Precambrian clastic sedimentation systems

Tropical deserts have existed sporodically on our planet from the Precambrian to the Present, and seem not to have been a permanent feature of its surface (Glennie 1987). Depending on their definition, tropical deserts and semi-deserts currently occupy between approximately one fifth and one third of the Earth's land surface, of which only about 4 ~ is covered by that popular concept of what a desert consists of--sand dunes. The remaining area comprises barren rock (both hill and plateau) with a variable cover of sediments transported by ephemeral streams (wadis, arroyos) to form the deposits at the terminal points of these streams. Varying with the ratio of water supply to the annual potential rate of evaporation, those terminal areas may be occupied by desert lakes that are generally of a temporary nature (permanent only if the water is provided from beyond the margins of the desert or is fed by groundwater), and become more saline as they become desiccated. The end product of such a situation is a salina or sabkha (area of sand, silt or clay, commonly encrusted with halite). There is no universally accepted definition of a desert. In its simplest form it can be defined as a barren tract of land over which rainfall is too limited or spasmodic to support vegetation adequately. Very few desert areas are completely devoid of vegetation, and many areas that fall within a desert in terms of average annual rainfall may have an even though sparse cover of plants that have adapted to the relatively arid environment in which they live. In this context, some writers define deserts as areas that have an average upper limit of 250 mm of annual rainfall, even though it may all fall in one storm and rainfall may not recur for several years. Perhaps more important than the rainfall itself is the ratio between it and the potential rate of evaporat ion-the aridity or desiccation factor. This is about 1 : 10 in some parts of the Australian Desert and up to 1:500 in areas of the Sahara (Cooke & Warren 1973). Tropical deserts exist either because the winds that cross their surfaces have a low relative humidity or there are no hills to push the winds up to colder cloud-forming altitudes; the Southwest Monsoon gives torrential rain over the Western Ghats of peninsular India but little over the arid low-lying Rajasthan Desert further to the N. Drawn from the high-pressure area of the Mediterranean Sea, the North African trade winds blow south and southwestward under clear skies, warming as they cross the Sahara, and so becoming capable of absorbing more moisture without condensation. By contrast, the southern hemisphere coastal deserts of the Namib and Atacama are often shrouded in mist but rarely experience rain. Cold deserts also occur in areas of low precipitation but lack a protective cover of vegetation because the moisture necessary for growth is frozen for a large part of the year. Allied to the low temperature, peri-glacial dune fields exist because of adequate supplies of fluvioglacial sediments that are unprotected by vegetation and have a grain size that is suitable for deflation once the summer melt-floods have ceased. Dunes are formed because strong dry winds can transport sand-size particles as a saltation load within a few metres of the land surface (sand storm). Because winds can drive sand over a hard immobile surface faster than over one of loose sand (Bagnold 1941), sand tends to accumulate in areas that are already sand covered. These sand patches grow in size whenever the velocity of the wind lessens and is no longer capable of keeping the grains in motion; dunes eventually result. Silt and clay-size particles, however, can be carried in suspension (dust storm) and by this means may be transported right out of the desert to be deposited elsewhere, as a marine horizon in the ocean or on land as loess. A cover of vegetation on the land surface tends to inhibit the aeolian transport of sand. Some ancient dune sequences that were deposited before higher plants began to colonize extensive areas of land during the Devonian (eg Precambrian dunes; Ross 1983) may reflect the ease with which sand could be moved by the wind in the absence of vegetation rather than any long-term aridity. The Devonian Old Red Sandstone sequences of Britain, for instance, are dominated by fluvial and lacustrine sediment that must indicate a fairly high annual rainfall, and yet dune sands have accumulated locally (Mykura 1983; Carruthers, this volume). To confirm a true desert origin for ancient sequences, additional evidence of aridity is needed. In hot deserts this is most easily recog-

Desert sediments: ancient and modern

Silica diagenesis: origin of inorganic and replacement cherts

Publisher Summary This chapter summarizes the sedimentary characteristics of non-marine environments. Non-marine evaporite deposits are common features of modern arid closed basins, but relatively few have been recognized in the geologic record. These evaporites are important because their depositional setting reflects both climatic and tectonic conditions and they may contain economically important mineral resources. Most modern non-marine evaporites and almost all ancient non-marine evaporates occur in saline lake deposits or their associated facies. The term saline lake includes perennial lake, ephemeral lake, playa-lake, playa, salina, salar, chott, salt pan, inland sabkha, and any other terms used to refer to areas that, at least intermittently, hold standing bodies of water. The general distribution of modern non-marine evaporites is shown in a diagram and references are presented in a table in the chapter. The distinction between marine and non-marine evaporites is not trivial and many ancient evaporites that are literally assumed to be marine may be, at least in part, non-marine in origin Furthermore, some of the deposits interpreted as non-marine may actually be marine (for instance, the evaporites of the Precambrian McArthur Basin, Australia).

