Cretaceous-Palaeogene Paleoenvironment Evolution And Paleoceanographic Events In The Tethyan Ocean

The southern Tibet of eastern Tethys comprise continuous stratigraphic range from late Cretaceous-Paleogene which preserved important information related to the evolution and closure of the Tethys,the India-Asia collision and the paleoceanographic events.Up to now,many geological studies were carried on Tethys,but,the relationship between the evolution of Tethys and late Mesozoic ocean climate change doesn't seem to have progressed much for a long time.The important reason is lack of the understanding of the geological events in eastern Tethys.Therefore studying the late Cretaceous to Eocene marine sediments in southern Tibet is critical to constrain the age of initial India-Asia continental collision and to reconstruct the paleogeography of southern Tibet.This study presents combined stratigraphic,sedimentological,total organic carbon(TOC),CaCO3,magnetic susceptibility,diffuse reflectance spectrophotometry and the stable carbon and oxygen isotopic composition of both bulk samples and organic matter data for the Cretaceous-Palaeogene succession from the Tethys Himalayas of southern Tibet and Late Barrenmian-Middle Aptian succession from Gorgo a Cerbara section of Central Italy to constrain the evolution and closure of the Tethys,the India-Asia collision and the paleoceanographic events.The succession from bottom to top includes Gambacunkou,Jiubao,Zhepure Shanpo,Jidula,Zongpu,Enba and Zhaguo formations.Fourteen distinct microfacies identify three sedimentary environments in Zongpu Formation and the lower part of Enba Formation.Mudstone,wackestone with Udoteacean algae,bioclastic-peloidal packstone,packstone with Rotaliids and green algae,floatstone with Alveolina and Orbitolites were deposited in restricted lagoonal environments.Bioclastic packstone and grainstone with Rotaliids were deposited in high-energy shoal environments.Floatstones with Nummulitids or Alveolinids were deposited in shallow open-marine environments.Six distinct microfacies identify two sedimentary environments in the uppermost of Jiubao Formation and lowermost of Zhepure shanpo Formation including packstone with foraminifera which documents outer shelf environment,bioclastic packstone,calcisphere packstone,sandy bioclastic packstone and bindstone documenting slope environment.Four distinct microfacies identify two sedimentary environments in Bolinxiala Formation.Mudstone was deposited in hemi-pelagic to pelagic environment.Calcisphere packstone,packstone with foraminifera and filament wackestone were deposited in out shelf environment.The Bolinxiala,Gambacunkou and Jiubao formations were accumulated in hemi-pelagic and pelagic,outer shelf environment.The Bolinxiala formation corresponds to Jiubao and Zhepure Shanpo formations in ages.The uppermost Cretaceous Zhepure Shanpo formation records the pelagic into upper slope environments.The overlying Jidula Formation is interpreted as a prodelta environment in the lower part and a prograding delta plain in the upper part.The Palaeocene-lower Eocene Zongpu Formation records a carbonate ramp and documents two deepening-upward sequences separated by an unconformity corresponding to the Palaeocene/Eocene boundary and marked by a conglomerate with limestone clasts.The Enba formation is interpreted to have been relatively deep-water outer shelf,the overlying Zhaguo formation was interpreted as fluvial channel and floodplain environment.Four major lithological and paleoenvironmental changes occurred at three stratigraphic levels:the Zhepure Shanpo/Jidula boundary,the Jidula/Zongpu boundary,the Paleocene/Eocene boundary and the Zongpu/Enba boundary.Petrographic analysis,detrital zircon geochronology and Hf isotopic data indicate that detritus in the Enba and Zhaguo formations,deposited on the Indian passive margin,was derived from the Asian active margin in the north.These clastic units were thus deposited after the onset of the India-Asia continental collision in the early Himalayan foreland basin.Major lithological and paleoenvironmental changes occurred at three stratigraphic levels:the Jidula/Zongpu boundary(?62 Ma),the Paleocene/Eocene boundary(?56 Ma)and the Zongpu/Enba boundary(?51 Ma).Our provenance study confirms that the India-Asia collision was already under way during the deposition of the Enba Member(i.e.,by?5]Ma)and,along