TECTONIC FINGERPRINTS IN SIDERITE CEMENT, TIRRAWARRA SANDSTONE, SOUTHERN COOPER BASIN, AUSTRALIA

Compositional zoning and dissolution in cement is a direct response to the fluctuation of pore water chemistry, the variation of which durin g 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 chemis try and lead to dissolution of cement, made clear by zoning within sid erite crystals. Three different stages of siderite cement have been de scribed from the Tirrawarra Sandstone in the Moorari and Fly Lake fiel ds of the southern Cooper Basin, here referred to as S1 (early), S2 (m iddle), and S3 (late). Ragged dissolution surfaces separate the main p hases, occurring after precipitation of S1 and S2 with incipient disso lution suggested within S2. Back-scattered electron (BSE) images and e lectron microprobe analyses clearly differentiate each main phase of s iderite. S1 is a homogeneous, iron-rich siderite whereas S2 displays p atchy compositional zoning associated with several minor dissolution s tages, and S3 commences with even compositional banding and grades int o a thick homogeneous phase in the terms of composition. Isotope analy ses and fluid inclusion studies indicate that S1 formed at a temperatu re around 30 degrees C, S2 precipitated at a minimum temperature of 68 degrees C, and S3 formed around 102 degrees C. The heterogeneous, pit ted and zoned S2 is thought to have formed during a time of active tec tonism in the Cooper Basin, whereas the evenly banded nature of S3 sug gests that it precipitated during a quiet tectonic period when pore wa ters largely remained relatively constant. It appears that siderite ce ments in the Tirrawarra Sandstone record tectonic activity in the form of irregular growth and dissolution highlighted by compositional zoni ng with stages of strong dissolution recording particularly active tim es when pore waters changed composition dramatically. Some zoning coul d be related in part to tectonic pulses. The temperature recorded by e ach 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 ad ditional tool for calibrating the thermal history of an area.