Local Dip Research Articles

Tabular and Trough Cross-bedding: Comparison of Dip Azimuth Variability

ABSTRACT Two well exposed outcrops of the Homewood Sandstone in western Pennsylvania provide an excellent opportunity to document and compare local dip azimuth variability of tabular and trough types of cross-bedding. Tabular-shaped cross-beds occur at one locality; trough-shaped units at the other. All measures of scatter--consistency ratio, standard deviation, and spread--are appreciably greater for the randomly measured foreset dip azimuths of trough cross-bedding. If readings along trough axes only are considered, the scatter is greatly reduced and is comparable to that displayed by tabular cross-bedding foresets. At each locality the current direction is uniform both stratigraphically and areally. Thus, the markedly different distributions of foreset azimuths for these two cross-bedding types is related to an inherent property of the sedimentary structure, the curvature of foresets within individual units.

Structural History of Houma Embayment: ABSTRACT

The Houma embayment is a structural downwarping of the middle Miocene continental shelf which was filled contemporaneously with a northward-thickening wedge of deltaic-plain sandstone and deep-marine shale. The embayment wedge is terminated on the north by an arcuate, down-to-the-coast, growth-fault End_Page 2323------------------------------ system. Toward the south, the wedge thins to a few hundred feet where it is down-faulted below existing well control by post-embayment faulting. A series of paleostructural maps demonstrates that the basin subsided by northward rotational tilting into the boundary-fault system. Hydrocarbons accumulate where regional north dip is interrupted by local south dip or, as in most cases, where northward-plunging noses are terminated on the south by faulting. It is the thesis of this paper that regional north dip and thickening of rotational blocks into arcuate down-to-the-coast growth faults can be observed in all similar embayment-type structures, of which there are many in south Louisiana. The Houma embayment is at relatively shallow depths, and the presence of major hydrocarbon accumulati ns in the embayment wedge has encouraged a great amount of deep drilling. Information obtained from the deep drilling allows the structural history of the Houma embayment to be reconstructed accurately and used as a model for deep exploratory drilling in other areas. End_of_Article - Last_Page 2324------------

The Beech Gaps of the Great Smoky Mountains

The gaps are one of several striking vegetation types of higher altitudes in the Great Smoky Mountains of Tennessee and North Carolina. The term beech gap 1 refers to relatively high (above 5,000 feet elevation) forests of (Fagus grandifolia 2), which usually are found on the south-facing slopes of local dips in east-west or northeast-southwest ridges. Their boundaries are very sharply marked. The dominant forest type at elevations greater than about 5,000 feet in the Great Smokies is spruce-fir (Picea rubens-Abies fraseri). The forests occur as small stands or deciduous islands in this forest (Fig. 1). Present over considerable areas also are successional burn forests, composed of a variety of young trees. These occur generally where spruce-fir or spruce had previously stood. The area covered by high elevation forest is quite small. Ashe (1922) stated that in North Carolina it was doubtful if the aggregate area exceeds 10,000 acres. The total acreage in Tennessee is in all probability less. Some of the areas that formerly supported forests have been cleared in historic times. Cain (1937) cites as examples Newfound Gap and Indian Gap, the two lowest and widest passes through Forney Ridge between Knoxville, Tennessee, and Asheville, North Carolina. Some still exists at Newfound Gap, but the forest is now completely absent from Indian Gap. In 1947 and 1948, studies were made by the present author of a number of

The tectonics of the Tintagel area, North Cornwall

Summary The paper describes the structure of the coastal region of North Cornwall in the neighbourhood of Boscastle, Tintagel and Trebarwith Strand. The area has been remapped, and the tectonics originally outlined by Dewey (1909) have been reinterpreted. The revision has shown that the epidiorite at Tintagel is more extensive than hitherto believed. Massive epidiorite grades into chlorite-schist which is often very similar in appearance to the sheared volcanic rocks of the area; but the schist derived from the intrusive rock can be identified by its local thermal metamorphism of the phyllites which lie above it. The important tectonic episode in the area was that of thrusting, in which the upper beds moved from the south-south-east towards the north-north-west. The direction and sense of the translation is indicated by various minor structures, e.g. slickenside striations, elongations of lava pillows, amygdules etc., in a (the movement direction); and by drag-folds, zig-zag folds, boudinage structure, tension fractures etc. in b (normal to a ). Three main thrust-zones are recognized, and they separate the rocks into three tectonic units or thrust-slices which lie on an autochtonous foundation. The higher thrust-slices lie to the west of the lower ones because they have been preserved from erosion by the local westerly dip aided by normal faulting which downthrows to the west-north-west. The more important of these normal faults pass through Tintagel Haven and form, inter alia , the great fault complex seen in the cliffs of the Island. Similar faulting occurs at Trebarwith Strand, Lill Cove and Hole Beach. In each case the downthrow is such that the flat thrust-plane there is dropped to the west. The general direction of strike in the area is controlled by the gentle doming due to the Davidstow Anticline. This hemidome pitches north-westerly, and it developed later than the thrusting episode. The evidence shows that there is no connexion between the thrust movements which carried the beds forward at least 2½ miles (or more) and the warping of the anticline. The tectonics at Tintagel are thus no longer to be considered the result of thrusting from the west-north-west, but are rather the result of a major movement from the south-south-east, and so become an integral part of the Hercynian tectonic pattern of Cornwall.