Chapter 3 Depositional Environments of Non-Marine Evaporites

Pure quartz arenites are especially characteristic of lower Paleozoic and Proterozoic strata deposited in nonorogenic settings. A century‐long debate over the origin of these remarkably pure sandstones has remained unresolved, largely because they seem nonactualistic. The much greater importance of wind and fluvial processes prior to the Silurian appearance of macroscopic vegetation supported a physical origin, but it is now clear that both multicycling and intense chemical weathering can produce them. Multicycling seemed essential to account for their extreme textural maturity, with the exceptional rounding of many examples pointing to important eolian abrasion. Other attributes such as evidences of mixed sources, upward maturation, association with major unconformities, and an inverse relationship between labile grain content and grain size also were consistent with recycling. A single‐cycle origin proven in the modern humid tropics, however, is supported in the ancient record by examples with ...

https://www.jstor.org/stable/10.1086/375286

The Importance of Eolian Abrasion in Supermature Quartz Sandstones and the Paradox of Weathering on Vegetation‐Free Landscapes

Publisher Summary This chapter examines the eolian origin for part of a sequence of Proterozoic redbeds of the Hornby bay group in the Northwest Territories of Canada. Recognition of the eolianites and reconstruction of bedforms is based on the comparison of observed stratification with that described from modern, and some ancient, eolian sediment. Lateral changes in stratification styles within the eolianites are interpreted in terms of changing bedforms within the basin. This facies change reflects the complex interplay of wind regime, sand supply, air flow-bedform interaction, and climactic factors. The resultant basinal bedform profiles, which extend for 40 km perpendicular to depositional strike, are similar to those described from modern arid environment sand seas. The Bigbear system, which contains the eolianites, is 1.6 km thick, and records infill of an intermontane basin that had a southeast-dipping paleoslope. Prolonged erosion of the Bigbear system source area resulted in formation of a low-relief planation surface with an area of at least 3000 km 2 . Synsedimentary normal fault activity and deposition of intraformational conglomerates during later periods of Bigbear system deposition marked the beginning of crustal uplift and change from a southeast-dipping to southwest-dipping paleoslope.

Bigbear Erg: A Proterozoic Intermontane Eolian Sand Sea in the Hornby Bay Group, Northwest Territories, Canada.

Proterozoic aeolian quartz arenites from the Hornby Bay Group, Northwest Territories, Canada: Implications for Precambrian aeolian processes

Eruptive style and construction of shallow marine mafic tuff cones in the Narakay Volcanic Complex (Proterozoic, Hornby Bay Group, Northwest Territories, Canada)

Silcrete is widespread in the Thelon Basin and lower Coppermine Homocline, two Early to Middle Proterozoic cratonic cover sequences in the northwest Canadian Shield In the Thelon Basin, silcrete occurs as silicified lateritic weathering profiles, weathered tops of rhyolite flows, and silicified terrestrial volcaniclastic sediments In Coppermine Homocline, silcrete comprises silica-cemented rock fans at the basal unconformity, silicified evaporitic fracture-fills in basement, and silicified ephemeral lake sediments Petrographic studies of the silcretes indicate that they are strikingly similar to Cenozoic silcretes from southern Africa and Australia formed under arid climates The silcretes in both Proterozoic sequences are associated with climate-sensitive sediments, such as eolian tes and evaporites, that also suggest an arid environment of deposition Geochronologic data indicate that the silcretes represent part of an arid paleogeographic province that existed in the northwest Canadian Shield at about 185-17 Ga Although the Proterozoic silcretes are petrographically similar to Cenozoic arid climate analogs, the lack of land plants in the Precambrian suggests that geochemical criteria used to distinguish Cenozoic arid and humid climate silcretes may not be applicable to Precambrian examples

Paleoclimatic Significance of Widespread Proterozoic Silcretes in the Bear and Churchill Provinces of the Northwestern Canadian Shield

Handle everyday research tasks with reliable, citation-backed results

Your personal Research Agent to handle research tasks with citation-backed results

Popular Tasks used by Researchers

How can I help with your research?

Meet SciSpace

Get more enhanced response by uploading the PDFs you want me to reference.

No relevant PDFs in your library

SciSpace is the AI research assistant for academics. Run systematic literature reviews on 280M+ papers, and write papers with cited sources. Trusted by 1M+ students, PhDs & researchers.

SciSpace | AI for Research

Analyze PDFs

Code & Manuscripts

Funding & Grants

Literature & Patents

Medical & Clinical Data

Systematic Review

Visualize & Present

Web & Data

Build a Google Scholar-like website for your research.

Build a website

Create charts and images for your research

Create a Chart

Write a paper for submission to a journal

Draft a manuscript

Patent Search

Design eye-catching scientific posters in minutes.

Scientific Poster Generation

Systematic Literature Review

One task is running at the moment. Your messages will be shown right after.

Drag and drop or click here to browse

Loved by <highlight>1 million+</highlight> researchers

Extract a list of specific topics and their sources from unstructured text

Topics

Compare and analyze relevant papers that matches with your search

Papers

Get insights from PDFs and bookmarked papers from your library

My library

Recent searches

Try searching for:

Catch AI-generated content in scholarly and non-scholarly content

{ai} Detector

Ai Writer

Get PDF Summaries, highlighted text explanations 

Chat with PDF

Effortlessly create in-text citations and bibliographies in APA and 2,500 other formats

Citation generator

Get explanations, summaries, and answers on academic papers

Ease up your research workflow with {scispace}'s cohort of exciting AI tools

Elevate your academic writing skills and convey your ideas the way you want

Paraphraser

Explore our range of reading and writing tools

Your file is being prepared and should be ready in a few minutes. If it's a large file, it might take a bit longer. You can close this window, and we'll email you the file when it's done.

You have reached a maximum limit of <strong>{limit}</strong> columns in the table. Remove at least <strong>1</strong> column to add or create another one.

Gerald M. Ross

Author Tools

Chat about Author

Papers

Bigbear Erg: A Proterozoic Intermontane Eolian Sand Sea in the Hornby Bay Group, Northwest Territories, Canada.

Proterozoic aeolian quartz arenites from the Hornby Bay Group, Northwest Territories, Canada: Implications for Precambrian aeolian processes

Eruptive style and construction of shallow marine mafic tuff cones in the Narakay Volcanic Complex (Proterozoic, Hornby Bay Group, Northwest Territories, Canada)

Paleoclimatic Significance of Widespread Proterozoic Silcretes in the Bear and Churchill Provinces of the Northwestern Canadian Shield