Distribution of Small Scale Sedimentary Cycles Throughout Several Selected Basins

Authors: Attila ÇİNER

Abstract: Sedimentary cyclicity of different orders of magnitude is known to occur in several environments going from fluvial to deep sea, especially in carbonates but also in siliciclastic systems. In this paper the distribution of small scale sedimentary cycles in different basins has been studied through some selected examples. Here the smallest cyclic unit is called an elementary unit and defined as a shallowing or deepening upward unit several meters to several tens of meters thick. An elementary unit is closely related to environment and therefore for each sedimentary environment one or several types of elementary unit can be described. A higher order of cyclicity named elementary sequence made-up of the succession of genetically related elementary units is also recognized. In the alluvial plain, elementary units show a fining upward tendency starting with an erosive channel filled with sandstones and conglomerates and overlain by flood plain mudstones as is the case in the alluvial plains of Beldede and Karayün fluvial systems in Haymana and Sivas Tertiary Basins. On the contrary, shallowing upward cycles seem to control the development of deltaic successions. Elementary units embedded in elementary sequences are very common in Beldede braid delta (Haymana Basin) and Roda tidal delta (NE Spain). Shallowing upward cycles are also very common in the carbonate platforms. Nummulitic Çayraz and Alaman platforms (Haymana and Sivas Basins) are good examples where nummulite banks alternate with shelf lagoonal mudstones. The cyclic pattern is also recognizable in the basin plain. In the examples given from Vesc (SW France) few meters thick pelagic-hemipelagic bedding rhythms are very repetitive. Several types of elementary units and elementary sequences are also identified in the Yamak submarine fan (Haymana Basin). The processes governing the small scale cyclicities may have an autocyclic or allocyclic origin. Autocylic ones result from local changes in mechanical sedimentation such as the shifting of a deltaic lobe through time. On the other hand, allocyclic ones are the product of external driving mechanisms such as the rythmical change in climate, sea level and tectonic regime. Glacio-eustatic sea level changes is the most powerful mechanism for generating rapid sea level fluctuations and most of the small scale cyclicities. However, in tectonically active basins such as Haymana and Sivas Basins tectonic forces as well may create similar fluctuations in sea level and cause cyclicity.

Keywords: