AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Ichnology of the Early Devonian Jauf Formation in Northern Saudi Arabia
(1) Institute of Geological Sciences, Jagiellonian University, Kraków, Poland.
(2) Geological Technical Services Division, Saudi Aramco, Dhahran, Saudi Arabia.
The Jauf Formation in northern Saudi Arabia embraces a several hundred meters thick succession of mixed siliclastic-carbonate marginal marine and shallow marine deposits dated as late Pragian to late Emsian. Sedimentological logging of two pairs of core holes (JNDL-3, JNDL-4 and BAQA-1, BAQA-2), located about 350 km apart, has supplied original data on bioturbation structures recorded in the Jauf formation. In northern Saudi Arabia, the formation is divided into five members differing in lithofacies. Combined cores form a 270 m thick composite section of the Jauf Formation. This study aims primarily to show the most distinct types of bioturbation structures recorded in the examined cores, their distribution in the succession and relationship to lithofacies, palynofacies, and depositional environments. The investigated deposits display highly variable, lithofacies controlled bioturbation style and intensity. The most intense burrowing occurs in deposits dominated by fine-grained sand and green mud. A tendency of an increase of bioturbation intensity in heteroliths consisting of interbedded very thin sandstone, siltstone and mudstone layers suggest that the totally burrowed beds originally also were heteroliths. Their total burrowing results from slow sedimentation rate, rather high fertility, low salinity and satisfactory aeration of the depositional setting. The ichnofossils most distinct in succession divisions dominated with fine-grained sand and mud which in vertical sections show patterns corresponding to Spirophyton-Zoophycos, Rhizocorallium and Phycodes flabellum were produced by opportunistic organisms adapted for areas strongly influenced by fresh water and land (in brackish water). Their distribution in the succession corresponds with the distribution of Spirophyton, Rhizocorallium and Phycodes flabellum in other areas. The absence or subordinate occurrence of burrows in mudstone to grainstone type limestones results in part from their mass deposition by storm processes. Common interbedding of non burrowed black mudstones and restriction of bioturbation structures to faint sediment mottling, indicate sedimentation in areas hostile for macrobenthos, and particularly for the deep sediment penetration. The boundary between the Jauf and the underlying Tawil formations is distinctively marked by plant root structures.