Composition and Origin of Jurassic Ammonite Concretions at Gerzen, Germany

Abstract

<p> Study of the ecology of concretion and host sediment fossils from a shell bed in middle Bajocian clays of northwestern Germany indicates a predominantly epifaunal suspension-feeding community living on a firm mud bottom. The shell bed, firm bottom and low turbidity required by suspension feeders suggests a hiatus or reduced sedimentation at the time. Depth estimates of 50 to 150 m are indicated by ammonite and belemnite siphuncle and septal strength indices.</p> <p> Preservation of calcitic fossils is excellent in both concretions and host sediments. Aragonitic fossils show good preservation in the concretions. Aragonite has been replaced by at least two generations of calcite. Preservation in the host sediments is poor. Pyrite is common in void spaces of concretion fossils but less so in those from the host sediments.</p> <p> Eight concretions were studied, containing numerous Stephanoceras mutabile (macroconch) and Stephanoceras quenstedti (microconch). These are of opposite sexes but are not a dimorphic pair.</p> <p> Distribution of shell debris and other fossils within the concretions suggests that the ammonites were swept by currents into shallow depressions in the sea floor lined with shell debris. Such depressions have been observed in modern sediments as the result of the feeding activity of rays. The presence of currents is indicated by the southwest orientation of belemnite rostra in the host sediments. Carbonate content of concretion matrix is high indicating concretion growth in very fluid muds. This contrast with the firm bottom indicated by fossil ecology suggests rapid burial of the benthic community by either a mudflow or a sudden, large increase in sedimentation. Concretion growth was initiated by decomposition of organic matter within the mud.</p>