Sedimentology and Lithostratigraphy of the Pila Spi Formation in Koi Sanjaq area, NE Iraq; New insight for depositional Environment and Basin Configuration

Abstract

The Pila Spi formation composed of seven lithotypes; carbonates (dolomite and
dolomitic limestone), marl, shale/claystone, red argillaceous mudstone, sandstone,
carbonate breccias and debris flow, which are arranged in repeated cycles of mixed
siliciclastic-calciturbidites in a range of gravity-flow regime in the Koi Sanjaq area.
Sedimentologic and facies evidences suggest developed marine environment for the
Pila Spi Formation. Facies analysis and associated sedimentary structures including
graded beddings decide turbidity and gravity flow regime origin of the rocks.
Marine environment is supported by the identifying glauconite and fossils types,
which is reported here for the first time rather than lagoon environment.
The Pila Spi sequences are grouped into four facies associations confirming
developed marine depositional systems, these are (from bottom to top) channeledslumped
fine and mega carbonate breccias, calciturbidites, faulted and slump
carbonate mega-breccias, slump/slide carbonate mega-breccias facies associations
respectively.
Petrographic analysis of carbonate units of the Pila Spi rocks in Koi Sanjaq area
indicated that they are composed of skeletal grains with subordinate non-skeletal
grains of tuffaceous fragments, chert, chalcedony, volcanic ash, metamorphic and
detrital iron oxides grains, with noticeable grains of glauconite. Varieties of marine
fossils are identified includes planktonic bivalves and benthic forams of cool water,
which support the deeper marine environment.
Microfacies analysis reveals (15) types, these are; green algal boundstone, algal
wackestone dolomudstone, algal dolostone/mudstone, framestone-mudstone, small
benthic foraminiferal wackestone, algal packstone-mudstone, foraminiferal
packstone-mudstone, algal dolostone, fossiliferous grainstone, foraminiferal
packstone, stromatolite framestone, algal boundstone, calci-microbial grainstonepackstone,
and large benthic Foraminiferal bioclasts floatstone. These microfacies
types are grouped in four facies associations representing the depositional
environments of the formation, these are; outer shelf, reef, forereef, reef, backreef.