Sedimentology Of Halite Evaporites From The Palaeozoic Carribuddy Formation, Canning Basin, Western Australia.

Cathro, Donna

Honours Degree, 1989

University of Adelaide

Abstract

The early to mid-Palaeozoic Carribuddy Formation in the Canning Basin, Western Australia, comprises halite evaporites, mudstones, anhydrite and dolomite that have maximum thickness exceeding 2000m and which subcrop within an area of about 200,000 km² of the onshore basin. The rocksalt member ("Unit B") occurs between predominantly mudstone sequences of the overlying Unit A and underlying Units C and D, and attains a thickness of up to 800m in the Willara Sub-basin in the southwestern Canning Basin.

BHP-Utah Minerals 1988 potash exploration drill hole Brooke #l cored 675 m of halite evaporites of Unit 8 (depth 977 to 1652m) in the central Willara Sub-basin. The unit consists of two types of sedimentary cycle. Type 1 cycles, although volumetrically less important, are more complete and comprise:

• mudstone (top)
• halite with variable, upward-increasing mudstone content
• clear, coarsely crystalline halite
• anhydrite
• dolomite (base).

Type 2 cycles, which range from about 2 to 5m in thickness, are much more important volumetrically. They consist of:

• mudstone (top)
• halite with variable, upward-increasing mudstone content
• clear, coarsely crystalline halite (base).

The halite occurs in four main varieties:

1. Clear coarsely crystalline halite at or near the base of cycles. The lack of clay content may be a primarily depositional feature or result from diagenetic karstic void infilling.
2. Halite with minor mudstone content, that typically is colour banded. The halite comprises either randomly interlocking crystals of diagenetic origin, or vertically oriented crystals locally displaying chevrons indicating primary origin.
3. Muddy halite usually consisting of large, anhedral aggregates of halite, and local crystals with cubic form, embedded in a mudstone matrix. The halite is interpreted as growing displacively in the brine-soaked muds.
4. Vein halite is a diagenetic form, typically consisting of red, fibrous crystal arranged perpendicular to the vein wall and confined to mudstone beds.

The bromine content of halite samples from selected portions of the core ranges from 60 ppm to 270 ppm, suggesting marine to restricted basin conditions. Overall, the Carribuddy Formation Unit B halite was deposited in an extensive, shallow-water saline basin or "saltern" that was subjected to repeated evaporation cycles producing type 2 cycles and the formation of emphemeral mudflats, and occasional recharges of sea water producing type 1 cycles. The likely setting is similar to that envisaged for other ancient cyclic halite deposits, notably the Permian Salado Formation in New Mexico and the Permian San Andres Formation in Texas.