Quartz Cement Origins and Budget in the Tumblagooda Sandstone, Western Australia

Summary

The Tumblagooda Sandstone (late Silurian) includes facies of mixed fluvial and aeolian sandsheet origin. It is over 1 km thick in the type outcrop area and comprises quartz-rich medium to coarse grained red-bed sandstones which are thought to rest unconformably on metamorphic basement. Cementation is dominantly by quartz with minor illite and Fe-rich grain coatings. Fluvial facies have extensive quartz overgrowths (8–24% rock volume) and low fabric compaction due to net introduction of quartz. Aeolian facies show extensive quartz dissolution at grain contacts and contain only a minor (up to 5%) quartz cement volume. Point-count data imply compactional volume losses due to grain-to-grain dissolution of up to 3% for fluvial and 8–19% for aeolian facies. Calculation of pre-cement porosity gives figures of 21–33% (av. 27.5%) for fluvial and 22–34% (av. 29.3%) for aeolian sandstones. Silica is generally conserved within the formation, some being transferred from aeolian to fluvial facies with migration distances in the range of centimetres to tens of metres. However, other sources of silica are required to account for the quantity of quartz cement present within the formation; such sources are considered to be the redistribution of products from abrasion of detritus during transport, and solution products from stylolite seams. Feldspar dissolution could only have supplied a minor quantity of silica for cementation, dissolution of minor feldspar present took place later than the development of quartz overgrowths.

Quartz cementation of fluvial sandstones was aided by relatively clean detrital grain surfaces, but grain coatings of clay and oxides on aeolian grains encouraged dissolution at grain contacts during burial and inhibited overgrowth formation. Macroporosity (primary intergranular) values in the two facies are broadly similar (fluvial 1–12%, av. 6.6%; aeolian 2–17%, av. 9.1%) but pore configuration and connectivity varies and would greatly affect gas or fluid production from the major facies.

Oxygen isotope data measured on separated overgrowths range from δ¹⁸O–14.2‰ to 17.6‰ SMOW with no apparent relationship to locality, facies or stratigraphic position. For precipitation temperatures of 125°C and lower, calculated water δ¹⁸O values are less than zero, implying a meteoric origin. It is concluded that silica transfer took place at depths of ≈ 3 km and a temperature of ≈ 100°C, and commenced when porosity was generally 25–30% in both major facies.