Олигоценовая стратиграфия Сев Приаралья Зап Казахс

    
    Lucas, S.G., Kordikova, E.G. & Emry, R.J. 1998.*
Oligocene stratigraphy, sequence stratigraphy and mammalian biochronology north of the Aral Sea, Western Kazakhstan. - Bull. Carn. Mus. Nat. Hist., 34: 313-348.

ABSTRACT
Oligocene marginal marine and nonmarine strata exposed north of the Aral Sea (Kumbulak Cliffs to Altyn Chokysu) comprise two unconformity-bounded sequences. The lower sequence rests disconformably on bentonitic marine shale of the upper Eocene-lower Oligocene Chegan Formation and consists of the Kutanbulak Formation, as much as 28 m of fine-grained quartzarenite, overlain conformably by the Chilikta Formation, as much as 23 m of shale and thinly interbedded shale/sandstone. The younger sequence begins with the Chagray Formation, as much as 33 m of micaceous sandstone that disconformably overlies the Chilikta. The Chagray grades upward into the Aral Formation, >24 m of calcareous shale and claystone. Stratigraphy, facies architecture, and paleontology support interpretation of the Kutanbulak and Chagray as lowstand and/or early transgressive systems tracts composed of river and delta deposits that infilled incised topography that previously developed during lowstand.

Chilikta and Aral strata are late transgressive systems tracts and highstand systems tracts—mostly deposits of estuaries and lakes. Homotaxis to marine strata on the Ustyurt Plateau and marine bivalves indicate the Chilikta is late Rupelian, and the Chagray is Chattian. The oldest Oligocene fossil mammals in these strata occur in the lower Chilikta—fragmentary remains of sirenians and of the giant rhinoceros Paraceratherium. This is the oldest local record of Paraceratherium and its only direct tie to marine biochronology. The classic mammal assemblage collected near Agyspe comes from the Aral Formation, as do recently collected micromammal assemblages from Altyn Chokysu and vicinity.

Some Russian workers now regard these assemblages as of early Miocene age, but we identify them as late Oligocene because:
1) the Aral Formation is part of the same sequence as the Chattian Chagray Formation;
2) most of the genera and many of the species of Aral insectivores, lagomorphs, and rodents are known from the late Oligocene Tabenbulukian faunas of China and Mongolia; and
3) Aral megamammals include genera such as Lophiomeryx, Prodremotherium, Protaceratherium, Aprotodon, and Paraceratherium found in the European and/or Chinese late Oligocene. The Aral Formation fossil mammal assemblage represents one of the best documented late Oligocene mammal faunas known from Asia.

* https://www.palaeovertebrata.com/Articles/view/113
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