Rock Cycle Research Articles

Diagenesis and Mass Transfer in Sandstone-Shale Sequences and the Sedimentary Cycle: ABSTRACT

An analysis of diagenesis and mass transfer is made drawing on the literature and our work from the Brazilian shelf and Barbados. It is shown (although not unanticipated) that the initial sedimentary mineral composition is a major control of diagenetic products. For example, dioctahedral clay minerals, chlorite, and quartz characterize arkoses, whereas trioctahedral clays and zeolites are most commonly found in lithic sandstone. Dioctahedral smectite-rich shales exhibit the classical smectite/illite to illite burial pattern. However, mafic, trioctahedral clay-rich shales show a burial sequence of saponite to chlorite/saponite mixed layer, a progressive increase of chlorite-rich phases with increasing burial depth. Other compositionally dependent reaction paths are also di cussed. To assess mass transfer between shale and sandstone during burial, all major diagenetic pathways must be known for both rock types. A model for the Brazilian shelf sandstone-shale sequence is used as an example of quantification of mass transfer. Both sands and shales act as nearly isochemical systems; sandstones lose less than 2% K+ to shales, and gain less than 3% H2O, H+, and CO2 during burial diagenesis. It is shown using data from Barbados and the literature that burial diagenetic reactions are essentially irreversible, at least until the stage of weathering. Thus, these reactions can be used to assess the amount of overburden removed. Comparison of the diagenetically produced trend of illite/smectite compositions with depth in Barbados to trends produced in areas which have undergone only subsidence (e.g., Gulf Coast) suggests that about 3,300 to 9,800 ft (2 to 3 km) of overburden have been removed in Barbados. The irreversible and nearly isochemical nature of burial diagenetic reactions places constraints on the role of diagenesis in the sedimentary rock cycle. An attempt is made to quantify the global importance of these reactions in the rock cycle. End_of_Article - Last_Page 1445------------

Benioff, Bowen, and Popcorn

For college students in introductory earth science, popcorn is an effective analogy for igneous processes and products. Specific analogies include: (1) pouring popcorn into a pan — subduction, (2) popping kernels — Benioff zone seismicity, (3) removing the lid during popping — coeval plutonism and vulcanism, also geologic hazards, (4) retrieving a previously-hidden, popcorn ball from the pot — crystallization and magmatic differentiation, and (5) eating the popcorn ball — the next steps in the rock cycle, weathering and erosion.

A quantitative model for the sedimentary rock cycle

A steady-state quantitative model for the sedimentary rock cycle is presented. The cycling of 11 major elements through the oceanic, atmospheric, biospheric, and rock reservoirs is shown. Flux rates are based on the estimated average geologic rates of transfer; the total flux of material through the oceans is about 1 3 that of today. The model is consistent with current estimates of the chemical composition of the average dissolved and suspended loads of streams, with the present-day composition of the oceans, that of the average sedimentary rock, and that of the average composition of precipitation. The mean residence time of the major elements in the cycle is about 400 million years; estimates are given for the cycling times of the individual elements.

The Karroo Volcanic Cycle

The petrogenesis of all the Mesozoic igneous rocks of south-eastern Africa is considered in relation to a single unifying thermotectonic event postulated as initiating the disruption of Gondwanaland. The cycle began with the rise of a body of potassium-rich picrific magma from a depth of at least 500 km. This is the predominating source material for most of the rocks in the northern part of the province (e.g. Rhodesia). Overlying and peripheral to the main magma body a zone of normal (sodic) magma was generated and gave rise to some of the northern rocks and most of the southern rocks (e.g. Karroo dolerites, basalts of Lesotho and Swaziland). Initial eruptive products were locally well fractionated but gave way rapidly to highly primitive picritic lavas representing what is termed the culminatory stage of the cycle. A subsequent stage in which the basalts emitted remained relatively uniform and moderately fractionated over a long period of time is referred to as the 'steady state' phase. A waning stage may be represented by more salic rocks such as the phonolites of Lupata (Mozambique) and the associated plutons of the Chilwa province (Malawi). The Cretaceous kimberlites of Southern Africa may represent the terminal igneous activity of the cycle. Superimposed on the cycle of mantle-derived rocks is a minor cycle of anatexis in the crust which gave rise to most of the acid rocks of the province, particularly the rhyolites of the Lebombo monocline. It is suggested that other provinces such as the Deccan Traps and the Tertiary of Greenland may show patterns of igneous activity which are similar to the Karroo cycle in important respects.

The Rock Cycle

The rock cycle is the succession of processes of differentiation that include weathering, sedimentation, selective mobilization, and fractionation. These processes cause a regrouping of the major elements of the earth's crust. The regrouping appears to be in the direction of producing granitic rock.

Geomorphic and Structural Relations of Tertiary Volcanics in the Northern Teton Range and Jackson Hole, Wyoming

The relations of Yellowstone Tertiary volcanics to Tertiary faults in the Teton Range indicate that major movements probably occurred in early Pliocene time, following deposition of Miocene basic volcanics and associated sediments and prior to outpourings of plateau rhyolite, and that minor movements continued into Recent time. Erosion surfaces representing the late-mature stage Black Rock cycle are covered by the rhyolite. This relationship and the youthful geomorphic aspect of the rhyolite plateau indicate that the rhyolite and younger valley basalt are probably of early, or early middle, Pleistocene age. More precise dating of the rhyolite depends upon the age assigned to the Buffalo drift, which rests on these volcanics.

A modified classification of economic deposits

In the proposed classification certain modifications are made in that of Bateman (2), 1 which relate it more closely to the rock cycle. It is a classification based on geochemical and petrologic concepts. Consideration is also given to the morphological characteristics such as structure, texture, and composition, as well as to the genetic relationships.Economic geology deals with the upper part of the lithosphere. Here the processes of differentiation and recombination form the various rocks, some of which have economically valuable constituents. In this work-rocks have been divided on the basis of their position at the time of formation, in relationship to the upper geospheres of the earth. If the rock or the valuable constituents of a rock are formed solely within the upper-most lithosphere it is endogenic in nature and belongs to the endogenic or major cycle. Those rocks that are formed by the involvement of the uppermost lithosphere with the atmosphere, hydrosphere or biosphere are exogenic in nature and belong to the exogenic or minor cycle.The endogenic cycle is subdivided into the magmatic (early and late),contact metasomatic, pegmatite, hydrothermal (cavity filling, metasomatic), metamorphic, and granitization deposits. The exogenic cycle is subdivided into surficial volcanic emanations, soils, water supply or resistate, hydrolyzate, oxidate, precipitate, evaporate, oxidation with super-gene enrichment and biolith (reduzate and incombustible).