Geologic Time Scales

geologic time scale
diagram from the United States Geological Survey

The Geologic Time Scale, as shown above, documents intervals of geologic time relative to one another, and has been continuously developed and updated over the last two centuries. In addition to the relative dating of periods in Earth's history for which we have rocks preserved, geologists are now able to assign absolute age dates to critical intervals. In the Geologic Time Scale, time is generally divided on the basis of the earth's biotic composition, with the Phanerozoic Eon (i.e. the Paleozoic, Mesozoic and Cenozoic Eras) representing the period of Earth's history with advanced life forms, and the Pre Cambrian (or Proterozoic and Hadean Eras) representing the period before advanced life.

Within the context of the Phanerozoic Eon, geologists beginning in the late 1700's recognized that fossils appeared in an orderly fashion in stratigraphic units. Moreover, these geologists recognized that the fossilized biota demonstrated rather large changes in overall composition and showed both similarities with, and differences from living taxonomic groups. The majority of fossil organisms, however, did not match with modern groups; this led to the classification of three major eras within the period of time when the Earth's surface was populated with advanced life forms. These eras were referred to as the Paleozoic (meaning ancient life), the Mesozoic (meaning middle life), and the Cenozoic (meaning recent life) based on their relative similarity with modern taxa.

Much of the relative geologic time scale had been constructed prior to the 20th century, but much progress has been made in the last half-century toward defining the absolute ages of the rocks on which the relative time scale is based. The modern Geologic Time Scale as shown above is a compendium of both relative and absolute age dating and represents the most up-to-date assessment of Earth's history. Using a variety of techniques and dating methods, geologists have been able to ascertain the age of the Earth, as well as major eras, periods, and epochs within Earth's history. These dates are used to study, among other things, the tempo or rates of environmental and biologic change occurring on Earth. 

Ordovician time scale

The Ordovician Period is stratigraphically younger than the Cambrian Period and stratigraphically older than the Silurian, which overlies it. The Ordovician is now dated between 488.3 and 443.7 Ma (million years ago), which makes its duration 44.6 million years (Webby, 2003).

The Ordovician, like most periods, has traditionally been sub-divided into Early, Middle and Late epochs, and Lower, Middle and Upper Series, but in practice, this classification is of little use except for the application of broad general patterns. In order to make a more useful and practical time-scale the Ordovician has been sub-divided into distinctive, well-defined intervals of time often referred to as series and stages. Adding to the complexity of stratigraphic nomenclature of the geologic time scale, two main systems are employed to designate these time units. These two separate classification systems are presently in use for the establishment of chronostratigraphic age, one called the North American Stage Classification and the other the British or European Stage Classification system. The British classification system is more often used and represents the preferred chronostratigraphic system for the Ordovician. The relationship between the British and North American stage nomenclature is provided in the figure above. This site will follow the North American Classification system, as Trenton Falls is one of the primary type sections for this dating system.

Ordovician Time Scale
Figure modifed from Webby, 2003

Upper Ordovician time scale

The Upper Ordovician has been divided in the European System into the Caradoc and Ashgill Series and in the North American regional system into the upper part of the Whiterockian, the Mohawkian and the Cincinnatian Series.

Historically, the identification of European global series and stage boundaries has been difficult in North American carbonate-dominated rocks, such as the Trenton limestone. This is due, in part, to the lack of the key diagnostic graptolite and conondont species used in the British system. Paleontologists have developed an equally well-defined chronostratigraphic framework for use in the cratonic rocks of North America. Both relative dating systems are shown in the figure above so that the reader can be familiarized with the classification systems now employed globally for this period of time. Key index taxa (graptolites and conondonts species) that are commonly used for correlation are also provided in stratigraphic succession.

The North American series and stage classification has been constructed in part based on the rock exposures in central New York State. The Mohawkian Series, of which the Trenton limestone is a part, is now in use as the lower part of the Upper Ordovician. Although the duration of the Late Ordovician is now established at approximately 17.2 million years, the total amount of time contained within the Mohawkian Series is nearly 10 million years (or almost 1/4 the total duration of the Ordovician Period).

The Mohawkian Series is further subdivided into the Ashbyan, Turinian, Rocklandian (Ro. above), Kirkfieldian (Ki. above), and Shermanian stages, so the total duration of time represented within the Trenton limestone (Rocklandian, Kirkfieldian and Shermanian stages) is significantly less. No absolute dates have been published establishing the depositional time of the Trenton, but current estimates by Holland and Patzkowsky (1998) suggest that the total time for this interval is approximately 4 to 5 million years.

upper cordoviian time chart
Upper Ordovician Chronostratigraphic Time Scale. Image integrated from Webby et al., (2003) and Holland, (2003)


Upper Mohawkian time scale

The most recent publication dealing with the chronostratigraphic framework of the Trenton Group and its lateral equivalents for the New York State region was published in 2002 by Brett and Baird. In their figure, as shown below, there are no specific absolute age restraints placed on the strata from the type section of the Trenton limestone. However, it does show the specific relative age position of the Trenton Group carbonates relative to other key stratigraphic intervals in the Upper Ordovician of New York. These stratigraphic intervals are highlighted yellow.

Trenton Group Chronostratigraphic Chart  Image Modified from Brett and Baird (2002)
Trenton Group Chronostratigraphic Chart, modified from Brett and Baird (2002)

In their diagram, Brett and Baird have summarized the current biostratigraphic and lithostratigraphic units on a spatial scale from central to eastern New York State , which approximates the depositional strike of the Trenton Group during the Upper Ordovician. This diagram shows several key features which will be discussed in detail in other sections of this website. However, for the purpose of introducing them, the most critical observations shown in the diagram are: 1) the presence of several (there are 3 major ones shown) disconformities in the rocks in this area as shown by the vertical striping. These include: a) the basal Trenton Unconformity which is accentuated (and of longer duration) in the central Mohawk Valley in the neighborhood of the Canajoharie Arch; b) the upper Trenton Lower Utica disconformity centered in the Western Mohawk Valley region in the vicinity of Middleville, New York; and c) the top Schenectady disconformity centered in the region to the east of Little Falls New York; 2). The large-scale westward shift in facies belts through time beginning near the end of the Diplograputus multidens (Middle Mohawkian) chronozone and progressing through the Geniculograptus pygmaeus graptolite chronozone; and 3) the relative dominance of carbonate facies (in blue) in the Western Mohawk Valley in the Mohawkian and the dominance of shale and flysch facies (in black and stippled gray) in the Eastern Mohawk Valley.

In order to fully understand the complexities of the diagram shown above, the reader is encouraged to visit discussions on the tectonic setting of the Trenton Group, as well as sections on the lithostratigraphy and biostratigraphy as found in the section on sedimentary geology.