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Cross-Bedding Robert T University of North Dakota UND Scholarly Commons Undergraduate Theses and Senior Projects Theses, Dissertations, and Senior Projects 1958 Cross-Bedding Robert T. Peterson Follow this and additional works at: https://commons.und.edu/senior-projects Recommended Citation Peterson, Robert T., "Cross-Bedding" (1958). Undergraduate Theses and Senior Projects. 35. https://commons.und.edu/senior-projects/35 This Thesis is brought to you for free and open access by the Theses, Dissertations, and Senior Projects at UND Scholarly Commons. It has been accepted for inclusion in Undergraduate Theses and Senior Projects by an authorized administrator of UND Scholarly Commons. For more information, please contact [email protected]. CROSS-BEDDING A thesis Presented to the Faculty of the Department of Geology University of North Dakota In Partial F\llfillment of the Requirements for the Degree BacheJ.or of Science of Geology by Robert Thomas Peterson Ivey 1958 Abstract Cross-bedding is formed in both tetrestrial and marine environ­ ments. The littoral deltaic type is probably the most common. It also occurs in lake deltas, £1.uvial bars and channel fills, beaches, sand dunes, and as eolian deposits modifying any of the others. It ranges in size from microscopic to tens of yards thick. Ripple­ marks are considered to be miniature dunes. The most important uses of cross-beading are as a top and bottom indicator and as a cuITent direction indicator. A genetic inference from t ~e shape of any cross-bedded structure cannot be absolutely made. ii TABLE OF CONTENTS Page ABSTRACT • • • • • • • • • • • • • • • • • • • • • • • • • • .• . ii LIST OF JLLUSTRATIONS •••• • •. • • • • • • • • • • • •. iv INTROIUCTION • • • • • • • • • • • • • •• • • • • • • • • • • 1 General Terminology Classification TYPES OF CROSS-B.1~DDING • • • • • • • • • • •••••••••• 3 Mtaic Use Origin and desription F.Luvial Sand bars Festoon type Wave-built R-efs Ripple-marks Eolian CROSS-BEDDING TO Dt,TERMINE SOURCE AREA • • • • • • • • • • • • 14 SUNI1ARY • • • • • • • • • • • • • • • • • • • • • • • • • • • 19 BIBLIOGRAPHY • .. • • • • • • • • • • • • • • • • • • • • • 20 iii LIST OF ILLUSTRATIONS F.i.gure Page 1. Cross-bedding Classification •••••••••• •. •. 2 2. Topsets, Bottomsets, and Foresets ••• • • ••• • •• • 2 3. Compound Foreset Bedding •••• • ••••••••••• 8 4. Tabular Cross-bedding •••••••• • • • • ••• • • 8 Variety of Compound Foreset Bedding ••••••••••• 8 Festoon Cross-bedding •••••••••••••• •. • 10 1. Compass Showing Directions of Dip ••••••••••• 10 B. ster eographic Net •••••••••••••••••• • 17 9.. Dip-direction Sylllbol • • • • • • • • • • • • • • • • • • 17 iv Introduction General terminology Cross-bedding is a common primary structure of elastic sedimentary rocks. It is formed by current action, either wind or water, moving and depositing sediment at an origin§]. slope. McKee and Weir (1953, p. 382) have defined cross-stratum as a single layer of homogenous ar gradational lithology deposited at an angle to the original dip of the formation and separated from adjacent layers by surfaces of erosion, nondeposition, ar abrubt change in character. .Angular, torrential, and regular are terms which have been applied to cross-strata which appear in cross-section as nearly straight lines. The term tangential has been applied to cross-strata which, in cross-section, appear as smooth arcs meeting the underlying surface at low angles. 11 A set is a group of essentiaJJ.y conformable strata or cross-strata, separated from other sedimentary units by surfaces of erosion, nondeposition, or abrubt change in character" {11cKee and weir, 1953, p. 382-383). Classification McKee and Weir {1953, P• 387) have developed a classification 1-mich utilizes the character of the lower bounding surface of sets as the major criterion (fig. 1). The three types of cross-bedding under this classification are simple, planar, and trough. The lower bounding surfaces are surfaces of nondeposition, planar surfaces of erosion, and curved surfaces of erosion, respectively. Subordinate c~teria are the shape of the sets, arching of cross-strata, angle of dip of cross-strata, and length of the cross-strata. Cross-laminations range .from microscopic to single units several 2 l;.Lassi1·ication 01· Cross-stratiried Um.t.s Basic criterion Suborpj_nate criteria Character of lower shape attitude symnetry arching I dip I length bounding surface (set) (set) (set) ( cros~-strata) Simple lentic- plunging symmetric concave high- small- ular angle scale Planar tabular non- asymrnet- straight medium- Plungiruz ric scale Trough wedge- convex l~T large- shaped anp...J.c scale Fig. 1 from McKee and Weir, 1953 .:. F.i.g. 2 A-topsets, B-foresets, C-bottomsets Modified from Lahee, 1952, p. 89 3 tens-of yards in coarse sands am gravels. The larger units are usually either of eolian or deltaic origin. The larger units