The Middle Devonian in the Pine Point Area, N.W.T.: ABSTRACT

Extensive drilling on the south shore of Great Slave Lake has yielded new information on the structure and lithology of the Pine Point, Presqu'ile and Slave Point formations of Middle Devonian age. In most places along a 5 mile-wide belt, traced 30 miles in a N. 70° E. direction from a point 11 miles south of the mouth of the Buffalo River, the Slave Point formation varies from an upper phase of thin-bedded fossiliferous limestones to a lower phase which is mainly sub-lithographic limestone. These beds overlie coarsely recrystallized, vuggy dolomite of the upper part of the Presqu'ile which rests upon a considerable thickness of fine, sugary-textured and compact crystalline dolomites. The Presqu'ile dolomites overlie the shaly limestones of the Pine Point formation. North of this belt, the coarse-grained dolomite lenses out and, in the western part of the area, the stratigraphic interval is occupied by green shales. The lower dolomites interfinger toward the north with shaly limestones lithologically similar to the Pine Point formation. South of the belt, the coarse dolomite grades into finely crystalline dolomite. Although the regional dip is westerly, the structure is complicated by local folding and initial dips. North of the belt, northerly dips up to 125 feet per mile are found. South of the belt, the beds commonly dip more gently south. Sphalerite, galena, and marcasite occur in greatest abundance at three horizons within the Presqu'ile formation. Sulphur, gypsum, pitch-like bitumen and small traces of oil are found in increasing abundance toward the west in the Slave Point and Presqu'ile formation. The Presqu'ile formation shows many of the characteristics of a biostromal deposit with off-shelf facies extending into the basin of Great Slave Lake. The structure and development of the Paleozoic rocks may be related initially to the structure and topography of the underlying pre-Cambrian rocks. End_of_Article - Last_Page 2252------------

Notes on Isomagnetic Charts: V—The Occurrence of Local Dip‐ Poles

Summary—It is shown that local magnetic dip‐poles occur in pairs or groups of pairs, and the nature of the isomagnetic lines and magnetic meridians is illustrated in the simpler cases, It is also shown that a local magnetic disturbance due to a single magnetic pole, if sufficiently intense, can produce one pair of dip‐poles, and that if the disturbance is due to the field of a sufficiently intense local dipole, there may be either one or two local pole‐pairs, according to the orientation of the dipole. If the dipole‐field is due to magnetization induced in a roughly spherical mass of magnetite or iron pyrites, by the existing normal field, the necessary susceptibility of the mineral, if it is to produce local dip‐poles, is shown to depend on the ratio of its radius to its depth, and on the local value of the normal horizontal intensity H. A classification of pie‐pairs according to this value of H and the distance between the poles of a pole‐pair is suggested.

Buried and Resurrected Hills of Central Ozarks

It is pointed out in this article that in the central Ozark region there are exceptional opportunities to study the relation of buried hills to the resultant structure in superjacent sediments. The first part of the article is a presentation of the actually observed facts, without discussion. In the second part, the writers present their interpretation. Among the observed facts are the great relief of the pre-Cambrian surface; the thinness and character of the overlapping sediments; the observed dips, in some places as steep as 30°, in the adjacent limestones; the vertical magnitude of the resulting domes, with a maximum of more than 500 feet; the discovery of otherwise unsuspected porphyry knobs from the abnormal elevation of sedimentary contacts; and the remarkable lack of alignment of the dips, which follow a pattern of pre-Cambrian dendritic drainage. In explanation of these facts, the writers point out that the dips are not the result of intrusion, as the igneous rocks are older than the sediments; that isostasy is not an adequate explanation of such limited areal units; that recurrent vertical uplifts can not explain such local dips with the observed arrangement; that the total lack of alignment precludes lateral crustal deformation; that the influence of compaction is at best wholly inadequate; that there is no evidence of sufficient solution to have produced the observed results; and that finally, when all the known factors are given due consideration, original deposition seems to be the most adequate explanation. The persistence of the structures upward is held not to be much greater than twice the height of the buried hill. /P>

Stratigraphy and Structure of the Smoky Hill Chalk in Western Kansas

The general section of the Smoky Hill is described in detail. The key horizons consist of bentonite streaks and hard ledges with definite succession and intervals. The difficulties introduced by the effects of weathering and by the complicated structures are considered. The faulting in the Smoky Hill probably occurred during the Tertiary, and was presumably produced by the adjustment of the brittle chalk to deformation and not by compaction. Structure maps based on local dips in the chalk are unreliable.

Subsurface Structure of Some Unsymmetrical Anticlines in the Rocky Mountains

In the Rocky Mountain states the location of deep test wells on unsymmetrical anticlines has been a hazardous undertaking, and wells in surprisingly large numbers have been poorly located. This paper is a study by means of cross sections of many such folds. Past mistakes are analysed and the suggestion made that more attention be paid to regional conditions and the character of the entire anticline and less to the local dips at the surface axis. It will be necessary to survey the course of deep wells and determine the amount and direction of their deviation from vertical, in order thoroughly to understand subsurface structure on unsymmetrical anticlines.

The Relationships of the Olentangy Shale and Associated Devonian Deposits of Northern Ohio

In the flat glaciated region to the south and east of Sandusky there are few outcrops of the older rocks. The drainage is mostly by small, weak streams which have not yet had time to erode extensive valleys, and railroads have not been compelled to cut deeply in order to establish their grades. About seven miles south of Sandusky, however, where the land is a little higher than in the city and the mantle rock is exceedingly thin, some of the creeks have exposed small sections of bedrock which are somewhat exaggerated by a considerable local dip. Two of the more important of these are to be found along Plum and Pipe creeks. These sections have been discussed elsewhere,2 but a recent study of the region has added some valuable facts to those formerly given and has made it possible to correlate this Ohio Hamilton with the Devonian deposits of the same age to the north of Lake Erie. Plum Creek heads about nine miles southeast of Sandusky and flows, in a general northerly direction, to the lake. At a point about two miles east-northeast of Prout station, on the Baltimore & Ohio Railroad, it cuts into Huron shale, and a little farther north into the Hamilton beds, exposing the following section: