Breccia-Conglomerate: A "Clast Act" EarthCache
Breccia-Conglomerate: A "Clast Act"
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This earthcache takes you to an abandoned pier in North Rustico, a picturesque village along Prince Edward Island’s North Shore. The purpose of this earthcache is to examine the formation and characteristics of a sedimentary rock common to the Island — conglomerate.
PEI Geology
Prince Edward Island (PEI) forms part of the Maritime Carboniferous Basin. Beginning some 220-300 million years ago, and continuing for more than 75 million years, this lowland region or plain received massive accumulations of eroded material from the surrounding highlands. Brown and cloudy with sand, silt and gravel, ancient rivers drained these areas that presently form western Nova Scotia and parts of New Brunswick , Quebec and Newfoundland. PEI and its surrounding islands was formed as a result of this deposition as well as tectonic upheaval which elevated the Island above sea level. Today, a mantle of loose material known as glacial till, laid down 10,000 to 15,000 years ago, covers much of the Island. The underlying sedimentary bedrock consists of soft red sandstone intermixed with shale. Rock exposures are predominantly sandstone with lesser amounts of siltstone, claystone, and conglomerate also present. The only outcropping of igneous rock (dolerite) (click) found on PEI is on the northeast tip of George's (Hog) island, which is located in northern Malpeque Bay.
Conglomerate
Conglomerate (click)(click) is a coarse-grained sedimentary rock formed from rounded to sub-angular gravel-sized clasts (click) (e.g., granules, pebbles, cobbles, and boulders) larger than 2 mm (0.079 in) in diameter. Conglomerates form by the consolidation and lithification (click) of gravel. Conglomerates typically contain finer grained sediment (e.g., either sand, silt, clay or combination of them), called a matrix (click), filling their interstices or spaces, and are often cemented by calcite, iron oxide, silica, or hardened clay.
The size and composition of the gravel-size fraction of a conglomerate may or may not vary in composition, sorting, and size. In some conglomerates, the gravel-size class consist almost entirely of what were clay clasts at the time of deposition. Conglomerates can be found in sedimentary rock sequences of all ages but probably make up less than 1% by weight of all sedimentary rocks. In terms of origin and depositional mechanisms, they are closely related to sandstones [the predominant rock on PEI -- red-sandstone, due to its iron oxide content] and exhibit many of the same types of sedimentary structures (click).
Classification
Conglomerates may be named and classified by the:
(1) Amount and type of matrix present
(2) Composition of gravel-size clasts they contain
(3) Size range of gravel-size clasts present
A sedimentary rock composed largely of gravel is first named according to the roundness of the gravel. If the gravel clasts that comprise it is largely well-rounded to subrounded, it is a conglomerate. If the gravel clasts are largely angular, it is a breccia (click). Sedimentary rocks that contain a mixture of rounded and angular gravel clasts are referred to as breccio-conglomerate. The rounding of the clasts indicates that they have been transported some distance from their original source (e.g., by a river or glacier), or that they have resided in a high energy environment for some time (e.g. on a beach subject to wave action).
Texture
Conglomerates are rarely composed entirely of gravel-sized clasts. Typically, the space between the clasts is filled by the matrix. If the individual gravel clasts in a conglomerate are separated from each other by an abundance of matrix such that they are not in contact with each other and float within the matrix, it is said to be matrix supported and is called a paraconglomerate. Paraconglomerates are also often unstratified and can contain more matrix than gravel clasts. If the gravel clasts of a conglomerate are in contact with each other, it is said to be clast supported and is called an orthoconglomerate. Unlike paraconglomerates, orthoconglomerates are typically cross-bedded and often well-cemented and lithified by either calcite, hematite, quartz, or clay.
The differences between paraconglomerates and orthoconglomerates reflect differences in how they are deposited. Paraconglomerates are commonly either glacial tills or debris flow deposits. Orthoconglomerates are typically associated with aqueous currents, like rivers.
Clast Composition
Conglomerates are also classified according to the composition of their clasts. A conglomerate or any clastic sedimentary rock that consists of a single rock or mineral is known as either a monomict, monomictic, oligomict, or oligomictic conglomerate. If the conglomerate consists of two or more different types of rocks, minerals, or combination of both, it is known as either a polymict or polymictic conglomerate. If a polymictic conglomerate contains an assortment of the clasts of metastable and unstable rocks and minerals, it is called either a petromict or petromictic conglomerate.
In addition, conglomerates are classified by source as indicated by the lithology of the gravel-size clasts. If these clasts consist of rocks and minerals that are significantly different in lithology from the enclosing matrix and, thus, older and derived from outside the basin of deposition, the conglomerate is known as an extraformational conglomerate. If these clasts consist of rocks and minerals that are identical to or consistent with the lithology of the enclosing matrix and, thus, derived from within the basin of deposition, the conglomerate is known as an intraformational conglomerate.
Clast Size
Finally, conglomerates are often differentiated and named according to the dominant clast size comprising them. In this classification, a conglomerate composed largely of granule-size clasts would be called a granule conglomerate, and so forth. Four other different size designations are fine (2-6 mm), medium (6-20 mm), coarse (20-60 mm), and very coarse (> 60 mm). A conglomerate comprising a mixture of clast sizes is poorly sorted, while a\one comprising mostly clasts of the same size is well sorted.
Miscellaneous
A conglomerate's hardness is variable, soft to hard, dependent on clast composition and strength of cement. Colour, too, is variable, dependent on clast and matrix composition. Clasts are generally smooth to the touch while the matrix is variable. Again, while variable, generally harder rock types and/or minerals dominate as clasts.
Conglomerates are often used as dimension stone for decoration of walls and floors; if hard, it can be used as aggregate, fill and so forth in the construction and roading industries.
Logging Requirements:
Please send the answers to the following required questions to me through my geocaching profile before logging as found. DO NOT post any answers on your log, encrypted or otherwise.
1. List the name GC68R0X Breccia-Conglomerate: A "Clast Act" in the first line of your email. Also list the number and name(s) of people in your group.
2. Examine the large conglomerate boulder at the posted coordinates and answer the following questions:
(a) Is the boulder primarily paraconglomerate or orthoconglomerate?
(b) Is the boulder primarily oligomictic or polymictic conglomerate?
(c) Is the boulder conglomerate poorly sorted or well sorted?
(d) Is the matrix of the boulder conglomerate primarily smooth or rough in texture?
(e) What is the approximate number of clasts in a 30 cm (12 inch) diameter, and how big is the largest clast?
(f) Would the conglomerate most likely have originated from glacial till or a debris flow deposit? (PEI Geology section)
(g) What clastic feature distinguishes conglomerate from breccia?
3. (Optional) Per current www.geocaching.com guidelines, photos are no longer required. However, they are encouraged since they can help clarify that you have visited the location if your other logging requirement answers are vague. Post a picture of yourself and your GPSr with your log that shows the conglomerate in the background.
Additional Hints
(No hints available.)