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Lakeshore beach gravel (upper Holocene; derived from the Split Rock intrusive felsite, Proterozoic; Iona's Beach, northeastern Minnesota, USA) 13 | by James St. John
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Lakeshore beach gravel (upper Holocene; derived from the Split Rock intrusive felsite, Proterozoic; Iona's Beach, northeastern Minnesota, USA) 13

This is Iona's Beach, which is famous for its abundant pinkish-reddish gravel derived from a cliff of Proterozoic-aged Split Rock intrusive felsite just north of this site. The cliff has been extensively eroded by lakeshore wave action. The wave erosion has produced an abundance of felsite gravel, which has been deposited on the shoreline immediately south of the cliff. The gravel here consists principally of subrounded pebbles and cobbles. Subangular gravel is also present.

 

Felsite is the general term for a finely-crystalline igneous rock having felsic chemistry (= greater than 65% or 70% silica & rich in potassium (K) and sodium (Na) & dominated by the minerals quartz (SiO2) and K-feldspar (KAlSi3O8). These rocks have also been referred to as rhyolite, which is the most common finely-crystalline, felsic igneous rock.

 

Description of the Split Rock intrusive felsite from Miller et al. (2001): "Pink, sparsely porphyritic, aphanitic to fine-grained felsite; intrusive into Gooseberry River basalts (unit ngrb) in Split Rock River area.”

 

Description of the Split Rock intrusive felsite from Miller & Green (2002): "The Split Rock intrusive felsite (unit srif, M-119) is a pink, fine-grained, sparsely porphyritic rock also described as a rhyolite that shows local flow banding. . . . The rock splits readily into shingles, and locally has columnar jointings; these allow it to be relatively easily eroded by waves. . . . Compositionally, it is a quartz monzodiorite to granite.”

 

Description of the Split Rock intrusive felsite from Boerboom (2004): "The Split Rock intrusive felsite (rhyolite) is a pink, fine-grained, weakly porphyritic, flow-banded rock that has a prominent shingle parting to it (hence: Split Rock River). At a given location, the rhyolite has all the physical attributes of a rhyolite flow, but detailed mapping shows that it forms a north-striking, 3.4 kilometer wide body that cuts north from Lake Superior across the stratigraphy of the surrounding volcanic rocks. The felsite forms cliffs on Lake Superior south of the Split Rock River, and in the Split Rock River and nearby streams forms small, sharp canyons. Field measurements show that the flow banding dips gently toward the central axis of the body, implying that the intrusion is keel-shaped. The lower margins of the rhyolite are intruded by, or commingled with, a dark gray, fine-grained ferrodiabase or ferrodiorite.”

 

Locality: Iona's Beach, ~5.5 air kilometers southwest of Split Rock Lighthouse, northern shore of western Lake Superior, northeastern Minnesota, USA (47° 10’ 01.35” North latitude, 91° 25’ 17.69” West longitude)

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References on Split Rock intrusive felsite geology:

 

Miller et al. (2001) - Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Minnesota Geological Survey Miscellaneous Map Series Map M-119.

 

Miller & Green (2002) - Geology of the Beaver Bay Complex and related hypabyssal intrusions. in Geology and mineral potential of the Duluth Complex and related rocks of northeastern Minnesota. Minnesota Geological Survey Report of Investigations 58: 144-163.

 

Boerboom (2004) - Intrusions within the southwestern sequence of the NSVG. pp. 54-55 in 50th Annual Meeting, Institute on Lake Superior Geology, May 4-9, 2004, Duluth, Minnesota.

 

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Taken on June 9, 2015