Edwin Dinwiddie McKee

TERMINOLOGY FOR STRATIFICATION AND CROSS-STRATIFICATION IN SEDIMENTARY ROCKS

A terminology is suggested to aid the field geologist in describing the structures of stratified and cross-stratified rock units. Qualitative terms describing the character of rock layering are stratification, stratum, cross-stratification, cross-stratum, set, coset, and composite set. Quantitative terms applying to the thickness of stratification are very thick-bedded, thick-bedded, thin-bedded, very thin-bedded, laminated, and thinly laminated. Quantitative terms applying to the thickness of units into which the rock splits are massive, blocky, slabby, flaggy, shaly, platy, and papery. A classification of cross-stratification is suggested, based primarily on whether the lower bounding surface of a set of cross-strata is one of erosion or nondeposition and, if erosional, whether it is plane or curved. Features of secondary importance in this classification are the shape of set of cross-strata, the attitude of the axis, the symmetry of the cross-strata with respect to the axis, the arching of the cross-strata, the dip of the cross-strata, and the length of individual cross-strata.

Deformation of Lee-Side Laminae in Eolian Dunes

Processes responsible for structures in sand dunes consist of (l) primary deposition by saltation and creep and by settling from suspension, (2) redeposition accompanying avalanching, and (3) penecontemporaneous erosion. Characteristics of dune structures were examined in the field by introducing marker beds of magnetite at times of sand deposition, thus recording original surfaces and making possible the determination of subsequent changes. Similar structures were examined in the laboratory by testing processes and comparing the resulting structural forms with corresponding natural features. Avalanching in sand is of two types: sand flow and slumping. Deformational structures characteristic of each were recorded in the field and were reproduced in the laboratory. Nine varieties of deformational structures are recognized and described. Analysis of these structures suggests criteria for distinguishing compressional types (lower dune slope) from tensional types (upper dune slope). The analysis of deformational structures also serves to distinguish between forms developed in cohesive sand and those in non-cohesive sand. Since the degree of cohesion is largely a function of the amount of moisture in the sand at the time of avalanching, the deformational structures provide a means for recognizing original dry sand, wet sand, sand crusts, and saturated sand surfaces in ancient deposits. A testing of these criteria was made by comparing laboratory samples with those of dry sand at White Sands, New Mexico, and with those of coastal dunes (probably wet sand) in southern Brazil.

Primary structures of a seif dune and associated deposits in Libya

ABSTRACT Examination of a seif dune, an interdune area, and an adjoining serir near Sebhah Oasis in Libya indicates marked differences in texture and structure that should assist in the recognition of such deposits where they are preserved in the geologic record. The seif dune is largely composed of rounded, moderately well sorted, fine- to coarse-grained sand forming large-scale cross strata which dip at high angles in two nearly opposite directions. The interdune sand is consistently less well sorted and forms horizontal laminae or strata dipping at low angles. The serir characteristically contains clay, silt, and lag gravels, in addition to sand, giving the samples a double maxima in grade size analyses; it is poorly sorted and is comprised of low angle to horizontal strata, commonly contai ing carbon fragments or other impurities. Comparison of stratification in the seif dune with that in barchan dunes suggests that it should be possible to differentiate between these two types, where they are represented in ancient rocks, through an analysis of dip directions. Distribution of the steeply dipping cross strata of seif dunes to form two groups, with essentially opposite dip directions, contrasts with the general unidirectional dips of the steeply dipping cross strata in barchans.

Pliocene Uplift of the Grand Canyon Region—Time of Drainage Adjustment

Tertiary gravel deposits in ancient stream channels along the southern margin of the Colorado Plateaus of northern Arizona show by composition and structure that these deposits came from sources to the south and southwest at a time when central Arizona stood higher than the present Grand Canyon region. Three cobbles of basalt included in the gravel deposits have K-Ar ages of about 10.0 m.y., 12.2, and 12.4 m.y. showing that the major uplift of the plateau in northern Arizona had not taken place at that time. The present south-flowing drainage of the Verde River and neighboring streams resulted from final elevation of the northern Arizona region relative to central Arizona, and must have developed well before about 5 m.y.—the age of some basalts that flowed into the Verde Valley. Thus, the major relative uplift of the southern part of the Colorado Plateaus must have occurred within the 5 to 10 m.y. interval, or in early to middle Pliocene time. This time of uplift also was the time of major canyon erosion, including the cutting of Grand Canyon, within the Plateaus province.

