Apresentando a xenoconformidade - ETs na geologia???

sábado, junho 10, 2017

Introducing the Xenoconformity

Galen P. Halverson

Published on July 2017, First Published on June 08, 2017

Figure 1: A: The xenoconformity at the contact between the glaciogenic Elatina Formation below and the Nuccaleena cap dolostone in Brachina Gorge (Flinders Ranges, South Australia) was chosen as the Global Stratotype Section and Point (i.e., GSSP or “golden spike”) for the ca. 635 Ma base of the Ediacaran Period (Knoll et al., 2006). B: This same surface can be identified with confidence globally, including in Svalbard, where it marks the contact (white arrow) between the Cryogenian Wilsonbreen Formation and the Ediacaran Dracoisen cap dolostone. 

Abrupt vertical changes are commonplace in sedimentary sequences and a basis for how we subdivide the stratigraphic record. These stratigraphic shifts may be obvious, such as when they are manifested in sharp differences in lithology or an erosional surface, or they may be subtle, such as when they are represented in the turnover of fossil assemblages or changes in geochemical signatures. Commonly, these abrupt changes correspond to an unconformity, which is a stratigraphic surface that represents a break in sediment accumulation, often accompanied by erosion. Unconformities are as important as the strata themselves in interpreting geological history.
Less common but arguably equally important in reconstructing geological history are surfaces that result from dramatic shifts in environmental conditions. Familiar examples are many major boundaries of the geological time scale such as the Permian-Triassic and Cretaceous-Paleogene (K-Pg) boundaries. In addition to the loss of a large number of fossil taxa, the K-Pg boundary is clearly identified by an Ir anomaly (Alvarez et al., 1980Smit and Hertogen, 1980), a decrease in carbonate content, and a negative carbon isotope anomaly (Hsü et al., 1982). It is intuitive that state changes in paleoenvironments, as exemplified by the K-Pg boundary, occur both globally and regionally and that these changes can generate unique surfaces or transition zones that can be correlated widely. Nonetheless, while clearly understood that these stratigraphic surfaces exist, the geological lexicon lacks a term to describe them. In this issue of Geology, Carroll (p. 639) has taken the first step in correcting this semantic oversight by introducing the term xenoconformity, defined as “a stratigraphic surface or gradational interval that records a fundamental, abrupt, and persistent change in sedimentary facies across basinal to global scales.”
In sedimentary geology, the term facies refers to a suite of lithological, physical, and biological features of a sediment or sedimentary rock that can be used to distinguish it from other sediments that occur laterally or rocks that occur above or below (Walker, 1992). The principal of uniformitarianism, coupled to observations of modern environments, allows sedimentologists to link facies preserved in the sedimentary record to depositional process. By extension, a suite of facies can be combined into a unique facies association, which represents a specific depositional environment, such as a tidal flat, lagoon, or delta front. The theoretic framework by which geologists then read Earth history from the stratigraphic record is provided by Walther’s Law, which elegantly asserts that vertically juxtaposed and conformable facies associations in sedimentary sections reflect laterally adjacent environments at the time of deposition (Walther, 1893Middleton, 1973).