with facies analysis and general palaeogeographic considerations,indicates that Neo-Tethys was still wide open during the Early-Middle Paleocene.It is thus argued,consistently with previous studies,that the Paleocene/Eocene disconformity documented in Tethyan Himalaya is likely to record flexural uplift consequent to initial under thrusting of the Indian continental margin beneath Asia.The first stratigraphic record of the Paleocene-Eocene thermal maximum(PETM)in the Tethys Himalaya was preserved in the carbonate-platform succession deposited originally along the southern margin of the eastern Tethys Ocean.In south Tibet,the expanded PETM interval is documented by a markedly negative carbon-isotope excursion across the boundary between Members 3 and 4 of the Zongpu Formation.The succession is truncated erosionally with deposition of an intraformational carbonate conglomerate,and only the upper part of the PETM interval is preserved.Below the disconformity,floatstones or packstones with Nummulitids document progressive transgression to open-marine environments.Above the disconformity,are mainly restricted-lagoonal inner-ramp deposits characterized by Alveolina and Orbitolites.Foraminiferal assemblages of Shallow Benthic Zone(SBZ)4 below the conglomerate bed are replaced by assemblages of SBZ 6 above.The prominent negative excursion observed in ?13C curves for bulk rock(3.4‰ at Zongpu,4.9‰ at Zengbudong,6‰ at Shenkezha)and organic matter(?3‰,at Zengbudong)is thus correlated to the carbon-isotope excursion characterizing the PETM.The strongly 13C depleted record of shallow-marine carbonates suggests oxidation of organic matter resulting from intensified weathering,runoff and organic-matter flux.The erosional unconformity dated here with precision at the lower part of the PETM(i.e.,between 56 and 55.5 Ma)is identified all along the Tethys Himalaya.This places a stringent constraint on the evolution of the northern margin of India during initial collision with Asia.In a period of warm climate and eustatic rise,the Paleocene/Eocene disconformity testifies to tectonic uplift associated with the southward migration of an orogenic wave,originated 3±1 Ma earlier in the Middle Paleocene at the first site of India-Asia continent-continent collision.We performed a detailed study of the stratigraphic transition from Oceanic Anoxic Event 1a(OAE1a)to oceanic red bed 1(ORB1)from the classic Gorgo a Cerbara section in the Umbria region of central Italy.We focused on a 25.5-m-thick stratigraphic succession,from which we analyzed 305 samples for total organic carbon(TOC),CaCO3,magnetic susceptibility,diffuse reflectance spectrophotometry and the stable carbon and oxygen isotopic composition of both bulk samples and organic matter.In the Gorgo a Cerbara section,the Selli Level of OAEla(approximately 1.81 m thick)consists of laminated to bioturbated dark gray to black mudstones and shales,with medium to dark gray radiolarian-rich silty to sandy layers and the TOC content reaches a maximum of 20.22%.The carbon isotopic values show a negative excursion(C3 stage,approximately 0.14 m)at the base of the Selli Level,followed by a stepwise positive excursion(C4-C6 stages,approximately 1.67 m)in the upper part of the Selli Level.The transition from OAE1a to ORB1(approximately 3.19 m thick)is characterized by bioturbated greenish gray cherty limestones and marly limestones with subordinate marls,corresponds to stable carbon isotopic C7 stage and lasts for approximately 0.75 Myr.The ORB1 interval(approximately 13.15 m)consists of reddish marly clay stones,dark red marlstones,red marly limestones and red calcareous shales which indicates the highly oxic environment.Our results reveal a stepwise transition from a predominantly mesotrophic and dysoxic to anoxic environment at the time that the OAE1a black shales were deposited to an oligotrophic and oxic environment during the transitional interval and finally to highly oxic conditions during the ORB1 interval.The nannoconid crisis occurs at the top of the C2 stage,just 0.34 m below the negative excursion in ?13C isotopic values.The massive CaCO3 dissolution phase occurs 0.25 m above the negative excursion;it lasted for 0.85 Myr and probably resulted from excess CO2,ocean acidification,and carbonate compensation depth(CCD)shoaling.Deposition of massive black shales occurs at the base of the C6 stage and lasted for 0.4 Myr.