Clasts consist entirely of shark teeth and bone fragments,
turtle plates, and pelecypods!
Shark Tooth
Bone
Phoptographed by Michael P. Klimetz
MONTANA
Photographed by Michael P. Klimetz
[Conglomeratic Limestone]
Fountain Formation
Late Pennsylvanian
COLORADO
Photographed by Michael P. Klimetz
[Basal Conglomerate]
Pocono Sandstone
Lower Mississippian
Little Mountain
Columbia County
PENNSYLVANIAN
Photographed by Michael P. Klimetz
MONTANA
A conglomerate is a clastic sedimentary rock consisting of comparatively large-sized, individual clasts supported within a finer-grained matrix that have become cemented together. Conglomerates possess of rounded clasts and are thus differentiated from breccias which possess angular clasts. Both conglomerates and breccias are characterized by clasts greater than that of sand (>2 mm). Furthermore, conglomerates are classified in terms of both the rounding and sorting of their clast population. Paraconglomerates are matrix-supported rocks that contains at least 15% sand-sized or smaller grains (<2 mm), the remainder of which are larger grains of varying sizes. Orthoconglomerates are clast-supported rocks with a matrix of sand and finer particles of less than 15%. Metamorphic alteration can transform a conglomerate into metaconglomerate with a characteristic flattened and stretched clast texture.
Conglomerates are classified on the basis of the lithologies of their component clasts as follows:
Monomict - clasts with only a single lithology
Oligomict - clasts of only a few different lithologies
Polymict - clasts of many different lithologies
Intraformational - clasts derived from the same formation in which they are found
Extraformational - clasts derived older rocks than the formation in which they are found
Furthermore, conglomerates are also classified on the basis of dominant clast size as follows:
Granule conglomerate 2–4 mm
Pebble conglomerate 4–64 mm
Cobble conglomerate 64–256 mm
Boulder conglomerate >256 mm
Conglomerates are deposited in a variety of sedimentary environments. The basal part of a bed is typically coarse-grained and sometimes conglomeratic in deep-water marine turbidites. Here, conglomerates are normally very well sorted with well-rounded clasts and often with a strong imbrication of the clasts. Shallow-water marine conglomerates are normally present at the base of sequences laid down during marine trangressions above an unconformity, and are known as basal conglomerates. They represent the position of the shoreline at a particular time and hence are diachronous (time-transgressive). Conglomerates deposited in fluvial environments possess typically large-size, well-rounded and well-sorted clasts. Clasts of this size are carried as bedload only at times of high flow-rate. Maximum clast size decreases with transport distance due to attrition. Hence such conglomerates are more characteristic of immature river systems. Conglomerates are generally confined to the basal part of channel fill deposited by mature rivers where they are known as pebble lags. Alluvial deposits are formed in areas of high relief, are typically coarse-grained, and accumulate in cone-shaped bodies called alluvial fans. At mountain fronts, individual alluvial fans merge together to form braidplains and produce thick deposits of conglomerate. The bulk of conglomerates deposited in this setting are clast-supported with a strong imbrication. Some matrix-supported conglomerates may also be present, and are through to be a result of debris-flow deposition on alluvial fans. Glaciers transport a considerable volume of coarse-grained material and many glacial deposits are conglomeratic. Tillites, the sediments deposited directly by a glacier, are typically poorly-sorted, matrix-supported conglomerates. The matrix is generally fine-grained, consisting of finely pulverized, clay-sized rock fragments. Waterlain deposits associated with glaciers are often conglomeratic and comprise structures such as eskers. When a succession of conglomerates accumulate into an alluvial fan in rapidly eroding environments, the resulting rock unit is often called a fanglomerate. These form the reservoir rocks of a many highly productive oil fields such as the Tiffany and Brae fields in the North Sea. These fanglomerates were interpreted to have been deposited in a deep marine setting adjacent to a rapidly moving fault which supplied an intermittent stream of debris into a conglomerate pile. These sediment fans are typically several kilometers in thickness close to the fault plane.
