First biostratigraphic (palynological) dating of Middle and Late Cambrian strata in the subsurface of northwestern Algeria, North Africa: Implications for regional stratigraphy

The first biostratigraphic (palynological) evidence of Middle to Late Cambrian sedimentation in the Algerian subsurface is presented and discussed in this paper, resulting from the palynological analysis of pre-Ordovician elastic sequences penetrated by borehole AMG-1 in northwestern Algeria. Well preserved and moderately diversified acritarchs occur in a transgressive sequence comprising the upper third of the Sebkhet el Mellah Formation and most part of the Ain en Nechea Formation. This acritarch suite corresponds to the Cristallinium cambriense Eliasum/Timofeevia acritarch Superzone established in Morocco as well as to the Adara alea acritarch Zone of eastern Newfondland, both directly correlated to the Paradoxides paradoxissimus Trilobite Zone of Middle Cambrian age. This stratigraphic interval correlates clearly with the mixed carbonate-siliciclastic successions outcropping in the Tacheddirt Valley of the Anti Atlas, Morocco. The uppermost part of the Ain en Nechea Formation is possibly attributed to the Late Cambrian, based on the absence of the typical Middle Cambrian species, and the presence of taxa of Timofeevia and Cristallinium, associated to an increase in abundance and diversity of diacromorphs acritarchs, suggesting that the Late Cambrian stratal sequences must be considerably reduced and/or partly eroded in the study area. These results provide the basis for improved correlation with the acritarch-bearing Middle Cambrian sediments of the Anti Atlas in Morocco, and with the outcrops of the Ougarta Ranges further south in Algeria, facilitating future sequence stratigraphic studies of the pre-Ordovician elastic successions of the North Saharan Platform,with a higher resolution than previuosly available. (c) 2007 Elsevier B.V. All rights reserved

The Cambrian-Ordovician succession in the Ougarta Range (western Algeria, North Africa) and interference of the Late Ordovician glaciation on the development of Lower Paleozoic transgression on northern Gondwana.

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Biostratigraphical calibration of third order Ordovician sequences on the northern Gondwana platform

This study illustrates bathymetric trends across the northern Gondwana platform during the Ordovician. The sedimentary records of the Algerian Sahara, central Morocco. and western France are investigated to document related environmental changes through the period Outcrop and subcrop sections have been selected with regard to their completeness and to the precision of the available biostrarigraphical data, based on chitinozoans. This group provides the most continuous biostratigraphical record in the terrigenous Ordovician successions of northern Gondwana. For each area, the chitinozoan data have been assembled, updated and referred to one of the 26 Ordovician chitinozoan biozones defined in northern Gondwana regions A sequential analysis based on the identification of genetic units is presented The stacking pattern of these units is established for each locality, and the resulting curves express variations in sedimentary environments partly driven by relative sea-level changes. From successive smoothings of these curves, we define 16 main cycles at lower frequencies Uncertainties concerning the precise chronostratigraphic position of some of these cycles are discussed The proposed northern Gondwana relative sea-level variation curves are compared to the recently-published global Ordovician sea-level curve and to other regional curves available for the major Ordovician palaeoplates (Laurentia, Baltica, South China, north-eastern Gondwana) (C) 2010 Elsevier B V All rights reserve

The Late Ordovician glacio-eustatic record from a high-latitude storm-dominated shelf succession: The Bou Ingarf section (Anti-Atlas, Southern Morocco)

