Origins of Carbonate Spherulites in Sedimentary Rocks, Examples From Early Carboniferous Lacustrine Sediments (East Kirkton of Scotland) and Laboratory Experiments

Abstract

Origins of Carbonate Spherulites in Sedimentary Rocks, Examples From Early Carboniferous Lacustrine Sediments (East Kirkton of Scotland) and Laboratory Experiments

Carbonate spherulitic deposits are excellent reservoirs in Early Cretaceous lacustrine pre-salt fields in the South Atlantic. Their origin is still debated due to a lack of good outcrop analogues. Debated issues include: 1) whether organic matter and microbial activities influence spherulite formation (Mercedes-Martin et al., 2016); 2) whether Mg-silicate gels (stevensite) act as a substrate for spherulite precipitation (Tosca and Wright, 2015), and 3) whether spherulites formed by displacive crystal growth mechanisms within the host sediment (Dorobek et al., 2012). This debate impacts on our understanding of the palaeo-environmental significance of spherulites and affects geological facies modelling in hydrocarbon exploration. More studies are therefore needed.

This study assesses the origin of spherulites from the Early Carboniferous alkaline lacustrine East Kirkton Limestone in Scotland, together with laboratory experiments. The outcrop study shows that the spherulites are widely distributed and mainly occur in 1) thin, low energy organic-rich laminites and 2) thicker, higher energy lithoclastic limestones. Detailed petrographic, SEM-CL, electron microprobe and stable isotopic investigation suggests that spherulites in this locality were formed in a number of ways and supports elements of all the debated-models for their growth. The laboratory experiments were carried out to test whether the spherulite formation is influenced by biotical or abiotical factors, and/or by the Mg-rich clay stevensite. They concluded that organic matter plays a critical role in spherulite growth. Details of both studies will be given in our presentation.

These results suggest there maybe multiple mechanisms for spherulite development in the Pre-Salt fields of the South Atlantic.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019