Using Elemental Chemostratigraphy on Mid–Late Frasnian Platform-Top Successions from the Lennard Shelf Outcrops, Canning Basin, Western Australia

Abstract

High-resolution chronostratigraphic correlation using elemental chemostratigraphy in platform carbonates is typically difficult to achieve. Here, elemental chemostratigraphy is used to correlate between two platform-top, carbonate-dominated field sections from the narrow Lennard Shelf that existed on the NE margin of the Canning Basin, Western Australia, during the mid-late Frasnian. The correlation, constrained by magnetic polarity reversals and physical ground truthing, is based on recognition of distinctive cyclical ''stacking patterns'' defined by changes in concentrations of the trace element zirconium (Zr). Zr concentrations are controlled by the amount of the heavy mineral zircon in the sediments, which is derived from a terrigenous source and is diagenetically very stable. The stacking patterns in the lower part of the study sections display gradually upward-increasing values of Zr to a maximum, followed by an almost immediate fall to a minimum. In the upper part of the study interval, the cycles are more symmetrical, with both gradually increasing and decreasing portions. The point at which the change in Zr stacking pattern occurs in the two sections is synchronous and occurs in association with a supersequence maximum flooding surface. The correlation based on maximum and minimum Zr values throughout the two sections is demonstrated to be chronostratigraphic by comparison with correlations based upon paleomagnetism and physical ground truthing.When element ratios commonly used as provenance and paleoclimate proxies are plotted, the variations between closely spaced samples are greater than any systematic variations throughout the study intervals. Therefore, no isochemical chemozones can be defined, implying that during deposition of the study intervals, there were no long-lived changes in sediment provenance or paleoclimate that the elemental chemistry can detect.The work presented here shows that the standard approach of defining isochemical chemozones for chemostratigraphic correlation is not always appropriate. However, an approach using cyclical changes in elemental variables for chemostratigraphic correlation between two closely spaced sections is chronostratigraphically valid. The greater challenge is in application of the same approach to more widely spaced sections, potentially in different facies of a carbonate setting.

Chemostratigraphic Approach to the Study of Resources’ Deposit in the Upper Silesian Coal Basin (Poland)

“…In a stratigraphic context, differences in mineralogy and/or chemistry can lead to the formation of a geochemical system, which provides valuable information about geological settings, climate, and the processes of sediment production and preservation. The stratigraphic record exhibited changes in the concentration of certain elements over time as a result of geological conditions, such as tectonic, climatic, redox, oceanographic, biotic, and other processes [15][16][17]26,46].…”

“…The ratio U/Th or Th/U is also often applied in paleoredox conditions research [17,46,[64][65][66][67]. U/Th ratios <0.75 suggest oxic environments and U/Th ratios between 0.75 and 1.25 suggest dysoxic environments, while a U/Th ratio >1.25 suggests anoxic conditions [63].…”

“…Chemostratigraphy is based on the recognition of stratigraphical variations in the geochemistry of the depositional sequences, but the success of the technique depends on relating chemical variations to changes in mineralogy. The inorganic geochemistry of sediments is highly variable and depends on the type of clastic material and its provenance, paleoenvironmental conditions, and diagenesis [9][10][11][12][13][14][15][16][17].…”

“…Even apparently homogeneous sequences demonstrate some chemical composition differences, which make chemocorrelation a commonly usable technique to determine stratigraphy and correlate boreholes [11][12][13][14][15][16][17][18][19][20].…”

“…The application of geochemistry and chemostratigraphy in hydrocarbon exploration and reservoir characterization has become increasingly common in recent years, particularly with the advent of developments in analytical instrumentation [10,[15][16][17][19][20][21].…”