POSSIBLE SEISMIC ORIGIN OF MOLAR TOOTH STRUCTURES IN NEOPROTEROZOIC CARBONATE RAMP DEPOSITS, NORTH CHINA

The Xinmincun Formation forms the uppermost unit of a thick Neoprotero zoic section which accumulated near the east margin of the North China Block and is overlain by two thin nearshore to continental formations below fossiliferous Lower Cambrian sediments. Although tectonically d eformed, sedimentary structures are preserved undeformed on cleavage-p arallel surfaces, and an 80 m section has been reconstructed by correl ation across minor folds and faults in the Golden Stone beach area, 50 km NE of the city of Dalian, southern Liaoning province. The measured section shows 65 m of storm-dominated deposits, consisting of alterna tions of micrites and sharp-based graded intraclastic grainstone beds (tempestites), some with rudaceous, commonly erosional or guttered, ba ses. The top 15 m of section shows three alternations of similar subti dal lithofacies with partly to completely dolomitized peritidal deposi ts (laminated, sometimes fenestral and desiccated, micrite beds, and i ntraclastic rudite and grainstone beds). Tempestite beds become thinne r and less abundant upwards towards a muddy upper shoreface zone. This environment was characterized in part by the occurrence of micrite wi th thin or streaky lamination (probable storm-resuspended sediment), i nterbedded tempestites, numerous erosion surfaces, and evidence of liq uidization and sediment slumping into hollows. Molar tooth structures are widespread in micrite beds of the Xinmincun Formation and are pres ent in lesser abundance in tempestites and liquefied channel-fills. Or iginally the structures were sub-vertical cracks, 1-20 cm long, taperi ng upwards and downwards. Subsequently they became filled with microsp ar cement and buckled rigidly during compaction of surrounding sedimen t during burial. Evidence of repeated episodes of cracking, presence o f brecciated cracks and localization of cracking within beds, together with variable degree of development and variation of preferential ali gnment in plan indicates a mechanical origin. Crack generation may hav e been by seismic surface waves generated by movement along faults def ining either the basin's margins or its internal structure, or possibl y by wave action during storms. A seismic origin for molar tooth struc ture is consistent with other Neoproterozoic occurrences. Their prefer ential occurrence in Precambrian deposits arises from the relative rig idity of micritic sediment at this time related to lack of bioturbatio n, incipient cementation and possibly microbial binding.