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Influence of regional extensional tectonics on the Jurassic ammonoid lineages as exampled on the Tethyan margins

Krishna J.

Volumina Jurassica

2006

Vol. 4, no. 1

180-180

In another research (Krishna 2006) demonstrated the governance of the 1st and 2nd order sequence stratigraphic frameworks by regional extensional tectonics contrary to the global applicability of the eustatically driven 3rd order framework. In an effort to test the crucial role of extensional tectonics on the 1st to 2nd order sequence framework is investigated the influence of such tectonics on the Jurassic ammonoid lineages in the Tethys. The well resolved 1st to 2nd SB’s and MFS’s of the recently developed sequence framework in the Tethys are analysed vis-a-vis FAD’s and LAD’s of respective Tethyan ammonoid lineages. The lineages selected for the exercise belong to Stephanoceratacea and Perisphinctacea: – Macrocephalitinae Chrysoolithicus lineage (late Late Bathonian Triangularis Zone to mid Early Callovian Diadematus Subzone), Mayatinae (Early Oxfordian Obliqueplicatus Zone to Middle Oxfordian Orientalis Zone) of the Gondwanian Tethyan margin, and the broadly corresponding – Kosmoceratidae and – Cardioceratinae lineage of Stephanoceratacea, and – Torquatisphinctinae (late Late Oxfordian Wagurensis Zone to Early Tithonian Virgatosphinctoides Zone) – Virgatosphinctinae (late Late Kimmeridgian Infundibulum Subzone to Late Tithonian Densiplicatus Zone) of the Gondwanian Tethyan margin and the broadly corresponding – Perisphinctinae (late Late Callovian Lamberti Zone to early Late Oxfordian Bifurcatus Zone), – Ataxioceratinae (mid Late Oxfordian Bimammatum Zone to Early Kimmeridgian Hypselocyclum Zone), – Lithacoceratinae (Late Kimmeridgian Beckeri Zone to Late Tithonian Durangites Zone) of Perisphinctacea. The investigations reveal that the lineages either originate or become extinct or drastically reduced in density with marked change in evolutionary framework either at or near the 1st or 2nd order MFS’s or SB’s. For example, there is extinction of the Gondwanian Chrysoolithicus lineage at a 2nd order MFS, of the corresponding European Kosmoceratinae also at a 2nd order MFS, of the Gondwanian Mayaitinae at the 1st order MFS and of the corresponding European Cardioceratinae also at the 1st order MFS, drastic reduction in the density of the Gondwanian Torquatisphinctinae at a 2nd order MFS and origination of the corresponding European Ataxioceratinae at a 2nd order SB, of the Gondwanian Virgatosphinctinae at the 1st order MFS and of the European Lithacoceratinae near the 1st order MFS. The investigations as above explicitly demonstrate that the ranges of ammonoids lineages are controlled by the 1st and 2nd order sequence stratigraphic framework which in turn are exclusively governed by relatively low frequency high magnitude regional extensional tectonics. The regional extensional tectonics lacks globality on account of widely differing stress buildup and timing of the resultant extensional tectonic event. On the contrary the high frequency low magnitude 3rd order framework is guided by eustatic fluctuations without any interference by the low frequency high magnitude extensional tectonics.

Exclusive governance of the 1st and 2nd order sequence framework by regional extensional tectonics as demonstrated on the Gondwanian Tethyan margin

Krishna J.

Volumina Jurassica

2006

Vol. 4, no. 1

51-51

Krishna et al. (2000) invoked the role of extensional tectonics to explain the explicit discordance of the 1st order MFS of the Jurassic-Cretaceous depositional sequences on the two margins of the Tethys, respectively i. e. above the Late Kimmeridgian Eudoxus Zone on the European Tethyan margin, and above the Middle Oxfordian Schilli Subzone in Kachchh (India). By way of further demonstration of the exclusing role of the regional extensional tectonics is here interpreted the basal SB of the sequence near the Pliensbachian/Toarcian boundary in different basins of the Gondwanian Tethyan margin from East Africa and Madagascar in the west to Indonesia and Australia in the east inclusive of the entire Tethyan margin of the Indian subcontinent. The so determined basal SB is again found unambiguously discordant with the corresponding SB on the European Tethyan margin. The interpretation of the basal SB is based on the presence of tectonically induced marked change of environment and litho-biofacies along with subaerial stratigraphic gap of varying duration between the Triassic-Liassic stratigraphic units below, and Toarcian or younger units above with or without ammonoids in Somalia, Ethiopea, Kenya, Tanzania, Madagascar; Baluchistan, Attock and Hazara regions of Pakistan; Kachachh, Jaisalmer, Malla-Johar and Spiti regions of India; areas of Nepal; Indonesia and Australia. These facies changes and subaerial stratigraphic gaps are also evident in the intra-cratonic non-marine Gondwana basins, as also in the exposed and onshore/offshore well sections on the Indian East coast. Igneous activity is also manifested in a few regions of the Gondwanian Tethyan margin. In the East African countries, Madagascar and Baluchistan the non-marine Triassic sands are uncoformably overlain by marls and limestones with basal Toarcian Bouleiceras. In Kachchh the late Late Bajocian Leptosphinctes bearing sediments are underlain by several hundred meters of sediments before reaching the Precambrian basement. In Spiti the marine ammonoid devoid Liassic limestones are overlain by marls with Early Toarcian Alocolytoceras. In the Indian intra-cratonic non-marine successions too, there is evidenced a large stratigraphic gap between Triassic-Liassic and Late Jurassic - Early Cretaceous stratigraphic units. In the Indian East coast Krishna-Godavari and other basins the non-marine Triassic units are overlain by Late(?) Jurassic - Early Cretaceous marine units. To conclude, the 1st and 2nd order sequence stratigraphic framework on the two margins of the Tethys is exclusively governed by the respective highly differing regional/basinal extensional tectonics and so is highly discordant. In contrast the agreement at the 3rd order is solely guided by the global sea-level changes. Further, the tectonically governed high magnitude and low frequency 1st and 2nd order framework is unable to distort the eustatically guided low magnitude high frequency 3rd order framework.