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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.