Petrological and geochemical interpretation of Triassic-Jurassic boundary sections from northern Iraq

Triassic-Jurassic sedimentary successions (Baluti and Sarki formations) in northern Iraq record a variety of environmental changes that may be related to global Triassic-Jurassic (Tr/J) boundary events. The diversity of some benthic fauna decreases through the transitional boundary beds. The coastal marine environment of the lower part of the Baluti Formation is followed by shallower tidal flat and supratidal marginal marine environments at the transitional boundary with the Jurassic-age Sarki Formation. The alternating calcareous mudrocks and dolomitic limestones of the transitional succession are overlain by a succession of calcareous mudrocks and dolomicrites that form a dolocrete bed in the latest Triassic. The early Jurassic carbonates (lower part of Sarki Formation) were deposited in a shallow-marine to lagoonal environment. Geochemical evidence supports this interpretation. TOC% increases towards the Tr/J boundary and the lower part of the Sarki Formation. This increase can be interpreted as resulting from the primary precipitation of dolocrete as palaeosol horizons. The variations in the oxygen isotope ratios mainly reflect the facies and diagenetic effects. Th/K ratio is generally constant and shows an increase in the calcareous mudrock beds of the upper part of the Baluti Formation, possibly related to the degradation of K-bearing clay minerals. Low Th/U ratios are due to the depletion in thorium, typical of many marine carbonates rather than to an increase in authigenic uranium. This explanation is also corroborated by the presence of abundant fossils in some of the studied carbonates. Copyright (c) 2013 John Wiley & Sons, Ltd.