Post by 1dave on Feb 11, 2020 15:44:40 GMT -5
We Rockhounds seldom think about sedimentary rocks, but this may change your mind.
If it were not for sedimentation those treasures would never be available to us.
I think you may find information in this PDF as interesting as I did.
www.researchgate.net/publication/234111861_Geology_Morphology_and_Sedimentology_of_Estuaries_and_Coasts
Geology, Morphology, and Sedimentology of Estuaries and Coasts by Burghard W. Flemming 2011
I had never thought about the survival rate of ancient sediments.
A new way of looking at igneous rocks - that have to erode to release the geodes etc. we love -
Discharge rates of the major rivers of the world.
How dams destroy beaches.
Rates of coastal erosion -
If it were not for sedimentation those treasures would never be available to us.
I think you may find information in this PDF as interesting as I did.
www.researchgate.net/publication/234111861_Geology_Morphology_and_Sedimentology_of_Estuaries_and_Coasts
Geology, Morphology, and Sedimentology of Estuaries and Coasts by Burghard W. Flemming 2011
Abstract
The sedimentary rocks exposed at the surface of the Earth today consist primarily of volcaniclastics (~17.6%), graywackes (~2.6%), arkoses (~5.3%), quartz sandstones (~7.5%), mudrocks (~60%), limestones and dolomites (~6.0%), and evaporites (~1.0%). Together with the weathering products of igneous rocks, they form the source of the sediments supplied to the coasts of the world by river and glacier discharge. It is estimated that between 20 and 70 109 t of sediment are delivered to the coast every year. While the majority of beaches consist of gravels and sand, including various proportions of bioclastic material, some coasts remain muddy because wave action is unable to remove the large amounts of fine-grained sediments supplied by some rivers.
The morphodynamic behavior of beaches is finely tuned between local grain size and wave climate, beach slope generally increasing with increasing grain size, but decreasing with higher wave energy so that for any given grain size, high-energy beaches tend to be flatter than low-energy beaches. Estuaries show a marked tripartite longitudinal zonation that is independent of tidal range. Both lower and upper estuaries are sandy and/or gravely, bioclastic material being restricted to the lower part.
Middle estuaries, by contrast, consist of muddy sediments formed by flocculation processes in the mixing zone between saltwater and freshwater. Aggregates predominately consist of particles smaller than about 8 μm and are the main components of fluid muds found in many estuaries. Muds have higher water contents and hence lower bulk densities than sands so that 50:50 sand–mud mixtures contain more mud per unit volume than a 100% pure mud.
The sedimentary rocks exposed at the surface of the Earth today consist primarily of volcaniclastics (~17.6%), graywackes (~2.6%), arkoses (~5.3%), quartz sandstones (~7.5%), mudrocks (~60%), limestones and dolomites (~6.0%), and evaporites (~1.0%). Together with the weathering products of igneous rocks, they form the source of the sediments supplied to the coasts of the world by river and glacier discharge. It is estimated that between 20 and 70 109 t of sediment are delivered to the coast every year. While the majority of beaches consist of gravels and sand, including various proportions of bioclastic material, some coasts remain muddy because wave action is unable to remove the large amounts of fine-grained sediments supplied by some rivers.
The morphodynamic behavior of beaches is finely tuned between local grain size and wave climate, beach slope generally increasing with increasing grain size, but decreasing with higher wave energy so that for any given grain size, high-energy beaches tend to be flatter than low-energy beaches. Estuaries show a marked tripartite longitudinal zonation that is independent of tidal range. Both lower and upper estuaries are sandy and/or gravely, bioclastic material being restricted to the lower part.
Middle estuaries, by contrast, consist of muddy sediments formed by flocculation processes in the mixing zone between saltwater and freshwater. Aggregates predominately consist of particles smaller than about 8 μm and are the main components of fluid muds found in many estuaries. Muds have higher water contents and hence lower bulk densities than sands so that 50:50 sand–mud mixtures contain more mud per unit volume than a 100% pure mud.
I had never thought about the survival rate of ancient sediments.
Examination of geological maps will reveal that the different types of sedimentary rocks forming
the upper crust of the earth today are not all of the same age. In a general sense, young deposits occur
more frequently than old deposits. However, looking at the survival of sedimentary rocks over time
(Garrels and Mackenzie, 1971), the trend from older to younger rocks is not characterized by a
steadily increasing progression, but instead by a saw-tooth pattern which suggests that rocks of some
geological periods have had a higher survival rate than others (Fig. 2).
Thus, from the relatively low survival of late Precambrian rocks (>600 Ma BP), the rate increases rapidly to reach a first peak in rocks of Devonian age (345–395 Ma BP). Thereafter, the survival rate decreases just as rapidly to reach a marked low in rocks of Permian age (230–280 Ma BP).
