Geo- and biodynamic evolution at the northern margin of Gondwana during Late Silurian to Early Devonian time (Hazro Area, SE Turkey)

A global literature survey of 24 worldwide occurring Silurian/Devonian boundary sites and a subsequent organic geochemical evaluation of eight of these locations from Turkey, Morocco, Ukraine, Poland as well as from the stratotype section in the Czech Republic revealed that the organic matter of sediments from the Hazro area (SE Turkey) exhibits a comparatively low thermal stage equivalent to 0.53% Rr. Only minor diagenetic effects as well as the outstanding preservation and low thermal maturity of the organic matter provided a combined sedimentological and organic geochemical approach to Upper Silurian/Lower Devonian sediments from the Dadas Formation (SE Turkey) in order to reconstruct the ancient depositional environment, its changes and the associated biotic response through time. Ludlovian through Lochkovian strata of SE Turkey are well exposed with a thickness of about 140m in the northwestern part of the broadly eroded Hazro Anticline, located 75 kilometres northeast of the city of Diyarbakir. The prevailing fine-grained siliciclastic and calcareous sediments were deposited within a marine shoreline environment that belonged to a pericontinental shelf platform adjacent to the northern margin of Gondwana. Over approximately two million years, an overall oxic environment evolved twice from (1) offshore storm affected mid to outer shelf through (2) offshore transition or inner shelf to (3) a storm and tide affected shoreface zone, indicating two superimposed progradational processes in the range of the 5th order (1000-100.000 years) cyclicity. Sediment provenance studies reveal that the detrital influx was invariably sourced from high silicic parent rocks of the Mardin-Kahta uplift region, which was situated approximately 100 kilometres south of the study area. Depositional processes in the outer shelf environment (1) took place under low energy conditions which led to the accumulation of very fine silt-sized muds settling out of a uniform suspension. Occasionally, the sea floor was affected by storm driven currents, transporting coarse silts and brachiopod shells from a shallower environment into the offshore zone. Relatively high sedimentation rates hindered disturbance of the sea floor by benthic organisms and led to the accumulation of diverse and excellently preserved palynomorphs which originated mainly from phyto- and zooplankton. The occurrence of numerous algal fragments (phycomata of the genus Tasmanites) in the transition from the outer to the inner shelf zone documents a period of high bioproductivity. Due to the gradual drop of relative sea level, the sea floor was subjected to more frequent disruptive storms within the inner shelf zone (2). Partly turbulent depositional conditions caused a decrease in planktonic activity, whereas the infaunal activity (i.e. the benthic and microbial activity) contemporaneously increased. At least for the benthic activity, this trend proceeded further in the storm and tide dominated shoreface zone (3), where traces of sinuous tracks and vertical burrow systems often occur on wavy to lenticular bedding planes. The environment was affected by both storm and tide driven forces as documented by swaley cross stratification, mud drapings and a high mineralogical and textural maturity. The two successive progradational sequences of the 5th order were separated from each other by a period of non-deposition and erosion, which is reflected by the prominent occurrence of several transgressive lag deposits highlighting a sequence boundary. This boundary most likely separates the Silurian from the Devonian strata, whereas the Silurian/Devonian boundary (sensu strictu) seems to be not preserved within the Dadas Formation. The change from a siliciclastic dominated environment in the uppermost Silurian to a mixed calcareous and siliciclastic environment in the lowermost Devonian indicates a variation in provenance due to the relative sea-level change and the subsequent onset of high frequency storm episodes, documented by six autocyclic coarsening and shallowing-upward sequences in the range of 6th order (100-10.000 years) cyclicity.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2004

Author dc:creator

• Kranendonck, Oliver

Contributors dc:contributor

• Kukla, Peter A.

• info:eu-repo/classification/ddc/550
• Südostanatolien
• Silur-Devon-Grenze
• Antiklinale
• Sedimentologie
• Organische Geochemie
• Geowissenschaften
• Sedimentology
• Silurian
• Devonian boundary
• Organic Geochemistry

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-121392%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:59623