Storm Sediments on a Pacific Atoll

ABSTRACT Deposits resulting from a single violent typhoon that struck a Pacific atoll consist of (1) coarse gravel ridges on the reef flat, (2) new or augmented beach ridges of gravel, (3) gravel sheets, ranging from one-half to three feet in thickness, across two-thirds of some islets, and (4) sediment redeposited on the lagoon sides of many islets. These storm deposits differ greatly in texture and structure from those formed in corresponding places under normal environmental conditions. In addition, storm sediments of the lagoon floor are relatively coarser at any particular depth than those formed under other conditions. Both storm and normal types are recognized in sections that expose earlier islet deposits.

SEDIMENTARY BASINS OF ARIZONA AND ADJOINING AREAS

An analysis has been made of the thickness of sediments accumulated in various parts of Arizona and adjoining areas during each of the periods of geologic history. From these and other sedimentary data, conclusions are drawn concerning the times and places of crustal movement in the region. For the Paleozoic and Mesozoic eras, data have been compiled, period by period, in the form of isopach maps. For the Cenozoic era, information on thickness of sediments in individual basins is shown on a base map. Five basic structures appear in Arizona on all of the Paleozoic maps. These consist of two positive areas (in the northeast and southwest respectively), the margins of two geosynclines (in the northwest and southeast), and a sag in central Arizona between the positive areas and connecting the two geosynclines. Prongs or submarine ridges extending basinward from the positive areas, and areas of more rapid sinking between them, appear to have shifted position from period to period. Four major changes in isopach pattern are as follows: (1) The Uncompahgre-San Luis highlands of Colorado were formed and north central New Mexico became an area of more active and extensive sedimentation starting with the Pennsylvanian. (2) Central and southern Arizona became a high area contributing sediments to the north during Triassic time. (3) Northern Arizona received no sediments in Lower Cretaceous time, but in southern Arizona an uplift was followed by development of a deep, northwestward-trending trough from Mexico. (4) Deposits of Upper Cretaceous age were accumulated across northern Arizona in and near a sea that expanded westward from the Rocky Mountain trough.

Original structures in Colorado River flood deposits of Grand Canyon

ABSTRACT Original structures of distinctive character are developed during deposition of flood deposits along the Colorado River in Grand Canyon. A description of the types of bedding and cross-lamination in these deposits is given and a comparison is made between them and the original structures formed in certain other environments including the Colorado River delta and typical sea beaches. Two forms of contemporaneous deformation developed in Colorado River flood deposits are described and discussed.

K-Ar Age of Lava Dam in Grand Canyon

The K-Ar age of the basal basalt flow at the bottom of the “Lower Canyon group” of lavas near Toroweap fault is 1.16 ± 0.18 standard deviation (sd) m.y. This represents a minimum age of Grand Canyon, for at the time the lava formed, the canyon was essentially as deep as it is today. Since that time the Colorado River has cut through the 550-ft lava dam at the mouth of Toroweap Valley, an additional 50 ft of Paleozoic strata below, and through one or more younger lava dams in the area; downstream it has cut through 100 feet of younger intracanyon lavas.

CORRELATION OF THE PERMIAN FORMATIONS OF NORTH AMERICA

The chart (PI. 1) indicates the present stratigraphic classification of the Permian rocks in each important area of outcrop in North America and the time relations of the deposits in the several areas as now understood. Annotations in the text suggest the evidence for many of the correlations and point out unsolved and controversial problems.

Experiments on Formation of Contorted Structures in Mud

Contorted structures can be formed in mud or sand as a result of differential loading. Fifteen sets of experiments were conducted in water tanks to test various factors of possible significance in the contortion of mud by loading. Of six factors tested, the most significant was distribution of load, but others affecting the type of structure under certain conditions were (1) the manner of depositing the mud, (2) the form of the underlying surface, (3) the direction of loading, and (4) the movement or lack of movement of water during loading. Organic material was shown to be unneccessary in forming conical structure or convolute bedding. Strength of base had little or no influence on convolute-structure development. Contortions ranged from the simple anticlinal type with vertical axial plane, commonly referred to as convolute, to structures with gently dipping axial planes, to others with lateral extensions or “flames” from the apexes, and, finally, to those with complex overturned folds. Causes of these variations were determined in terms of the factors listed above. Some additional forms of contorted bedding result from other types of penecontemporaneous deformation such as slumping from undermining or from oversteepening, differential lateral movement, and surface drag; these forms differ from those structures formed by loading.