What is Conglomerate?
Conglomerate is a coarse-grained sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts, e.g., granules, pebbles, cobbles, and boulders, larger than 2 mm (0.079 in) in diameter. Conglomerates form by the consolidation and lithification of gravel. Mudslides and glaciation often help form conglomerate rock. Conglomerates typically contain finer-grained sediment, e.g., either sand, silt, clay, or a combination of them, called matrix by geologists, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay.
Sedimentary Rocks in NJ:
Note: The dot in the image above represents where Hanover Township is located on the map.
What is Puddingstone?
Puddingstone is a conglomerate that consists of distinctly rounded pebbles whose colors contrast sharply with the color of the finer-grained, often sandy, matrix or cement surrounding them. The rounded pebbles and the sharp contrast in color give this type of conglomerate the appearance of a raisin or pudding. There are a couple of different types of puddingstone, with different compositions, origins, and geographical distributions.
Roxbury Puddingstone- This conglomerate is composed of a grey feldspathic sand matrix and well-rounded pebbles and cobbles of quartzite, granite, felsite, and quartz monzonite. The beds of ‘puddingstone’ are complexly interbedded with layers of graded sandstone. It likely accumulated in deep rift basin submerged by marine waters.
Skunnemunk Puddingstone- is a conglomerate that is part of a 3,000 feet (910 m) thick geologic formation formally known as the Skunnemunk Conglomerate. This puddingstone is a distinctive, Late Devonian, grayish-purple to grayish-red, thin to very thick-bedded, cross-bedded, conglomerate. Within the Skunnemunk Conglomerate, it is interbedded with grayish-purple to grayish-red sandstone. This conglomerate consists of pebbles and cobbles of white vein quartz, red and green quartzite, sandstone, red and gray chert, and red shale. The grayish-purple to grayish-red conglomerate and sandstone is cemented largely by hematite and microcrystalline quartz.
Common Minerals found in Puddingstone:
Mineral 1: Quartz
Mineral 2: Feldspar
Mineral 3: Mica
Mafic or Felsic?
A mafic rock is rich in in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro.
A felsic rock is rich in feldspar and quartz. Most felsic rocks are lighter than mafic rocks and have beige or light brown hues. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, and orthoclase.
Udden-Wentworth Scale:
Logging Tasks:
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At the GZ, you will find an example of puddingstone. What type of puddingstone is it and why?
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What is the most common mineral (that's not a part of the matrix) in this piece of puddingstone? Describe the appearance of it in the rock.
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Is the rock at the GZ considered to be mafic or felsic? Why or why not?
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Based on the map above, do you think that puddingstone and other conglomerate rocks are naturally found in this part of NJ?
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Using the Udden-Wentworth scale, how large and what category does this rock at the GZ fall under?
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Congrats to tyva28 for the FTF!
Sources:
https://stevewoodruff.wordpress.com/2007/09/03/in-praise-of-puddingstone/
https://en.wikipedia.org/wiki/Quartz
http://www.hiddennj.com/2014/11/the-proof-is-in-puddingstone.html
https://www.state.nj.us/dep/njgs/enviroed/infocirc/glacial.pdf
https://en.wikipedia.org/wiki/Grain_size#/media/File:Wentworth_scale.png
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