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Snake Hill Formation
Geology
Introduction
The Snake Hill Formation is a sandstone-rich Upper Ordovician unit that is exposed on the eastern side of Saratoga Lake, New York.[1] The formation’s type section is bound above and below by severely deformed and faulted rock layers, making it difficult to identify the stratigraphic units that were originally deposited before and after.[1] “The Snake Hill Formation records the earliest known and easternmost shallow shelf facies of the Taconic synorogenic progradational succession in New York State.”[1] "The rapid disappearance of the Snake Hill Formation to the west and occurrences of debrites and prolific tool marks in the studied section indicate that the Snake Hill Formation records a narrow shelf margin that grades rapidly down- slope into deeper basinal facies."[1] This locality is very important because it “preserves a slice of the shallow basin margin immediately adjacent to the Taconic orogen, a record that has otherwise been lost through subduction and erosion.”[1]
Lithology and Depositional Analysis
The base of the Snake Hill Formation locality overlies a shale mélange unit; the Snake Hill Formation itself is a maximum of 150 meters thick.[1] The basal interval of the formation (about 14 meters), is mostly comprised of “shale with scattered, (<10%) lenticular, medium-grained litharenite beds.”[1] These coarser, litharenitic sandstone layers are turbidites, which suggest that this facies represents a deeper, low energy environment in which fine-grained mud deposition was interrupted by high- energy turbidity currents.[1]
Overlying the basal interval is about 30 meters of alternating "dark gray- black shale" and "fine-grained, medium gray, sublitharenite.”[1] Hummocky cross-stratification is noticeable in the sandstone layers, indicating a storm wave- dominated shelf environment; the alternating shale layers indicate periods of lower energy conditions on the shelf.[1] This facies also contains the highest concentration of fossils in the formation.[1]
The next 10 meters of section “consists entirely of massive, coarse-grained, subrounded, light gray quartz arenite.”[1] This section contains moderate bioturbation along with scattered mudstone clasts. It also has a massive appearance due to amalgamated bedding.[1] This facies therefore represents the shallowest depositional environment in the formation.[1] Above this, the lithology of the formation then alternates between that of the 14-46 meter section and the 46-52 meter section.[1]
The formation possesses a rich array of marine fossils. These fossils include fauna such as brachiopods (mostly Sowerbyella and Dalmanella),bivalves(mostly C. ventricosus, Ctenodonta, and L. schucherti), crinoids, and some cephalopods.[1] The formation’s numerous species of mollusks, combined with a lack of significant evidence for the abundant presence of trilobites, further suggests that the Snake Hill was deposited in a near-shore shelf environment.[1]
Controversies Over Identification/Extent
The term “Snake Hill” was first used to describe rock units in New York State by geologist Edward O. Ulrich in 1911.[2] However, throughout the 20th century, geologists loosely applied the term to various Ordovician rock units that did not match up with the Snake Hill Formation's type locality in terms of lithology.[1] In particular, shale units of similar age to the formation were identified as the Snake Hill Shale, despite the fact that the formation's type locality is predominantly sandstone. However, recent research has concluded that previous references to the Snake Hill outside of its locality on Saratoga Lake are incorrect, stating that "The Snake Hill Formation is the only known example of proximal, near-shore facies deposited on the western side of the outer Taconic arc, and represents easternmost deposition in the Taconic foreland basin. The Snake Hill Formation is a unique occurrence, and thus is restricted to its type locality at Snake Hill, New York."[1]
References
- ^ a b c d e f g h i j k l m n o p q r s t English, Adam M. (28). "Snake Hill — reconstructing eastern Taconic foreland basin litho- and biofacies from a giant mélange block in eastern New York, USA" (PDF). Palaeogeography, Palaeoclimatology, Palaeoecology. 242 (3–4): 201–213. doi:10.1016/j.palaeo.2006.05.014. Retrieved 12/3/13.
{{cite journal}}: Check date values in:|accessdate=,|date=, and|year=/|date=mismatch (help); Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help) - ^ "USGS National Geologic Maps Database". Geologic Unit: Snake HIll. United States Geological Survey. Retrieved December 9, 2013.