Ammonites from lumpy limestones (Lower Pliensbachian, Portugal). Taphonomic analysis and palaeoenvironmental implications.

Abstract

Preservational features of ammonites recorded in the Lower Pliensbachian lumpy limestones of the Lusitanian Basin confirm the deep marine origin previously established for this facies. These deposits can be subdivided into three main taphofacies which are distinguished by preservational ammonite features: 1) lumpy limestones and marly intervals with reelaborated ammonites, 2) laminated marls and bituminous shales with accumulated ammonites, and 3) homogeneous limestones with resedimented ammonites. The background sedimentation of suboxic (dysaerobic, bioturbated lumpy muds; taphofacies 1) to anoxic conditions (anaerobic, laminated muds; taphofacies 2) on deep zone was interrupted by depositional events related to distal gravity flows (taphofacies 3). Lumpy limestones containing reelaborated ammonites, and showing gradational boundaries and inverse grading developed in deep environments due to sedimentary starving. The stratigraphic intervals of taphofacies 1 represent the lowest values of sedimentation and accumulation rates. Taphofacies of type 1 alternate with taphofacies of type 2 composing stratigraphic cycles of metric order. Such cycles resulted from cyclical environmental changes of hundreds of thousands of years. Deepening episodes of 4th-order led to the development of dysaerobic to anaerobic environments, whilst subsequent shallowing episodes increased the levels of bottom oxygenation.