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Processes controlling the composition of clastic sediments
Book reviews
0264-8172(95)00065-8
Processes Controlling the Composition of Clastic Sediments Mark J. Johnsson and Abhijit Basu (Eds) Geological Society of America Special Paper 284; 1993; ISBN 0 8137 2284 5; 342 p p ; softbound. Price: £45.00 At some point in many sedimentologists' careers they are faced with the dreaded task of sandstone point counting. Many at this stage falter and shy away from the path commenced by Henry Clifton Sorby, whilst others realize that detailed sandstone petrography unlocks a wealth of information about weathering, transportation, source area geology, tectonic setting, climate, sediment dispersal patterns and diagenesis. However, even for the initiated, the complex inter-play of processes which affect the final composition of a clastic sediment is often not fully appreciated. This volume is a key future reference for the understanding of these processes. The volume arose following a thematic meeting held at the 1991 annual meeting of the Geological Society of America. The editors' stated aim was to assemble a group of papers that, taken together, sketched an outline of the integrated processes that shape the composition of clastic sediments. The editors have clearly reached that aim with a volume that will prove to be essential reading for anyone involved in sandstone provenance studies. The book is 342 pages long and contains 20 separate papers which have been separated into six sections: introductory papers (2), source rocks and tectonic setting (6), chemical weathering controls (3), mechanical controls (3), compositional controls in depositional systems (3) and, finally, three papers which deal with the estimation of rates and fluxes. There is an understandable bias towards North American examples, but this should not deter the European reader! As in any conference volume, the quality of the individual contributions is variable, although the overall standard is high. Included within the volume are a number of key papers well worth reading. The papers which I most enjoyed reading include the following. Johnsson, in the first paper, provides an excellent review of the inter-related processes, other than source rock composition, which ultimately control sediment composition. This paper should be compulsory reading for anyone working on sandstone provenance! Mclennan et al. show that geochemical and isotopic data, when combined with petrographic data, clearly aid the discrimination of the tectonic setting of the source rocks. Similarly, Girty et al. provide a good example of the use of REE/Th/Sc geochemistry to fingerprint the tectonic setting of an argillitechert sequence. Heins provides an interesting paper examining the relative importance of source rock texture versus climate as controls on the composition of modern sands derived from five separate granodiorite plutons. He shows that the source rock texture exerts a greater control than climate. This is an important concluson that merits further investigation by examining modern sands derived from plutons in a wider climatic range than in Heins's paper and, conversely, by examining the composition of sand derived from a texturally variable pluton in a single climatic regime. This paper leads on to several interesting studies on the chemical and mechanical controls on sandstone composition. Johnsson et al. examined the intensity and duration of chemical weathering of basalt on Hawaii. The soil clay mineralogy is a direct function of the level of precipitation (which controls the weathering intensity) and the duration of weathering. In a region with variable rainfall, different clay species may be produced from the same source terrain. Morton and Johnsson show that particular heavy mineral species are selectively removed due to chemical weathering during short-term alluvial storage. Dutta et al. provide a convincing theoretical and experimental study which shows that quartz enrichment in aeolian sands is a function of mechanical fracturing of feldspars as a consequence of impacting saltating grains and the subsequent
350
aeolian removal of the fractured feldspar grains. Again in an experimental study, Osborne et al. used a tumbler to simulate over 200 km of bedload transport for granodiorite samples to investigate the nature of grain fracturing. Whilst coarser particles do fracture along crystal boundaries as expected, finer grained particles fracture across mineralogical boundaries, implying that the commonly cited grain size control on sediment composition may not always hold true. Perhaps sedimentologists need to work with mineral process engineers to examine in more detail the mechanisms of grain liberation! Cavazzo et al. examined the recycling of sedimentary rocks by studying modem sands derived from a turbidite-dominated source terrain. Whilst the sandstone components of the turbidites were recorded within the modern fluvial sediments, the low preservation potential of the associated mudrocks meant that they were effectively lost during reworking. Hydrodynamic sorting, due to different depositional processes operating within different environments, also leads to a modification of the sandstone composition, as shown by Kairo et al. Soreghan and Cohen show that within