Tectonic fingerprints in siderite cement, Tirrawarra Sandstone, southern Cooper Basin, South Australia

Abstract

Compositional zoning and dissolution in cement is a direct response to the fluctuation of pore water chemistry, the variation of which during burial can be controlled by many factors, including the interaction between pore water and rock-forming minerals and the mixing of fluids from different origins. This paper suggests that tectonic activity can, by altering the hydraulic gradient, also influence pore water chemistry and lead to dissolution of cement, made clear by zoning within siderite crystals. Three different stages of siderite cement have been described from the Tirrawarra Sandstone in the Moorari and Fly Lake fields of the southern Cooper Basin, here referred to as S1 (early), S2 (middle), and S3 (late). Ragged dissolution surfaces separate the main phases, occurring after precipitation of S1 and S2 with incipient dissolution suggested within S2. Back-scattered electron (BSE) images and electron microprobe analyses clearly differentiate each main phase of siderite. S1 is a homogeneous, iron-rich siderite whereas S2 displays patchy compositional zoning associated with several minor dissolution stages, and S3 commences with even compositional banding and grades into a thick homogeneous phase in the terms of composition.Isotope analyses and fluid inclusion studies indicate that S1 formed at a temperature around 30 degrees celsius, S2 precipitated at a minimum temperature of 68 degrees celsius, and S3 formed around 102 degrees celsius. The heterogeneous, pitted and zoned S2 is thought to have formed during a time of active tectonism in the Cooper basin, whereas the evenly banded nature of S3 suggests that it precipitated during a quiet tectonic period when pore waters largely remained relatively constant. It appears that siderite cements in the Tirrawarra Sandstone record tectonic activity in the form of irregular growth and dissolution highlighted by compositional zoning with stages of strong dissolution recording particularly active times when pore waters changed composition dramatically. Some zoning could be related in part to tectonic pulses. The temperature recorded by each of the siderite stages allows their precipitation to be tied to a burial history curve, and by making some simple assumptions about that history, the timing of cementation can be estimated. This can be a additional tool for calibrating the thermal history of an area.