Devonian Rock Units of the Cayuga Lake Basin Name of Rock Unit Description Sherb

Question

Devonian Rock Units of the Cayuga Lake Basin

Name of Rock Unit

Description

Sherburne

shale with increasing sand

Geneseo

black shale; about 10 m thick

Tully

limestone; about 3 m thick

Moscow

shale with limestone at base; about 15 m thick

Ludlowville

shale; about 25 m thick

Skaneateles

shale with limestone on top; total thickness about 28 m|

Marcellus

primarily shale, black shale at base, then think limestone; nearly 4 m thick limestone at top, total thickness nearly 25 m

Onondaga

limestone; >25 m thick

Oriskany

sandstone; about 1.2 m thick

Manilus

limestone; about 7.6 m thick

Rondout

dolomite: about 7.6 m thick

1. One of the rock units in this stratigraphic column is thought to have originally been beach sand dating from the Middle Devonian. Which rock unit is most likely to have originated as a beach? How do you know?

2.         The Onondaga contains a few think layers of volcanic ash. Why is volcanic ash useful for correlating layers of sedimentary rock? (see your text if you don’t remember or can’t figure it out).

3.         The Onondaga contains an abundance of coral fossils, along with crinoids, brachiopods, and trilobites. Why are fossils of ferns, rushes, and primitive conifers (pines) unlikely to be found in these rocks?

Name of Rock Unit

Description

Sherburne

shale with increasing sand

Geneseo

black shale; about 10 m thick

Tully

limestone; about 3 m thick

Moscow

shale with limestone at base; about 15 m thick

Ludlowville

shale; about 25 m thick

Skaneateles

shale with limestone on top; total thickness about 28 m|

Marcellus

primarily shale, black shale at base, then think limestone; nearly 4 m thick limestone at top, total thickness nearly 25 m

Onondaga

limestone; >25 m thick

Oriskany

sandstone; about 1.2 m thick

Manilus

limestone; about 7.6 m thick

Rondout

dolomite: about 7.6 m thick

Explanation / Answer

1.Rock units in the stratigraphic column having sand are Sherburne(shale with increasing sand ) and Oriskany(sandstone). If the whole table denotes the devonian time then the Oriskany formation is most likely a beach depost as it is sort of middle in the table and Oriskany formation is underlain by limestone(Marine rock unit) and overlain by limestone(marine), This tells us that there must have taken some regression in the area after deposition of Manilus rock unit. In the regressed area sand was deposited as beach sand. Sherbune can also be a beach deposit but since a formation from middle devonian is asked Oriskany should be the answer.

2. Volcanic ash is useful for correlating layers of sedimentary rock as the deposition of volcanic ash takes place in a short geological time scale and mostly the geographical distribution is widespread. Such types of beds hacing short geological time scale deposition and widespread geographical distribution are called marker beds and are used for correlating strata.

3.Ferns, rushes and primitive conifers (pines) are all terrestial plants. While corals, crinoids , brachipods and trilobites are marine organisms. As the onondaga contains an abundance of coral fossils , along with crinoids, brachiopods and trilobites it indicates that onondaga must be a marine deposit. Hence probability of finding ferns , rushes and primitive confiers is moslty unlikely as onondaga was deposited in marine conditions and not in terrestial conditions.

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