the Lake Tanganiyka section of the strongly asymmetrical East African rift basin, the tectonics of each part of the basin margin influence the sandstone composition. The sandstone composition differs between the hanging wall and footwall margins of the rift basin as a result of factors such as drainage basin size, slope, etc. However, whether this tectonic influence could be recognized and clearly distinguished from the other controls in the ancient rock record seems unlikely. Molinaroli et al. show that Saharan dust is being deposited within the central Mediterranean basin and can be distinguished from anthropogenic aerosols by examining its grain size, mineralogy and chemistry. Palomares and Arribas provide an interesting study of recent sands derived from source areas comprising either only a single source lithology or a combination of two differing lithologies. By comparing the petrography of sand samples with the extent of the differing source lithologies within the respective drainage basins, it is possible to examine the relative contributions from the different sources. They show that an outcrop area of only 5% granitoid significantly modified the sediment modal composition and they estimated that, in their study area, granitoid rocks were 14-20 times more efficient in producing sand grains than slates and schists. This conclusion means that we should be cautious in the reconstruction of palaeogeographical/palaeogeological models on the basis of sandstone composition. The final paper in the volume, by Molinaroli and Basu, suggests that much of the traditional petrographic data only provides a qualitative assessment of the source rock assemblages. In a case study comparing the nominal ratio of garnet to zircon and by examining the texture and chemistry of detrital opaque oxide phases in both Holocene and Cretaceous-Tertiary sands derived from the same source area, they show that it is possible to obtain a quantitative estimate of the proportions of the different source rocks present. Developing techniques for quantitative provenance studies is one aim for future petrographers, So, in summary, the papers in this book should be read by anyone interested in sandstone petrography. In fact, they should also be read by anyone not interested in sandstone petrography so that they can find out what they are missing! By integrating the results of experimental studies with work on modern sands and from the ancient rock record, we are gaining a clearer insight into the processes controlling sandstone composition. The authors and editors are to be congratulated for producing this book.
Marine and Petroleum Geology 1996 Volume 13 Number 3
Dr D. Pirrie Camborne School of Mines, University of Exeter, UK
0264-8172(95)00065-8
Processes Controlling the Composition of Clastic Sediments Mark J. Johnsson and Abhijit Basu (Eds) Geological Society of America Special Paper 284; 1993; ISBN 0 8137 2284 5; 342 p p ; softbound. Price: £45.00 At some point in many sedimentologists' careers they are faced with the dreaded task of sandstone point counting. Many at this stage falter and shy away from the path commenced by Henry Clifton Sorby, whilst others realize that detailed sandstone petrography unlocks a wealth of information about weathering, transportation, source area geology, tectonic setting, climate, sediment dispersal patterns and diagenesis. However, even for the initiated, the complex inter-play of processes which affect the final composition of a clastic sediment is often not fully appreciated. This volume is a key future reference for the understanding of these processes. The volume arose following a thematic meeting held at the 1991 annual meeting of the Geological Society of America. The editors' stated aim was to assemble a group of papers that, taken together, sketched an outline of the integrated processes that shape the composition of clastic sediments. The editors have clearly reached that aim with a volume that will prove to be essential reading for anyone involved in sandstone provenance studies. The book is 342 pages long and contains 20 separate papers which have been separated into six sections: introductory papers (2), source rocks and tectonic setting (6), chemical weathering controls (3), mechanical controls (3), compositional controls in depositional systems (3) and, finally, three papers which deal with the estimation of rates and fluxes. There is an understandable bias towards North American examples, but this should not deter the European reader! As in any conference volume, the quality of the individual contributions is variable, although the overall standard is high. Included within the volume are a number of key papers well worth reading. The papers which I most enjoyed reading include the following. Johnsson, in the first paper, provides an excellent review of the inter-related processes, other than source rock composition, which ultimately control sediment composition. This paper should be compulsory reading for anyone working on sandstone provenance! Mclennan et al. show that geochemical and isotopic data, when combined with petrographic data, clearly aid the discrimination of the tectonic setting of the source rocks. Similarly, Girty et al. provide a good example of the use of REE/Th/Sc geochemistry to fingerprint the tectonic setting of an