Chemostratigraphic Proxy Records: Forward Modeling the Effects of Unconformities, Variable Sediment Accumulation Rates, and Sampling-Interval Bias
Paul M. Myrow, John P. Grotzinger, 2000. "Chemostratigraphic Proxy Records: Forward Modeling the Effects of Unconformities, Variable Sediment Accumulation Rates, and Sampling-Interval Bias", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
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Stratigraphic forward models are a significant asset in reconstructing the important processes controlling sedimentary basin development, stratigraphic architecture, and the distribution of facies. Generally, the models are used to make predictions concerning the rates and magnitudes of geologic processes operating at various temporal and spatial scales. Here, we use a recently developed stratigraphic forward modeling package (STRATA) to evaluate the impact of varying accommodation space and sediment fluxes, as well as sampling intervals, on the structure of chemostratigraphic curves preserved in the rock record.
The method assumes that a primary signal, say secular variations in the chemical composition of seawater, is embedded within the stratigraphic record of platform carbonate deposits. In principle, the method and underlying assumptions are generally independent of the particular primary signal that is being recorded (e.g., Sr isotopes), but for the sake of illustration we apply the approach to the terminal Proterozoic δ13C record. If enough is known about the processes that regulate the primary signal, then a map of the primary signal can be used to infer how the controlling processes have varied through time and space—the stratigraphic record itself may be of little interest, depending on the question that is being asked. However, stratigraphic processes can exert a fundamental control on the structure of the primary signal, particularly if they are unsteady in time. Therefore, a forward stratigraphic model can be an essential tool in illuminating which processes may have influenced the final form of the primary signal as it is preserved in the rock record. Geologic processes such as variations in sea level, subsidence, and sediment supply can clearly influence the form of δ13C curves as a result of variations in accommodation space and sediment preservation.
Another important influence on the construction of chemostratigraphic curves is the bias that is introduced through variations in sampling intervals. Samples are often collected in the field with spacings of 10–20 meters or more. The numerical experiments presented here show, particularly for epicratonic cover sequences (such as the Siberian platform), that sample spacings of >10 m can result in potentially severe distortion of the terminal Proterozoic and Early Cambrian δ13C primary signal. On the other hand, sample spacings of 1–2 m or less result in recovery of even short duration events—provided that the data gaps associated with unconformities and siliciclastic intervals can be accounted for, as well as the overprinting effects of diagenesis.
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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.