Mesoproterozoic Soft-sediment Deformational Structures In Central Yunnan, China And Their Geological Implications

Soft-sediment deformational structures and molar-tooth structures developed well inDalongkou Formation, Kunyang Group, which is a succession of marine carbonate rocks,outcropping mainly in Central Yunnan. The types of soft-sediment deformationalstructures include hydroplastic folds, sand dikes, mushroom-like intrusions of sand or silt,dish-like water-escape structures, ball-and-pillow structures, diapers and syn-depositionalfaults. They formed in temporally and stratigraphically constrained horizons bounded byundeformed strata, indicating that they are syn-depositional.Three event beds of soft-sediment deformational structures have been recognized inDalongkou Formation, and all of them are in Sanyuangong Member. The Characteristicsof soft-sediment deformational structures are not same at different sections, even indifferent parts of the same event bed. In the distal area to possible epicenter, thesoft-sediment deformation structures are simple and mainly hydroplastic folds, and themolar-tooth structures are small and simple too. While in the proximal area, thesoft-sediment deformational structures are larger and more complicated, and new types ofstructures appear, such as liquefied sand dikes and mushroom-like intrusions of sand orsilt, indicating that these structures were related to higher energy than that far away fromthe possible epicenter. The characteristics of molar-tooth structures, developed in theproximal area to possible epicenter, change analogously. The above facts lead to theconclusion that soft-sediment deformational structures and the molar-tooth structuresmight be triggered by the same agent—paleoearthquakes. A set of ash tuff and bedded tuff lies right beneath Dalongkou Formation. This set ofvolcanic rock, about 100m thick, belongs to Fuliangpeng Member, and is the first set ofAndesitic volcanic rock of Kunyang Group. This set of ash tuff is coeval with both thechange of the sedimentary facies from terrigenous detritus to carbonate and the beginningof much seismite to develop. In order to date seismites, two tuff samples were collectedrespectively from the bed's lower and uppermost parts. A weighed mean U-Pb age of1032±9Ma were obtained using zircons selected from the samples, which indicate thevolcanic rock is late Mesoproterozoic in age, and the earthquakes developed in lateMesoproterozoic. The result also support that the Mesoproterozoic orogen in KangdianArea may be related to Grenville orogeny.