engleski

Planktonic Foraminiferal Turnover at the Eocenel0ligocene Boundary (Palmyra Region, Syria)

A detailed micropaleontological investigation was performed on planktonic foraminiferal assemblages, spanning the interval from Zone P15 (Late Eocene) to Zone P22 (Late Oligocene) in three deep exploration wells (Jihar-1, Jihar-4 and Jihar-5) in Palmyra region in Syria. As all analyses have been done from drill-cuttings, biostratigraphical zonal boundaries are based on the last occurrence of the index taxa. Late Eocene sediments are composed of argillaceous limestones of wackestone type, sporadically silicified. Oligocene sediments consist of marls and argillaceous limestones, wackestone and wackestone/packstone types with occasional occurrences of calcareous marls. The changes of planktonic foraminiferal assemblages indicate a general cooling trend and eutrophication of the euphotic zone beriween Middle Eocene and Oligocene time. Highly diversified, subtropical to warm fauna, where K-mode life strategy predominated, were gradually replaced by low diversified assemblages with dominance of cooler r-strategists taxa. The observed extinctions began with the onset of global cooling in the middle Eocene, and culminated near the Eocene-Oligocene boundary. Changes in composition of planktonic foraminiferal assemblages occurred at the Middle/Upper Eocene boundary (Zones P14 and P15) are marked by the extinction of muricate forms, except small acarinids as Acarinina medizzai TOUMARKINE & BOLLI. In the Upper Eocene of the investigated wells some new species of the planktonic foraminifera made their first appearances, such as Tenuitella gemma (JENKINS), Tenuitellinata angustiumbilicata (BOLLI), Turborotalia coccoaensis (CUSHMAN), "Globigerina" venezuelana HEDBERG and Globoturborotalita ouachitaensis (HOWE & WALLACE). Beside these, Globigerinatheka index (FINLAY), Turborotalia cerroazzulensis (COLE), T. pomeroli (TOUMARKINE & BOLLI), Subotina eocaena (GUMBEL), Pseudohastigerina micra (COLE) and some other species have been found. Extinction of the Hantkenina alabamensis CUSHMAN signify the top of the Zone P15. Most of the previously mentioned species (with the exception of the H. alabamensis) exceeded in Zone P16/17. Furthermore, in this interval Turborotalia ampliapertura (BOLLI), Globigerina praebulloides BLOW, "Globigerina " tapuriensis BLOW & BANNER and Paragloborotalia nana (BOLLI) have been determined. The Eocene/Oligocene boundary is marked by ocean temperature droop, which is the result of the formation of sea ice sheets around the Antarctic, and of the development of heavy cold water circulation locally on the bottom of the ocean. Faunal overturn is indicated by the extinction of specialized Eocene forms (K-strategists), such as turborotalids, globigerinathekids, hantkeninids, and their replacement by generalized globigerinid form species. In addition, the decrease in the size of specimens is remarkable, as well as increased abundances of the opportunistic taxa and cool water indices, increased numbers of the non spinose, deeper dwelling forms and decrease of the diversity. The Early Oligocene (Zone P17/18) foraminiferal association of Palmyra region includes Tenuitella gemma (JENKINS), T. munda (JENKINS), "Globigerina" venezuelana HEDBERG, "G." rohri BOLLI, Paragloborotalia nana (BOLLI), Pseudohastigerina micra (COLE), Chiloguembelina cubensis (PALMER) and Turhorotalia ampliapertura (BOLLI). The end of Zone P18/19 is marked by the extinction of Pseudohastigerina micra (COLE). Planktonic foramiferal association of Zone P20 consists of "Globigerina" tapuriensis BLOW & BANNER, "G." rohri BOLLI, Turborotalia ampliapertura (BOLLI), Cassigerinella chipolensis (CUSHMAN & PONTON), Chilongumbelina cubensis (PALMER), Globoturborotalita ouchitaensis gnaucki (BLOW & BANNER), Catapsydrax martini (BLOW & BANNER), and tenuitelids. The association of the next biozone (P21a) is very well preserved and diversified. Species Globoturborotalita anguliofficinalic (BLOW), G. Ciperoensis (BOLLI), G. Angulisuturalic (BOLLI), Paragloborotalia opima opima (BOLLI) made their firs appearances, but Cassigerinella chipolensis (CUSHMAN & PONTON), Chiloguembelina cubensis (PALMER), globgerinids and tenuitelids have been observed as well. The disappearance of Chiloguembelina cubensis is signified in the Early/Late Oligocene boundary. In the investigated Syrian wells, the planktonic foraminiferal assemblage in the lower part of the Late Oligocene (Zone P21b) is characterised by the following species: Paragloborotalia opima opima (BOLLI), &laquo ; Globigerina&raquo ; rohri BOLLI, &laquo ; G.&raquo ; triapartita KOCH, Cassigerinela chipolensis (CUSHMAN & PONTON), Globigerina officinalis SUBBTINA, Globigerina praebulloides BLOW and some others forms. The planktonic foraminiferal association of the Late Oligocene (Zone P22) consists of globigerinids, globoturborotalitids, tenuitelids and some large &laquo ; globigerinids&raquo ; . Planktonic foraminiferal assemblages dominated by turborotalids, pseudohastigerinids, small acarinids and rare tenuitelids prevail in Late Eocene. Very low number of benthic foraminifera implies sedimentation in deep, open sea environment. Gradual increase of the benthic foraminifera proportions going upwards through the Oligocene interval indicates shallowing of the sedimentary environment. At the Eocene/Oligocene boundary specialized, subtropical planktonic foraminifera were replaced by opportunistic, cooler forms, such as globigerinids, cassigerinelids and globoturborotalitids that coincide with global maximum cooling event.

planktonic foraminifera; Eocenl0ligocene boundary

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