Wyniki wyszukiwania - BazTech

1

Redox conditions, glacio-eustasy, and the status of the Cenomanian-Turonian Anoxic Event: new evidence from the Upper Cretaceous Chalk of England

Jeans Christopher V., Wray David S., Williams C. Terry, Bland David J., Wood Christopher J.

Acta Geologica Polonica

|

2021

|

Vol. 71, no. 2

103--152

EN

The nature of the Cenomanian–Turonian Oceanic Anoxic Event (CTOAE) and its δ13 C Excursion is considered in the light of (1) the stratigraphical framework in which the CTOAE developed in the European shelf seas, (2) conclusions that can be drawn from new detailed investigations of the Chalk succession at three locations in England, at Melton Ross and Flixton in the Northern Province where organic-rich ‘black bands’ are present, and at Dover in the Southern Province (part of the Anglo-Paris Basin) where they are absent, and (3) how these conclusion fit in with the present understanding of the CTOAE. The application of the cerium anomaly method (German and Elderfield 1990) at Dover, Melton Ross and Flixton has allowed the varying palaeoredox conditions in the Chalk Sea and its sediments to be related to the acid insoluble residues, organic carbon, δ18O (calcite), δ13C (calcite), δ13C (organic matter), Fe 2+ and Mn2+ (calcite), and P/TiO2 (acid insoluble residue). This has provided evidence that the initial stages of the δ13C Excursion in England were related to (1) a drop of sea level estimated at between 45 and 85 metres, (2) influxes of terrestrial silicate and organic detritus from adjacent continental sources and the reworking of exposed marine sediments, and (3) the presence of three cold water phases (named the Wood, Jefferies and Black) associated with the appearance of the cold-water pulse fauna during the Plenus Cold Event. Conditions in the water column and in the chalk sediment were different in the two areas. In the Northern Province, cerium-enriched waters and anoxic conditions were widespread; the δ13C pattern reflects the interplay between the development of anoxia in the water column and the preservation of terrestrial and marine organic matter in the black bands; here the CTOAE was short-lived (~0.25 Ma) lasting only the length of the Upper Cenomanian Metoicoceras geslinianum Zone. In the Southern Province, water conditions were oxic and the δ13C Excursion lasted to the top of the Lower Turonian Watinoceras devonense Zone, much longer (~1.05 Ma) than in the Northern Province. These differences are discussed with respect to (1) the Cenomanian–Turonian Anoxic Event (CTAE) hypothesis when the ocean-continent-atmosphere systems were linked, (2) limitations of chemostratigraphic global correlation, and (3) the Cenomanian-Turonian Anoxic Event Recovery (CTOAER), a new term to define the varying lengths of time it took different oceans and seas to recover once the linked ocean-continent-atmosphere system was over. The possibility is considered that glacio-eustasy (the glacial control hypothesis of Jeans et al. 1991) with the waxing and waning of polar ice sheets, in association with the degassing of large igneous provinces, may have set the scene for the development of the Cenomanian-Turonian Anoxic Event (CTAE).

2

Ordovician Jeleniów Claystone Formation of the Holy Cross Mountains, Poland - Reconstruction of redox conditions using pyrite framboid study

Smolarek J., Marynowski L., Trela W.

Contemporary Trends in Geoscience

|

2014

|

Vol. 3, iss. 1

59--67

EN

The aim of this research is to reconstruct palaeoredox conditions during sedimentation of the Jeleniów Claystone Formation deposits, using framboid pyrite diameter measurements. Analysis of pyrite framboids diameter distribution is an effective method in the palaeoenvironmental interpretation which allow for a more detailed insight into the redox conditions, and thus the distinction between euxinic, dysoxic and anoxic conditions. Most of the samples is characterized by framboid indicators typical for anoxic/euxinic conditions in the water column, with average (mean) values ranging from 5.29 to 6.02 µm and quite low standard deviation (SD) values ranging from 1.49 to 3.0. The remaining samples have shown slightly higher values of framboid diameter typical for upper dysoxic conditions, with average values (6.37 to 7.20 µm) and low standard deviation (SD) values (1.88 to 2.88). From the depth of 75.5 m till the shallowest part of the Jeleniów Claystone Formation, two samples have been examined and no framboids has been detected. Because secondary weathering should be excluded, the lack of framboids possibly indicates oxic conditions in the water column. Oxic conditions continue within the Wólka Formation based on the lack of framboids in the ZB 51.6 sample.

3

Trace elements and mineral assemblage as palaeoenvironmental markers in the Cenomanian/Turonian Magierowa Member (Pieniny Klippen Belt, West Carpathians)

Wójcik-Tabol P.

Studia Geologica Polonica

|

2008

|

Vol. 131

247-268

EN

The studied sediments of the Magierowa Member corespond to the Cenomanian/Turonian OAE 2. A better understanding of the genesis and palaeoenvironmental setting of this unit was achieved by means of geochemical indicators integrated with mineral composition. It suggests a strong influence of palaeoproductivity/redox cycles. The black shales are characterised by the enhanced accumulation and preservation of marine-derived organic matter. Lamination and enrich- ment of redox-sensitive elements (e.g., Ag, Cd, Mo, V) imply periodic prevalence of anoxic/euxinic conditions within the sediment (pore water) occasionally extending to the sediment/water interface. The bioturbated claystone intervals record periods of low productivity and development of more normal-marine conditions on the bottom. Furthermore, low Mn-contents, broad pyrite size-distribu- tion and presence of marine-origin organic matter suggest that an oxygen minimum zone (OMZ) must have existed within the water column during accumulation. The mineral assemblage of the Magierowa Mbr includes clay minerals, quartz, feldspar and iron minerals. Mineralogical data comparing with some gechemical ratios: Ti/Al, K/Al, Rb/Al suggest pelagic regime of deposition. Well-crystallised illite corresponding with high kerogen maturation and large iron mineral point to advanced diagenesis.

4

Geochemistry of Middle Jurassic mudstones (Kraków-Częstochowa area, southern Poland): interpretation of the depositional redox conditions

Szczepanik P., Witkowska M., Sawłowicz Z.

Geological Quarterly

|

2007

|

Vol. 51, No 1

57-66

EN

Middle Bathonian iron-bearing dark grey mdstones and claystones from the Kraków-Czestochowa Upland (southern Poland) were subjected to geochemical and mineralogical study in order to evaluate palaeo-redox conditions of their deposition and diagenesis. They are mainly composed of kaolinite, with smaller amounts of other clay minerals and detrital quartz. Organic matter is composed of type III and/or IV kerogen and its d13C is between -23 and -24‰, suggesting a mainly terrigenous source, with a possible admixture of marine input. Most geochemical parameters: (total organic carbon) TOC/S, U/Th, Ni/Co, V/Cr, (Cu+Mo)/Zn ratios, authigenic uranium content, and Fe-TOC-S relationship, indicate deposition under oxygenated bottom water conditions. By contrast, DOP (degree of pyritization) and V/V+Ni indices suggest a dysoxic environment. However, DOP probably reflects redox conditions in the sediment during diagenesis rather than in the over laying water column. The composition of the organic matter and the domination of pyrite euhedra over framboids indicates that the V/V+Ni ratio may not be reliable for determination of redox conditions in the rocks studied. There is no significant difference between the geochemical indices of host rocks with nodules and those with out them.