Evidences of glaciation at the end of the Ordovician are widespread in western Gondwana Some authors consider the glaciation was restricted to the Hirnantian time, but occurrences of glacial deposits in the Lower Silurian strata of South America indicate that the Gondwana glaciers did not completely disappear after the Hirnantian glaciation. In addition, numerous studies based on palaeoecology, sedimentology, sequence stratigraphy and stable isotopes have suggested that ice sheets formed as soon as the Early/Middle Katian The objective of this work is to investigate the high-frequency eustatic signals in a stratigraphic succession corresponding to the Katian and the Hirnantian (similar to 10 My comprising the uppermost part of the Ordovician) The studied section (Bou Ingarf section >600 m) is located in the Central Anti-Atlas, southern Morocco It shows an almost continuous succession of siliciclastic platform deposits. The lower package, Katian to early Hirnantian in age, corresponds to a shelf succession, within which storm dynamics prevailed The upper package, middle to upper Hirnantian in age, comprises a glaciation-related succession made up of a suite of coastal to flood-dominated fluvio-glacial deposits The latter have essentially filled in large palaeochannels interpreted as subglacial tunnel valleys From high-resolution facies and sequential analysis, a curve of variation of depositional environments was established at very high, high and low frequencies Time calibration was performed based on a high-quality biostratigraphic control mainly derived from chitinozoan biozones through the whole succession. Assuming a constant tectonic subsidence and a bathymetric model (shoreface/upper offshore boundary 30 m, upper offshore/lower offshore boundary: 120 m), changes in facies-based water depths are converted into an eustatic sea-level curve using a one-dimensional backstripping procedure The eustatic sea-level curve shows that the stratigraphic succession is dominated by the stacking of sequences at very high- (40 m) and coeval forced regressions were identified during the Katian They represent glacial episodes of significant extent prior to the Hirnantian, but essentially lacking a glacial record Strata reflecting the Hirnantian glaciation include two differentiated events and an intermediate but important transgression. The second Hirnantian glacial event, the only one associated with subglacial erosion in the Bou Ingarf area, corresponds to the Late Ordovician glacial climax characterised by a continental-scale ice sheet Ice sheets permanently occupying the centre of the Gondwana landmass throughout the Late Ordovician may reconcile moderate Hirnantian eustatic sea-level fall amplitude (4080 m) and palaeoglacial reconstructions that show a Hirnantian ice sheet covering the main part of western Gondwana. (C) 2010 Elsevier B V All rights reserved

The Late Ordovician glacio-eustatic record from a high-latitude storm-dominated shelf succession: The Bou Ingarf section (Anti-Atlas, Southern Morocco)

International audienceEvidences of glaciation at the end of the Ordovician are widespread in western Gondwana. Some authors consider the glaciation was restricted to the Hirnantian time, but occurrences of glacial deposits in the Lower Silurian strata of South America indicate that the Gondwana glaciers did not completely disappear after the Hirnantian glaciation. In addition, numerous studies based on palaeoecology, sedimentology, sequence stratigraphy and stable isotopes have suggested that ice sheets formed as soon as the Early/Middle Katian. The objective of this work is to investigate the high-frequency eustatic signals in a stratigraphic succession corresponding to the Katian and the Hirnantian (not, vert, similar 10 My comprising the uppermost part of the Ordovician). The studied section (Bou Ingarf section > 600 m) is located in the Central Anti-Atlas, southern Morocco. It shows an almost continuous succession of siliciclastic platform deposits. The lower package, Katian to early Hirnantian in age, corresponds to a shelf succession, within which storm dynamics prevailed. The upper package, middle to upper Hirnantian in age, comprises a glaciation-related succession made up of a suite of coastal to flood-dominated fluvio-glacial deposits. The latter have essentially filled in large palaeochannels interpreted as subglacial tunnel valleys. From high-resolution facies and sequential analysis, a curve of variation of depositional environments was established at very high, high and low frequencies. Time calibration was performed based on a high-quality biostratigraphic control mainly derived from chitinozoan biozones through the whole succession. Assuming a constant tectonic subsidence and a bathymetric model (shoreface/upper offshore boundary: 30 m; upper offshore/lower offshore boundary: 120 m), changes in facies-based water depths are converted into an eustatic sea-level curve using a one-dimensional backstripping procedure. The eustatic sea-level curve shows that the stratigraphic succession is dominated by the stacking of sequences at very high- ( 40 m) and coeval forced regressions were identified during the Katian. They represent glacial episodes of significant extent prior to the Hirnantian, but essentially lacking a glacial record. Strata reflecting the Hirnantian glaciation include two differentiated events and an intermediate but important transgression. The second Hirnantian glacial event, the only one associated with subglacial erosion in the Bou Ingarf area, corresponds to the Late Ordovician glacial climax characterised by a continental-scale ice sheet. Ice sheets permanently occupying the centre of the Gondwana landmass throughout the Late Ordovician may reconcile moderate Hirnantian eustatic sea-level fall amplitude (40–80 m) and palaeoglacial reconstructions that show a Hirnantian ice sheet covering the main part of western Gondwana