could be mistaken for folded strata {Shrock, 1948, p. 248 ). Pettijohn {1957, p. 166) doubts that any genetic significance can be attached to the classification of McKee and Weir. Types of Cross-bedding Deltaic Use Cross-bedding is characteristic of littoral deltaic deposits and when found in outcrop is helpful for paleogeographic reconstruction. In general, this type of cross-laminae dips toward t he sea. This e makes it possible to determine the direction of origin of the sediment, and t hus to get an indication of the direction in which to expect changes in texture, composition, and t hickness of strata (Brett, 1955, P• 148 ). Origin and description A condition necessary for the building of a delta is that a sufficient volume of sediment must be transported by the stream. The ideal condition is an orogenic movement which will incise the streams and supply a great volume of sediment, at a fast rate, to a basin with a stable or sinking bottom. Under this condition the streams have a high gradient and can carry coarse particles. Most of t he sediment in traction is released immediately when the stream enters the sea and the material in suspension is carried farther out. Both of these deposits are laid down at a low angle en the basins edge. Since there is more coarse than fine- grained sediment supplied 4 • by a mountain buil ding orogeny., t his part of t he deposit becomes thicker t han t hat whose sediment i s suppli ed by t he material in suspension. This makes an inclined surf ace ·where t he two parts of t he deposit join. This surface continues to grow outward by de r osition of sediment being pushed over its upper edge. At this stage t hree distinct parts of the delta are known: (1) t he low­ angle surface of the coarser mat erial near t he stream mouth, (2) t he low-angle surface of t he suspension load deposit, anc (3) t he steeper sloping surface 'Which joins t hem. These individual parts of t he singl e layer are termed topset., bottomset., and foreset beds, respectively (fig. 2 ). When t he detritus i s mostly fine the foreset beds are built largely by material settling from suspension. The foreset beds are generally the ones preserved and they are t he laminae referred to as cross-laminae. Successive layers of varying lithology may be lain down, exposed by emergence of t he delta or eustatic lowering, and partly eroded away. Since t r:.e topset beds are t hose on t he top t hey are t he first to be removed. If t he sea l ater rises, t he deposition p~o~ess repeats and cross-strata are deposited upon underlying truncated foreset beds. The second set of cross-laminae may be entirely different in lithology and in the angles of dip of the topset, foreset, and bottomset beds. In this way t he sets are formed; "a group of essentially conformable strata or cross-strata, separated from other sedimentary units by surfaces of erosion, nondeposition., or abrubt change in character. 11 Originally t he dip of t he foreset beds decreases toward the top and toward t he bottom. If the plane of erosion is gelow the e. top of the foreset beds, a cross-section of the set will show the ar:tPc).e between the fore sets and this plane to be greater than the angle between the bottomsets and the lower bounding surface. Since most outcrops sho-wing cross-bedding occur with the plane of erosion between the topsets and bottomsets, the upper angle is greater than the lower, giving a concave upward appearance. This has long been used as an indicator of the top and bottom of beds (Twenhofel, 1950, P• 556). The angle of dip of the foresets is said by Nevin and Trainer (1927, p. ti55) to be dependent upon the coarseness of material and the stream velocity. In an artificial delta building experiment they found the dip to be that of the angle of repose: (1) sand-30 degrees and (2) clay-20 degrees or less. These angles were consider­ ably reduced by an increase of stream velocity. The clay beds showed a thickening downward and the sandy beds thickened upward. Tt-renhof el (1950, p. 556) states that the length and thickness of the foresets is determined by the volUllle of sediment and the stream velocity. An increase in volume of sediment causes an increase in thickness and an increase in stream velocity causes an increase in length. Schwarzacher (1953, p. 325-326, fide Potter and Seiver, 1956, P• 23) concluded that thin units represent a decrease in sediment and/or a deci:ease in grain size. Twenhofel (1950, p. 556) places :t,he maximum length of foresets at about 90 feet, and the average at about nine feet. Knight (1929, p. 20), in his study of the Fountain formation of the Laramie Basin, found a direct relationship between the texture and the scale of cross-bedding; the finer grit beds -- varying from a .few inches to five feet in length, the coarser grained ·6 beds from five to twenty feet, and the cobble comglomerates from forty to fifty feet in length.
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