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--- chronostratigraphy [19/03/2023 17:36] – mike_gss
+++ chronostratigraphy [19/03/2023 17:42] (current) – mike_gss
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 ==== Chronostratigraphy - Geological Time Scales ====
-Chronostratigraphy is the process by which geological time is divided up into meaningful "chunks" of named units. This is hierarchical with categories such as Eons, Eras, Periods, Epochs and Ages/Stages... together with various "Sub-" categories as well. Thanks to Mr. Spielberg and a rather confused //Tyrannosaurus rex//* (which didn't even //live// during the Jurassic!) almost everybody has heard of the Jurassic Period. But the Jurassic is but one of three Periods that make up the Mesozoic Era which itself is but one of three Eras that make up the Phanerozoic Eon.
+Chronostratigraphy is the process by which geological time is divided up into meaningful "chunks" of named units. This is hierarchical with categories such as Eons, Eras, Periods, Epochs and Ages/Stages... together with various "Sub-" categories as well. Thanks to Mr. Spielberg and a rather confused //Tyrannosaurus rex//* (which didn't even //live// during the Jurassic) almost everybody has heard of the Jurassic Period. But the Jurassic is but one of three Periods that make up the Mesozoic Era which itself is but one of three Eras that make up the Phanerozoic Eon.
 (* strictly speaking - if you have been following the notes on Taxonomy - **//Tyrannosaurus rex// Osborn, 1905**)
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 //Two geologists arguing over the placement of a chronostratigraphic boundary (thick black line). The dispute arises because each geologist is studying a different type of fossil which may be responding to different environmental/evolutionary pressures acting at different times. In theory, the rock layers between the two black lines could be assigned two different ages!//
-Despite all the work of many geologists in the 18th, 19th and the bulk of the 20th centuries, the first concerted, collaborative attempt to provide a global standard chronstratigraphic time scale did not occur until Harland et al., 1990, with subsequent new techniques and refinements in Gradstein et al. 2004 and subsequent publications
+Despite all the work of many geologists in the 18th, 19th and the bulk of the 20th centuries, the first concerted, collaborative attempt to provide a global standard chronstratigraphic time scale did not occur until Harland et al. (1990), with subsequent new techniques and refinements in Gradstein et al. (2004) and subsequent publications.
 Somewhat disbelievingly there are currently fourteen proposed separate datums (based on magnetostratigraphy and biostratigraphy) to define the boundary between the Jurassic and Cretaceous Periods (Gradstein et al., 2012). All other Period boundaries have been defined. Eight years later (Gradstein et al., 2020), although a formal proposal has been put forward, this was still "under consideration" by the International Commission on Stratigraphy.
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 **Why Does This Matter?**
-For practical purposes, the use of stage names and their derivatives – chronostratigraphy – as correlation tools is inherently problematic. For example, as the example above shows, correlating a Late Campanian section in one well with a Late Campanian section in another would be incorrect if the two wells were drilled in the 1990s and the 2000s. Conversely sections from two wells designated as Early Maastrichtian and Late Campanian respectively would not be correlated yet those two intervals would in fact be contemporary. The chronostratigraphic ages assigned to stratigraphic intervals in wells and outcrops are convenient, but they are only labels assigned by us based on current opinions.
+For practical purposes, the use of stage names and their derivatives – chronostratigraphy – as correlation tools is inherently problematic. For example, as the example at the top shows, correlating a Late Campanian section in one well with a Late Campanian section in another would be incorrect if the two wells were drilled in the 1990s and the 2000s. Conversely sections from two wells designated as Early Maastrichtian and Late Campanian respectively would not be correlated yet those two intervals would in fact be contemporary. The chronostratigraphic ages assigned to stratigraphic intervals in wells and outcrops are convenient, but they are only labels assigned by us based on current opinions.
 Put another way, ages are interpretations and, as we know, interpretations can and do change. This is frustrating for geoscientists of course – it seems that “the oracles are giving conflicting methods or changing their mind!” (Simmons, 2016). When geologists ask the relatively simple question “what age is that outcrop/formation/log pick/seismic marker?” they normally expect a relatively simple answer – “it’s Early Cretaceous” or “it’s upper Bajocian”.
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 “//It contains an assemblage of dinoflagellate fossils indicative of the Nannoceratopsis semex biozone which in turn is currently calibrated with the Strenoceras niortense to Parkinsonia parkinsoni Tethyan ammonite biozones which currently form the standard biozones of the upper half of the Bajocian stage within the Jurassic system.//”
 Even this answer has (to paraphrase a recent US secretary of State) “certain uncertainties”! For instance, how certain was the original interpreter that those marker species really were indicative of the //Nannoceratopsis semex// biozone?!