Evolution of Late Paleoproterozoic Ramp Systems, Lower McNamara Group, Northeastern Australia
Terry T. Sami, Noel P. James, T. Kurtis Kyser, Peter N. Southgate, M. Jim Jackson, Rod W. Page, 2000. "Evolution of Late Paleoproterozoic Ramp Systems, Lower McNamara Group, Northeastern Australia", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
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Rocks of the lower McNamara Group (including the Torpedo Creek, Gunpowder Creek, Paradise Creek, and Esperanza formations) form part of the extensive late Paleoproterozoic sedimentary cover in northwestern Queensland. The succession varies in thickness from 450 to 1250 m and consists largely of dolomite, chert, and siliciclastic rocks deposited in nonmarine, marginal marine, and marine environments. The package is divisible into two distinct basin-filling supersequences (second-order sequences): Prize, a lower suite of fault-bounded, storm-dominated siliciclastic ramps; and the overlying Gun, a regionally extensive, southeast-facing, storm-dominated mixed carbonate/siliciclastic ramp. A regionally correlative unconformity in the lower to middle Gunpowder Formation, representing a depositional hiatus of over 28 My, records the boundary between these two supersequences. Combining facies architecture with gamma-ray logs derived from hand-held spectrometers enables further subdivision into three third-order depositional sequences, (1) Prize 2, Torpedo Creek-lower Gunpowder Creek, (2) Gunl, upper Gunpowder Creek-Paradise Creek, and (3) Gun 2, upper Paradise Creek-Esperanza, each of which is bounded by regionally correlative disconformilies.
The Prize 2 depositional sequence consists largely of siliciclastic storm-dominated ramp sediments deposited in locally fault-controlled depocenters. The overlying Gun 1 sequence marks the transition from a siliciclastic to a carbonate ramp as storm-dominated shallow-water carbonate facies prograded easl/southeastward over the underlying siliciclaslics. Storm processes continued to dominate sedimentation. Formation of stromatolitic rim complexes had a significant hydrodynamic effect on this second ramp and promoted flattening of the inner ramp and steepening of the outer ramp to form a distally steepened ramp to rimmed platform. Unlike the underlying succession, the Gun 1 depositional sequence represents a regionally continuous depositional system with only minor thickness variations. The Gun 2 depositional sequence similarly represents a storm-dominated siliciclastic/carbonate ramp. The most striking difference between the Gun 1 and 2 sequences is the relatively abrupt shift from micrite-rich domal-columnar stromatolites to spar-rich microdigitate and digitate columnar stromatolites.
Unlike most other Paleoproterozoic carbonate successions documented to date, the lower McNamara platform was dominated by storm-deposited and reworked sediments, predominantly intraclasts, quartz and peloid silt, and muds, rather than precipitated carbonate cements. The preponderance of silt-size grains, rather than mud-size, differentiates this succession from most other detrital Proterozoic platforms. The internal architecture and evolution of the succession is instead similar to sediment-rich Phanerozoic carbonate ramps.
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Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.