Late Cretaceous dysoxia in a southern high latitude prodelta succession, the Otway Basin, southeastern Australia  

Authors / CoAuthors

Gallagher, S.J. | Taylor, D. | Apthorpe, M. | Stilwell, J.D. | Boreham, C.J. | Holdgate, G.R. | Wallace, M.W. | Quilty, P.G.

Abstract

The warm greenhouse world of the Late Cretaceous created an ocean that was poorly stratified latitudinally and vertically. Periodically these oceans experienced globally significant events where oxygen minimum zones enveloped the continental margins. Evidence of the effect of one of these Ocean Anoxic Events (OAE?s) is preserved in the southern high latitude strata of the Otway Basin in southeast Australia. During the Late Cretaceous, thick sequences of mudstone-dominated deltaic sediments (the Otway Delta) were deposited in an elongate inlet (ca. 500km wide) between Antarctica and Australia located at least 70?S. The initial Turonian strata of this delta (the Waarre Formation) were deposited in marginal marine delta plain to delta front conditions. The overlying Flaxman Formation and basal Belfast Mudstone preserve evidence of transgressive inner to middle shelf upper delta to prodelta conditions. These Turonian units were subject to periodic dysoxia. The conditions that created this dysoxia in the region were similar to those of the high northern latitude Cretaceous Interior Seaway of North America where intermittent freshwater input and deepening seas caused periods of thermohaline stratification and reduced bottom waters. The overlying Coniacian to Santonian Belfast Mudstone was deposited in outer shelf to upper slope prodelta conditions subject to periodic fluctuations in dysoxia with normal marine salinities. After a period when the oxygen minimum zone contracted, upward-increasing dysoxia in the Belfast Mudstone herald the onset of the Coniacian to Santonian OAE 3. This was the last OAE of the Late Cretaceous, prior to the onset of more ?modern? oceanic conditions. The fluctuations in TOC and hydrogen index in these strata reflect variable dysoxic conditions similar to that reported for OAE 3 in the tropical eastern Atlantic by Hofmann et al. (2003). This periodicity implies a very active and dynamic Late Cretaceous hydrosphere. Eventually, hyposaline conditions or higher sedimentation rates due to upper delta progradation and shallowing in the Santonian caused the local extinction and dissolution of many of the calcareous benthic taxa of the Belfast Mudstone.

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nonGeographicDataset

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61282

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• External Publication

( Theme )

• fossil

( Theme )

• hydrocarbons

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• organic geochemistry

( Theme )

• palaeontology

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• sequence stratigraphy

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• Earth Sciences

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2004-01-01T00:00:00

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