GSWA rock classification scheme

Submitted by chee.ng on Wed, 03/07/2024 - 11:40
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Listing of rock types and lithological nomenclature (and related codes) used by the Geological Survey of Western Australia (GSWA) for the dual purpose of recording lithologies during mapping / core logging, and for representing mappable lithostratigraphic/lithotectonic units in the State. Rather than attempting to cover for all existing rock types and lithologies, the GSWA’s scheme and terms in this vocabulary are specifically informed by Western Australian geology; a companion vocabulary covers lithological equivalents for regolith materials.

Apart from the approach to classification of some volcanic (pyroclastic) rocks, GSWA's lithological nomenclature largely conforms to internationally recognized classifications such as Le Maitre et al. (2005) for plutonic/intrusive rocks and Fettes and Desmons (2007) for metamorphic rocks. Note that: a) GSWA prefers the term intrusive to plutonic, as it carries no inference on depth of emplacement; b) GSWA's rock classification scheme prefers the use of volcanic terminology for any igneous rock formed on or near the Earth's surface, i.e. for those hypabyssal or sub-volcanic rocks such as dykes, sills, and cryptodomes that are demonstrably both spatially and temporally associated with extrusive volcanic rocks; c) some intrusive mafic/ultramafic terminology is allowed for cumulate rocks originating from thick volcanic successions. In the absence of a single unified scheme for sedimentary rocks, multiple sources were considered, and definitions rely heavily on the Glossary of geology (5th edition, revised; Neuendorf et al., 2011). Full references listed under History Note.

GSWA’s rock classification scheme is designed to provide consistent terminology within the organization, and primarily relies on features observable in all rocks at outcrop scale, in hand specimen, and in thin section. Additional terms cover for broader combinations of multiple lithologies encountered during mapping (especially for sedimentary rocks, e.g. ‘siliciclastic + lesser carbonate’), and to ensure a coherent approach across different rock types (e.g. use of ‘mylonitized’ for all metamorphic rock groups, even if some would be rare occurrences or difficult to identify in the field). Definitions in this vocabulary are largely compiled or derived from the quoted references and the Glossary of Geology (5th edition, revised, Neuendorf et al., 2011), with adaptions based on recommended GSWA usage; metamorphic lithologies are often derived from definitions of non-metamorphosed equivalents. Some mineralogical/petrological knowledge is expected of the reader.

Two separate notations are included for each lithology. In addition to the lithological codes for use in a digital environment (one to four lower-case letters), a second notation discriminates usage in different systems: a) ‘E’ denotes lithological codes uniquely reserved for use on maps, in reference to the Explanatory Notes System (ENS) that underpins GSWA’s geological framework for Western Australia; b) ‘W’ denotes lithological codes restricted to field observations / core logging, in reference to GSWA’s field observation database WAROX; c) ‘B’ denotes codes that can be used in both ENS and WAROX (the majority). Note that ENS-only codes also include some terms based on lithogeochemistry or genetic connotation (e.g. komatiitic basalt), and that codes in this system are supplemented by primary and secondary qualifiers (not covered in this vocabulary. A concerted effort has been made to match WAROX-only codes with those generated via qualifiers in ENS, but in a few instances alphabetical limitations have prevented this (i.e. the code used in WAROX has a different meaning from the same string of letters generated in ENS); these are highlighted in the concept definitions.

The approach behind GSWA’s rock classification scheme is documented in Tyler at al. (2004; https://dmpbookshop.eruditetechnologies.com.au/product/the-revised-gswa-rock-classification-scheme.do). Details of GSWA’s code construction for ENS can be found in Riganti et al. (2013; https://dmpbookshop.eruditetechnologies.com.au/product/gswa-code-builder-constructing-and-unravelling-gswa-geological-codes.do) and in a companion application, the GSWA Code Builder (ENS codes only), available from the Department’s Data and Software Centre (https://dasc.demirs.wa.gov.au/).

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