argillitechert sequence. Heins provides an interesting paper examining the relative importance of source rock texture versus climate as controls on the composition of modern sands derived from five separate granodiorite plutons. He shows that the source rock texture exerts a greater control than climate. This is an important concluson that merits further investigation by examining modern sands derived from plutons in a wider climatic range than in Heins's paper and, conversely, by examining the composition of sand derived from a texturally variable pluton in a single climatic regime. This paper leads on to several interesting studies on the chemical and mechanical controls on sandstone composition. Johnsson et al. examined the intensity and duration of chemical weathering of basalt on Hawaii. The soil clay mineralogy is a direct function of the level of precipitation (which controls the weathering intensity) and the duration of weathering. In a region with variable rainfall, different clay species may be produced from the same source terrain. Morton and Johnsson show that particular heavy mineral species are selectively removed due to chemical weathering during short-term alluvial storage. Dutta et al. provide a convincing theoretical and experimental study which shows that quartz enrichment in aeolian sands is a function of mechanical fracturing of feldspars as a consequence of impacting saltating grains and the subsequent
350
aeolian removal of the fractured feldspar grains. Again in an experimental study, Osborne et al. used a tumbler to simulate over 200 km of bedload transport for granodiorite samples to investigate the nature of grain fracturing. Whilst coarser particles do fracture along crystal boundaries as expected, finer grained particles fracture across mineralogical boundaries, implying that the commonly cited grain size control on sediment composition may not always hold true. Perhaps sedimentologists need to work with mineral process engineers to examine in more detail the mechanisms of grain liberation! Cavazzo et al. examined the recycling of sedimentary rocks by studying modem sands derived from a turbidite-dominated source terrain. Whilst the sandstone components of the turbidites were recorded within the modern fluvial sediments, the low preservation potential of the associated mudrocks meant that they were effectively lost during reworking. Hydrodynamic sorting, due to different depositional processes operating within different environments, also leads to a modification of the sandstone composition, as shown by Kairo et al. Soreghan and Cohen show that within the Lake Tanganiyka section of the strongly asymmetrical East African rift basin, the tectonics of each part of the basin margin influence the sandstone composition. The sandstone composition differs between the hanging wall and footwall margins of the rift basin as a result of factors such as drainage basin size, slope, etc. However, whether this tectonic influence could be recognized and clearly distinguished from the other controls in the ancient rock record seems unlikely. Molinaroli et al. show that Saharan dust is being deposited within the central Mediterranean basin and can be distinguished from anthropogenic aerosols by examining its grain size, mineralogy and chemistry. Palomares and Arribas provide an interesting study of recent sands derived from source areas comprising either only a single source lithology or a combination of two differing lithologies. By comparing the petrography of sand samples with the extent of the differing source lithologies within the respective drainage basins, it is possible to examine the relative contributions from the different sources. They show that an outcrop area of only 5% granitoid significantly modified the sediment modal composition and they estimated that, in their study area, granitoid rocks were 14-20 times more efficient in producing sand grains than slates and schists. This conclusion means that we should be cautious in the reconstruction of palaeogeographical/palaeogeological models on the basis of sandstone composition. The final paper in the volume, by Molinaroli and Basu, suggests that much of the traditional petrographic data only provides a qualitative assessment of the source rock assemblages. In a case study comparing the nominal ratio of garnet to zircon and by examining the texture and chemistry of detrital opaque oxide phases in both Holocene and Cretaceous-Tertiary sands derived from the same source area, they show that it is possible to obtain a quantitative estimate of the proportions of the different source rocks present. Developing techniques for quantitative provenance studies is one aim for future petrographers, So, in summary, the papers in this book should be read by anyone interested in sandstone petrography. In fact, they should also be read by anyone not interested in sandstone petrography so that they can find out what they are missing! By integrating the results of experimental studies with work on modern sands and from the ancient rock record, we are gaining a clearer insight into the processes controlling sandstone composition. The authors and editors are to be congratulated for producing this book.
Marine and Petroleum Geology 1996 Volume 13 Number 3
Dr D. Pirrie Camborne School of Mines, University of Exeter, UK