LARGE-SCALE SEISMOGENIC DEFORMATION OF A CARBONATE PLATFORM STRADDLING THE PRECAMBRIAN-CAMBRIAN BOUNDARY, KARATAU RANGE, KAZAKHSTAN

摘要

The Berkuty Member of the Ediacaran Kyrshabakty Formation in the Malyi Karatau Mountains of southern Kazakhstan immediately underlies some of the world's largest sedimentary phosphate deposits of earliest Cambrian age. This 5-30 m thick unit, exposed over. 40 km strike length, is a complex carbonate unit that was formed by regional seismic deformation of semiconsolidated carbonate. Three subdivisions are recognized. The lower unit (Unit 1), 3-5 m thick, is regionally shortened by meter-scale flexural folds, and shows thrusting and imbrication of underlying siliciclastic units; local extension is accommodated by soft-sediment boudinage. The middle unit (Unit 2), 2-5 m thick, consists largely of a megabreccia of angular to subrounded dolomite clasts up to a meter across. The upper unit (Unit 3), up to 3 m thick, consists of microbially laminated dolobindstone, dolorudstone, and flat-pebble conglomerate, cut by syndepositional, low-angle normal faults. Channelization and bank-margin slumping at the top of Unit 3 was followed by karsting, prior to the formation of a high-energy, shallow-water, carbonate-phosphatic coastal environment. We interpret the Berkuty Member as an example of an incompletely and unevenly lithified carbonate platform deformed by at least one large seismic event (Mb greater than approximately 8). Deformation of the lower unit included local sliding, imbrication, and folding, whereas the middle unit was deformed by in-situ disaggregation due to extensive shaking. Regional coseismic or early postseismic uplift of soft sediment, possibly above sea level, led to widespread gravitational collapse, rapid drainage incision, and drainage fill by bank collapse and conglomeratic wedges. While each of the deformational features and stratigraphic elements observed in this unit is not diagnostic of a seismogenic origin, the remarkable lateral extent in outcrop, the wide range of structures related to soft-sediment deformation, their systematic lateral variability, and the internal processrelated stratigraphy demonstrates in an exemplary way the manner in which single soft-sediment deformation events can produce a large array of features over a broad area, largely due to minor rheological differences.