Abstract

At least four hardgrounds are present throughout the 27-m-thick exposed, Oligo-Miocene, cool-water Point Addis Limestone of the Torquay Basin in southeastern Australia. They range from thin (a few centimeters thick), incipient layers that have a nodular fabric to thick (up to 1 m thick) continuous hardgrounds. The hardgrounds tend to be at the top of coarsening-upward cycles and are typically overlain by low-relief erosion surfaces. The uppermost hardground is the best developed and most widespread in the Point Addis Limestone, and can be observed in all exposures of the succession. At some localities, it shows evidence of karstification such as dissolution pits, clints and grikes, and dissolution pinnacles. Whether karstified or not, well-developed hardgrounds are overlain by a lag conglomerate that consists of hardground intraclasts with marine Fe-oxide crusts. The hardgrounds are cemented by a first-generation isopachous, inclusion-rich, columnar and fibrous radiaxial calcite of marine origin with trace-element compositions (Mg (super 2+) = 0.68-1.74 mole % MgCO 3 ; Fe (super 2+) = 800-4690 ppm; Sr (super 2+) = 0-260 ppm; Mn (super 2+) = 0-230 ppm) and cathodoluminescence (dull/blotchy) indicative of stabilized Mg-calcite. The isopachous cements are invariably overlain by homogeneous, peloidal, or microbioclastic micrite, having trace-element compositions (Mg (super 2+) = 0.77-1.54 mole % MgCO 3 ; Fe (super 2+) = 760-10,030 ppm; Sr (super 2+) = 0-240 ppm; Mn (super 2+) = 60-340 ppm) and cathodoluminescence (dull/blotchy) again indicative of stabilized Mg-calcite. This micrite is always closely associated with the isopachous cements and appears to be of marine origin, perhaps being analogous to the micritic precipitates described from reefal settings. Clear calcite is the last cement generation in the hardgrounds and has attributes of meteoric cements (nonluminescent; Mg (super 2+) = 0.12-0.87 mole % MgCO 3 ; Fe (super 2+) = 0-230 ppm; Sr (super 2+) and Mn (super 2+) below detection limits). The whole-rock stable-isotope compositions of the hardgrounds and host limestone indicate that the whole unit has been subjected to pervasive alteration by meteoric fluids. The least altered carbonates analyzed from the Point Addis Limestone are brachiopods (delta 13 C = -1.5 per thousand to +2.2 per thousand PDB; delta 18 O = -1.7 per thousand to +0.9 per thousand PDB) and the unkarstified hardgrounds (delta 13 C per thousand to -1.0 per thousand PDB; delta 18 O = -0.6 per thousand to +0.6 per thousand PDB). These hardground occurrences help promote the validity of a unique sea-level-driven model for the formation of hardgrounds in cool-water settings. We propose that the development of these hardgrounds was an entirely marine process, produced by relative sea-level drop and entry of the sea floor into the zone of wave reworking. Marine cementation may begin in the form of nodules, at and below the seawater/sediment interface, and involves both slow sedimentation and shallowing of the cool waters. The nodules can later merge to form continuous hardgrounds. In such a high-energy environment, nondeposition and erosion are the dominant processes and marine cementation can occur. Further sea-level drop would lead to subaerial exposure of the previously formed hardgrounds, as found in one instance in the Point Addis Limestone at coastal exposures.

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