A new provenance tool for the exploration of unconventional plays: the provenance and mineralogy of silt.

Abstract

Provenance analysis has traditionally focused on sandstones, which are much easier to analyse than conglomerates - which must be analysed in the field with limited tools -) and mudrocks - which cannot be dealt with easily with classical optical methods the source. However, it is important to recognise that the silt fraction transported in suspension actually represents the majority of the sediment in large river systems and the predominant grain-size in major deltas and submarine fans, as well as in most of ancient sedimentary basins and reservoirs.

The standard routine to determine the composition of the silt and clay fractions includes a series of geochemical techniques to analyse major elements, trace elements and rare earth elements. Data are generally plotted onto tectonic-setting discrimination diagrams and parent rocks are generically identified only in terms of mafic or felsic affinity. A number of studies have demonstrated that these plots can be inaccurate. Furthermore, the traditional approach is limited by the incorrect assumption that silt-sized sediments are refractory and not affected by diagenesis.

A sophisticated separation technique and the combination of optical analysis, RAMAN spectroscopy and quantitative X-Ray power diffraction (Andò et al., 2011) is used to quantitatively determine the composition of silt (heavy and light minerals). This technique has been successfully applied to determine the provenance of Upper Jurassic-Cretaceous silt sediments exposed in the Mandawa Basin in southern Tanzania. The analysis of light minerals identifies quartz as the dominant phase (Q83-41K25-17P37-0). Among feldspars, K-feldspar dominates over plagioclase; both minerals increase upsection. Heavy-mineral concentration (HMC) varies between 0.2% and 2.3%, with values increasing from the northern to the southern parts of the basin. Garnet and apatite are the most common minerals, together with stable zircon, tourmaline and in minor amounts rutile. Accessory but diagnostic phases are titanite, staurolite, epidote, monazite and glaucophane. Etch pits on garnet and cockscomb features on staurolite suggest a significant effect by intrastratal dissolution, which modified the pristine heavy-mineral assemblage. Multivariate statistical analysis highlights a close association between garnet and apatite, a moderate one for titanite and epidote and, not surprisingly, a close one between stable ZTR minerals (zircon, tourmaline, rutile). The garnet-apatite association characterizes samples from the Upper Jurassic, whereas the ZTR index reaches highest values at the Jurassic-Cretaceous boundary. Upper Cretaceous samples are relatively rich in less stable titanite and epidote. These features result from the combination of changes in provenance and/or drainage patterns with superposed diagenetic effects. Source rocks of Upper Jurassic siliciclastic included Archean/Proterozoic medium to high-grade schists and gneisses, with contributions from Palaeozoic sediments possibly increasing at the Jurassic-Cretaceous boundary and additional detritus from Proterozoic phyllites and schists during the Upper Cretaceous.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015