Sedimentology and Stratigraphy

Dunkirk Formation (Clarke, 1903):

Type Locality: Point Gratiot, Dunkirk, NY

Equivalent Names: to the west -- Huron Fm.

Thickness: 24.1 m (~79.05 ft)

Lithology: The Dunkirk Formation is characterized by black shales and interbedded siltstones grading upsection into interbeds of gray shales, siltstones, and thin sandstones.  Units of the Dunkirk Formation are planer-bedded, with the thin, fine-grained sandstones becoming rippled near the top of the unit.  The rippled sandstones display climbing ripples.  Upsection, there is an increase in the occurrence of small, 3-D ripples and HCS in the thin sandstones.  The basal 2.5 meters are thick black silty shale, forming a sharp contact with the Wiscoy Formation calcareous siltstones.  The upper contact is gradational, with the formation changing at the first appearance of the thicker sandstones of the South Wales Formation. 

Ichnology:  The Dunkirk Formation is nearly devoid of trace fossils.  This may be a product of the outcrop exposures or a result of the Frasnian-Famennian extinction.  The scattered examples of trace fossils are primarily Planolites and Teichichnus of a Cruziana assemblage.

Depositional Environment: The black interbedded shales and siltstones with storm deposits increasing upsection are interpreted to represent shallowing basinal deposits.  The abrupt change from the shallow deposits of the Wiscoy Formation to the black shales and siltstones of the Dunkirk Formation is reflected in the sharp deepening in the relative sea level curve.  However, the presence of thin sandstones and siltstones with HCS in the middle and upper Dunkirk Formation indicate that during the deposition of the sandstones and siltstones the depositional environment of the Dunkirk seafloor lay within the depth of maximum storm-wave base.  The planer bedded siltstones and climbing ripples in the thin sandstones near the top of the formation suggest turbidite deposition.  The black shales may have resulted from a change in sediment influx in an area of restricted oxygen.

 

Dunkirk Frmation
 


Wiscoy Fm.-Dunkirk Fm. contact at Wiscoy Creek. The contact is located near the top of the 121cm ruler (R. Bechtel for scale).

 


Dunkirk Fm.-South Wales Fm. contact at Wiscoy Creek. The contact is the base of the thick sandstone (top of the 60cm ruler).

 

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