%0 Journal Article
%A Fernández López, Sixto Rafael
%A Duarte, Luís Vitor
%A Henriques, María Helena Paiva
%T Ammonites from lumpy limestones (Lower Pliensbachian, Portugal). Taphonomic analysis and palaeoenvironmental implications.
%D 2000
%@ 0214-2708
%U https://hdl.handle.net/20.500.14352/58713
%X Preservational features of ammonites recorded in the Lower Pliensbachian lumpy limestonesof the Lusitanian Basin confirm the deep marine origin previously established for this facies. Thesedeposits can be subdivided into three main taphofacies which are distinguished by preservationalammonite features: 1) lumpy limestones and marly intervals with reelaborated ammonites, 2) laminatedmarls and bituminous shales with accumulated ammonites, and 3) homogeneous limestones withresedimented ammonites. The background sedimentation of suboxic (dysaerobic, bioturbated lumpymuds; taphofacies 1) to anoxic conditions (anaerobic, laminated muds; taphofacies 2) on deep zonewas interrupted by depositional events related to distal gravity flows (taphofacies 3). Lumpy limestonescontaining reelaborated ammonites, and showing gradational boundaries and inverse grading developedin deep environments due to sedimentary starving. The stratigraphic intervals of taphofacies 1 representthe lowest values of sedimentation and accumulation rates. Taphofacies of type 1 alternate withtaphofacies of type 2 composing stratigraphic cycles of metric order. Such cycles resulted from cyclicalenvironmental changes of hundreds of thousands of years. Deepening episodes of 4th-order led to thedevelopment of dysaerobic to anaerobic environments, whilst subsequent shallowing episodes increasedthe levels of bottom oxygenation.
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