A second peak, which is only marginally lower than that of Devonian rocks, is reached in rocks of Triassic age (195–230 Ma BP). This is followed by a small drop in the occurrence of Jurassic rocks (141–195 Ma BP), followed by a continuous rise in the survival rate of Tertiary (2–65 Ma BP) and Quaternary rocks (<2 Ma BP). Not
unexpectedly, the youngest deposits are the most prominent because of their relatively short exposure to weathering and denudation processes.
This peculiar zigzag-trend with pronounced peaks in Devonian and Triassic rock volumes requires some explanation. The higher survival rates of rocks from some periods may be because of more resistance to weathering and erosion, or there may have been more sediment being produced and deposited in these periods. Although differential weathering and erosion may have played a supporting role, the geological evidence suggests that higher sediment production is more plausible. The early Devonian was not only a period of major geotectonic activity associated with the Caledonian Orogeny (McKerrow et al., 2002) but, at the same time, the evolution of vascular plants reached the point where, for the first time in earth history, large tracts of land began to be covered by dense vegetation (e.g., Willis and McElwain, 2002).
Without a plant cover, large-scale erosion of weathered rocks and resulting formation of huge sedimentary basins were the order of the day (e.g., Schumm, 1968; Dott and Shaver, 1974). This can also be observed today where the destruction of the protective plant cover usually results in severe erosion. Up to the Devonian period erosion and deposition were not inhibited by vegetation and, as a result, massive alluvial fans, deltaic deposits and basin fills accumulated, often reaching many thousands of meters in thickness (e.g., Visser, 1974). This changed dramatically with the evolution of higher plants, the production of sediment decreasing by as much as 50% (Schumm, 1968). The widespread development of huge coal deposits during the Carboniferous (Pennsylvanian) bears witness to this effect that continued well into Permian times and may help explain the low apparent survival of sedimentary deposits of this age.
Fig 2.
the upper crust of the earth today are not all of the same age. In a general sense, young deposits occur
more frequently than old deposits. However, looking at the survival of sedimentary rocks over time
(Garrels and Mackenzie, 1971), the trend from older to younger rocks is not characterized by a
steadily increasing progression, but instead by a saw-tooth pattern which suggests that rocks of some
geological periods have had a higher survival rate than others (Fig. 2).
Thus, from the relatively low survival of late Precambrian rocks (>600 Ma BP), the rate increases rapidly to reach a first peak in rocks of Devonian age (345–395 Ma BP). Thereafter, the survival rate decreases just as rapidly to reach a marked low in rocks of Permian age (230–280 Ma BP).
A second peak, which is only marginally lower than that of Devonian rocks, is reached in rocks of Triassic age (195–230 Ma BP). This is followed by a small drop in the occurrence of Jurassic rocks (141–195 Ma BP), followed by a continuous rise in the survival rate of Tertiary (2–65 Ma BP) and Quaternary rocks (<2 Ma BP). Not
unexpectedly, the youngest deposits are the most prominent because of their relatively short exposure to weathering and denudation processes.
This peculiar zigzag-trend with pronounced peaks in Devonian and Triassic rock volumes requires some explanation. The higher survival rates of rocks from some periods may be because of more resistance to weathering and erosion, or there may have been more sediment being produced and deposited in these periods. Although differential weathering and erosion may have played a supporting role, the geological evidence suggests that higher sediment production is more plausible. The early Devonian was not only a period of major geotectonic activity associated with the Caledonian Orogeny (McKerrow et al., 2002) but, at the same time, the evolution of vascular plants reached the point where, for the first time in earth history, large tracts of land began to be covered by dense vegetation (e.g., Willis and McElwain, 2002).
Without a plant cover, large-scale erosion of weathered rocks and resulting formation of huge sedimentary basins were the order of the day (e.g., Schumm, 1968; Dott and Shaver, 1974). This can also be observed today where the destruction of the protective plant cover usually results in severe erosion. Up to the Devonian period erosion and deposition were not inhibited by vegetation and, as a result, massive alluvial fans, deltaic deposits and basin fills accumulated, often reaching many thousands of meters in thickness (e.g., Visser, 1974). This changed dramatically with the evolution of higher plants, the production of sediment decreasing by as much as 50% (Schumm, 1968). The widespread development of huge coal deposits during the Carboniferous (Pennsylvanian) bears witness to this effect that continued well into Permian times and may help explain the low apparent survival of sedimentary deposits of this age.
Fig 2.
A new way of looking at igneous rocks - that have to erode to release the geodes etc. we love -
Discharge rates of the major rivers of the world.
How dams destroy beaches.
Rates of coastal erosion -
the rate of coastal retreat, and hence the supply of new sediment to adjacent beaches, varies by several orders of magnitude depending on sediment or rock type and its degree of consolidation. This is well illustrated in Fig. 8 (modified after Woodroffe, 2002) where recent tuffaceous sands may erode at several tens of meters per year, whereas the retreat of a basalt cliff (on average) proceeds at fractions of a millimeter per year (based on Emery and Kuhn, 1980, and Sunamura, 1992).
Fig.8
Fig.8
