SYNSEDIMENTARY COLLAPSE OF PORTIONS OF THE LOWER BLOMIDON FORMATION (LATE TRIASSIC), FUNDY RIFT BASIN, NOVA-SCOTIA

A chaotic mudstone unit within the lower Blomidon Formation (Late Tria ssic) has been traced for 35 km in the Mesozoic Fundy rift basin of No va Scotia. This unit is characterized by highly disrupted bedding that is commonly cut by small (<0.5 m) domino-style synsedimentary normal faults, downward movement of material, geopetal structures, variable t hickness, and an irregular, partially faulted contact with the overlyi ng unit. The chaotic unit is locally overlain by a fluvial sandstone, which is overlain conformably by mudstone. Although the thickness of t he sandstone is highly variable, the overlying mudstone unit exhibits only gentle regional dip. The sandstone unit exhibits numerous soft-se diment deformation features, including dewatering structures, convolut ed bedding, kink bands, and convergent fault fans. The frequency and i ntensity of these features increase dramatically above low points at t he base of the sandstone unit. These stratigraphic relations suggest b uried interstratal karst, the subsurface dissolution of evaporites bou nded by insoluble sediments. We infer that the chaotic unit was formed by subsidence and collapse resulting from the dissolution of an evapo rite bed or evaporite-rich unit by groundwater, producing dewatering a nd synsedimentary deformation structures in the overlying sandstone un it, which infilled surface depressions resulting from collapse. In coe val Moroccan rift basins, facies similar to the Blomidon Formation are associated with halite and gypsum beds. The regional extent of the ch aotic unit indicates a marked period of desiccation of a playa lake of the appropriate water chemistry. The sedimentary features described h ere may be useful for inferring the former existence of evaporites or evaporite-rich units in predominantly elastic terrestrial environments .