Analysis of the Effects of Abrasion on the Test of Palaeonummulites venosus: Implications for the Origin of Nummulithoclastic Sediments

摘要

Many early Tertiary nummulitic limestones contain broken Nummulites tests (commonly referred to as nummulithoclastic debris) that display breakage ranging from damage to the terminal chamber, to disintegration into sand- and silt-sized (and probably finer) fragments. Little consideration previously has been given to the processes responsible for this damage, or whether test abrasion can be used as an indicator of the degree of transportation or wave reworking. Studies of modern larger benthic foraminifera suggest that transport-induced abrasion is a likely candidate for the test damage seen in many fossil Nummulites. However, experimental reconstruction of the transportation of Nummulites within a traction carpet of skeletal material, using the structurally similar and related extant form Palaeonummulites venosus, failed to reproduce the degree of test damage seen in fossil forms, despite simulating transport up to approximately 71 km. Evidence from experimental and field observations suggests that the additional damage noted in Eocene Nummulites possibly is the result of transportation within turbidity currents and/or predation by relatively large bioeroders, such as fish and echinoids. Processes such as dissolution and microboring are considered to have played only a minor, if any, role in the comminution of Nummulites. These observations have been used to define a scale of taphonomic features observed in fossil Nummulites, to aid identification of autochthonous and allochthonous Nummulites populations in thin-section studies of nummulitic limestones, and to facilitate comparison between different facies and carbonate-platform environments.