Introduction
Rudaceous rocks also known as rudites, are a type of sedimentary rock that is mainly consists of coarse fragments and clasts. These clasts are usually larger than 2 millimeters in size and can consist of various rock types, mineral grains and sometimes even fossils. The term "rudaceous" is derived from the Latin word "rudus," which means rubble or coarse material.
Characteristic :
1. Clast Size and Shape : Characterized by the presence of coarse-grained clasts, which are larger than 2 millimeters in diameter. These clasts size can range from pebbles and cobbles to boulders. The clasts may exhibit rounded or angular shapes, depending on factors such as the distance of transport and the degree of abrasion during transportation.
2. Clast Composition : It is composed of a variety of rock types or mineral fragments. They may include fragments of igneous, metamorphic or sedimentary rocks. The composition of the clasts provides information about the source area from which the sediment was derived.
3. Matrix or Cement : These rocks have a finer-grained matrix or cementing material that binds the clasts together, which can consist of sand, silt, clay or a mixture of these materials. The nature and composition of the matrix can vary, and it can influence the overall strength and durability of the rock.
4. Sorting : Rudaceous rocks can exhibit a range of sorting characteristics. Well sorted rudaceous rocks contain similar size of clasts that are clearly separated, whereas poorly sorted types include a broad range of clast sizes with less distinct separation between them.
5. Sedimentary Structures: Rudaceous rocks often exhibit sedimentary structures that provide information about their depositional environment. These structures can include cross-bedding, imbrication (overlapping of clasts) and grading (sorting of clasts within a single bed). These structures indicate the action of currents or gravity during the deposition of the sediment.
6. High Energy Deposition : Rudaceous rocks, with their coarse clasts and distinct sedimentary structures, suggest that deposition in high-energy settings such as river channels, alluvial fans, beaches, or gravel bars, where strong water flow or gravity dominated transport
Classification :
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| Fig.: Classification of Rudaceous rock |
Rudaceous rocks can be further classified based on the dominant type of clasts present. There are mainly two types -
1. Conglomerate
2. Breccia
1. Conglomerate :
Conglomerates are rudaceous rocks that consist primarily of rounded or subrounded gravel-sized clasts (>2 mm) embedded in a matrix of finer material.
Classification :
(Depending upon different sediment sources.)
1.1. Epicalstic Conglomerate
1.2. Pyroclastic Conglomerate
1.3. Cataclastic Conglomerate
1.4. Mesoclastic Conglomerate
A. Intra formational epiclastic conglomerate
B. Extra formational epiclastic conglomerate
The clasts of intraformational epiclastic conglomerates may different in size, shape and composition, reflecting the sedimentary processes within the formation. These conglomerates typically exhibit a close spatial association with the sedimentary rocks from which they were derived.
The formation of intraformational epiclastic conglomerates can occur through various processes. For example - sediment gravity flows such as turbidity currents or debris flows can transport and deposit coarse grained sediment within the same formation. These sediment gravity flows can result in the accumulation of conglomeratic deposits within the sedimentary sequence.
B. Extra formational epiclastic conglomerate : Extraformational epiclastic conglomerate is a specific type of conglomerate that forms as a result of sedimentary processes occurring outside of the primary depositional environment or formation. It refers to conglomerates that are derived from materials outside of the sedimentary unit in which they are found.
Formation: Extraformational epiclastic conglomerates are typically formed through the process of sedimentary recycling. This occurs when sediments from pre-existing rocks or formations are eroded, transported, and re-deposited in a different sedimentary unit. The clasts in extraformational epiclastic conglomerates are derived from these recycled sediments.
Transport and Deposition: The clasts in extraformational epiclastic conglomerates are transported by various geological processes such as rivers, gravity-driven flows, glaciers or wind and deposited in a different environment or sedimentary basin from where they were originally derived. This can occur due to changes in tectonic activity, sea level fluctuations or erosion and re deposition during sedimentary cycles.
Characteristics: Extraformational epiclastic conglomerates typically exhibit a mix of clasts from different rock types and lithologies. The clasts may vary in shape, size, and composition, depending on their original source rocks. The conglomerate may also contain matrix material, which can be composed of finer-grained sediment or cement that fills the spaces between the clasts.
Significance: Provide valuable information about the history and evolution of sedimentary basins and also indicates the presence of distant sediment sources, tectonic activity or changes in depositional environments over time. By studying the clasts and their composition, geologists can gain insights into the provenance of the sediments and reconstruct the geological processes that led to their deposition.
Classification of extraformational epiclastic conglomerate : These epiclastic conglomerate are divided into three subdivision –
a. Oligomict conglomerate
b. Polymict conglomerate
c. Diamictic conglomerate
b. Polymictic conglomerate : In contrast to an oligomict conglomerate, a polymict conglomerate is composed of clasts derived from multiple rock types. The clasts in a polymict conglomerate can vary in size, shape and composition, reflecting a diverse range of source rocks. This term implies a greater heterogeneity in the clast composition within the rock.
c. Diamictic conglomerate : The term "diamictic" is not commonly used to describe conglomerates specifically, but it is often used to refer to a type of unsorted or poorly sorted sedimentary deposit, which have higher content of matrix propotion. In the context of conglomerates, a diamictic conglomerate would imply a poorly sorted rock consisting of a variety of clasts with no specific dominant or significant proportion of any particular rock type ,i.e; higher content of matrix propotion.
1.2. Pyroclastic Conglomerate : Pyroclastic conglomerates are conglomerates that contain clasts derived from volcanic eruptions. The clasts can include volcanic ash, pumice, volcanic bombs and other pyroclastic material. These conglomerates are associated with explosive volcanic activity and are often found in volcanic terrains. Its indicates, its volcanic origin.
1.3. Cataclastic Conglomerate: Cataclastic conglomerates form as a result of fracturing and fragmentation of rocks due to intense tectonic forces. The clasts in cataclastic conglomerates are angular and consist of rock fragments derived from nearby source rocks that have undergone brittle deformation. Its indicates, its tectonic origin.
1.4. Mesoclastic Conglomerate: Mesoclastic conglomerates are conglomerates ,which are produced by meteoric impact.
2. Breccia: Breccias are rudaceous rocks characterized by angular, sharp-edged clasts that are larger than 2 mm. The clasts are cemented together by a finer-grained matrix.
Conclusion :
Rudaceous rocks are coarse-grained sedimentary rocks made of fragments larger than 2 mm, such as pebbles and boulders. These clasts are often held together by finer materials like sand or clay. They typically form in high-energy environments such as rivers, alluvial fans or landslide zones, where strong forces can move and deposit large particles.
Common types include:
- Conglomerates (rounded clasts),
- Breccias (angular clasts) and
- Diamictites (poorly sorted mixtures).
