in the shelf margin reef can be correlated with the platform reefs near Dushan County in sequence stratigraphy. The Reef Member of the lower Jiwozhai Formation, from the I-type sequence boundary on the bottom Jiwozhai Formation to the clastics, constitutes a complete II-type sequence. The reefs are developed above the starved section, in the early highstand systems tract. The bedded or lentoid platform reefs might be influenced by sea-level fluctuations, and the platform was, during maring transgression, a slow-flowed subtidal environment low to favor the growth and development of reefs. In addition, some deformed structures, including small-sized folds and faults, are found in the platform reef near Dushan County. The small-sized folds are mainly recumbent and compact folds; and the small-sized faults are mainly dominated by reverse faults. The characters of the deformed structures in the reef and the normally sedimentary reef base and cap imply that the reef slumped in between post-deposition and pre-diagenesis. The contemporaneous slump structure is possibly related with volcanic activity in the Guangxi-Guizhou Sea during the Devonian Givetian.