Petrology of palaeosols and other terrestrial sediments on Aldabra, Western Indian Ocean

Abstract

Rocks exposed on Aldabra include at least six groups of marine deposits separated by terrestrial horizons. Sediments considered are divided into three major groups following important marine formations, the Picard Calcarenites, the Takamaka Limestone and the Aldabra Limestone. Seven textural groups have been identified (1) homogeneous textures in unmodified sediments, (2) crumb-like aggregates of micrite believed to have developed in a dry system, (3) fractured sediments, resulting from drying of wet coherent micrites, (4) vesicular structures, (5) glaebular textures, including concentric glaebules, faecal pellets and bodies formed by inorganic processes of surface accretion, (6) pedotubules, resulting from penetration by rootlets or burrowing organisms and (7) laminate structures formed by incremental deposition of sediment controlled by organisms or, possibly, climatic factors. Identification of the environments of deposition is confirmed by the presence of terrestrial faunas, but textures might be used alone to identify similar horizons within sequences where fossils are absent. Deposits overlying the Picard Calcarenites include unmodified sediments consisting of marine bioclasts but containing terrestrial snails (Trophidophora), tortoise bones and rootlet horizons. With these are associated yellow-brown sometimes laminate soils with distinctive textures and a fauna which includes Succinea. The Takamaka Limestone is overlain by deposits which include dense micrites containing well preserved rootlets, and texturally disorganized materials with cavities which have opened by shrinkage and differential compaction. Some are again strongly laminated. Multiphase cementation sequences are present in limited areas. At least two discrete terrestrial events followed the formation of widespread marine erosion surfaces. The emergence which halted deposition of the Aldabra Limestone brought about the formation of laminated crusts, solution cavity fillings (some containing large numbers of tortoise and other bones), cave deposits, stromatolites and soils, probably in a series of discrete events rather than during a single extended time interval. Within the terrestrial sediments diagenetic evidence proves unreliable in indicating the complexity or frequency of environmental changes of individual samples. Once a stable mineralogy is established the potential for alteration is confined to the periphery of the rock unit. Mineralogically most of the sediments are calcite but small quantities of chlorites and phosphates have been detected. Insoluble residues appear not to contain clays and the bulk are believed to be derived ultimately from organic sources.