MEGATRENDS IN NORTH AMERICAN PALEOCURRENTS.
© 2000 Arthur V. Chadwick
Paleocurrents are widely used in attempting to define basinal geometry and sediment dispersal patterns. Potter and Pettijohn (1977) have suggested paleocurrents on a larger scale be used for studying crustal evolution of a continent including the defining of borderlands and the timing of uplift and tectonics. As part of a study aimed at refining basinal geometry we have accumulated data from over half a million measured paleocurrent vectors at fifteen thousand locations from the Phanerozoic in North America. This database in connection with a graphical program capable of displaying the data in a variety of formats has enabled us to consider suprabasinal trends at scales which have not been possible in the past.
We have verified the stable southwesterly pattern of paleocurrents across the craton documented by others [Potter and Pryor(1961), Seeland(1968)] and have chronicled its persistence with some variation throughout the Paleozoic. In the Mesozoic the currents exhibit increasing variability and shift from predominantly westerly to predominantly easterly. By mid Cenozoic there is no discernible continent-wide paleocurrent pattern, reflecting expected tertiary basinal sedimentation. These patterns and transitions must accompany major changes in global current trends.
We are presently using this database to further define the effects of major orogenic and tectonic influences on depositional patterns across the North American continent.
During the past thirty years, the determination of paleocurrent directions has become a more or less routine part of most sedimentological studies. However, the potential usefulness of this information (Potter and Pettijohn, 1977) appears not to have been generally appreciated as evidenced by the large amounts of data contained in theses and dissertations that have not been published elsewhere. Occasional papers have attempted to apply this information to the solution of regional and continental problems outside the constraints of a localized basin (Potter and Pryor, 1961; Seeland, 1968; and especially, Bigarella, 1973, etc.) but studies of this type are rare. In studies relevant for North America, Potter and Pryor recognized the vertical persistence of southwesterly directional indicators over the span of the Phanerozoic in the region including and surrounding the upper Mississippi Valley. Seeland documents similar southwesterly currents horizontally across most of the continent for the basal post unconformity (basal Sauk) transgression. A goal of this study was to integrate and extend these two studies in an attempt to better understand the nature of sedimentary process across the craton, and to further define what areas may or may not have been sediment source areas.
While the continental distribution of paleocurrents would be expected to yield interesting information with respect to the geometry of basins and direction of sediment sources, it would seemingly be irrelevant to sum the currents across the continent as a whole, since basins by definition are flanked by potential source areas on all sides, and there is no compelling reason to think that sediments transported by several different mechanisms over an extended time frame should sum to any given direction. That this turns out not always to be the case is the subject of this paper.
METHODS AND RESULTS
In the process of attempting to reconstruct basinal geometry using paleocurrent patterns we have obtained data from over half a million measured paleocurrent directions at 15,615 localities on the North American continent. These measurements span from Lower Precambrian to Holocene, and include data from 1020 formal units. The data were derived from numerous published sources, as well as unpublished theses and dissertations. Geographic location, directional data, lithology, depositional environment, areal extent of study, number of data points, type of indicators, dispersion, and stratigraphy are attached to each observation.
The diagrams presented here illustrate the cumulative data for each of the geologic intervals shown, as a current rose. Data were analyzed using the Rayleigh statistic for circular data. Most of the data sets far exceeded that confidence level. The top row of figures gives the direction of transport using all data for all of North America for that interval. The three sets of figures below this are divided according to the depositional environment designated by the original author for the particular units being studied. The marine category contains data designated as either shallow or deep marine environments, including turbidites. The fluvial category also includes those studies describing fluvial-deltaic deposition. The subaerial category contains data from formations described as dune deposits as well as those considered to be alluvial.
This paper presents the results of a study investigating and exploring the nature and distribution of paleocurrents on a continent wide scale over the entire sedimentary history of North America.
1. In the Cenozoic, as predicted, depositional processes reflect the predominantly basinal character of the Tertiary. With the noted exception of the Paleocene, both
Paleogene and Neogene sedimentation reflect no continent wide influence aside from the continuing diffuse influence of the Mississippi drainage. Cenozoic sedimentary patterns have more in common with the Precambrian than they do with either preceding era.
2. In the Mesozoic, generally as expected, currents are broadly dispersed, in all environments, with a gradual shift from predominantly westerly in the Triassic (but with much scatter) to southerly and southeasterly in Jurassic and Lower Cretaceous, to strongly easterly in the Upper Cretaceous. Sediments that cross the Cretaceous-Paleocene boundary, as well as sediments of the Paleocene proper continue this easterly flow, in marked contrast to the strongly westerly Paleozoic trend, and to the general Mesozoic scatter.
3. During the Paleozoic, in sharp contrast to Mesozoic, Cenozoic and Precambrian tendencies, clear and persistent continent-wide trends are normative. Sediments moved generally from east and northeast to west and southwest across the North American Continent. This trend persists throughout the Paleozoic and includes all sediment types and depositional environments. A gradual shift is seen from lower and mid Paleozoic westerly trends to upper Paleozoic southerly trends.
The Quaternary and Tertiary paleocurrents reflect prevalent basinal influences and thus the summation of paleocurrents across the continent yields expected white noise. The exception to this generality is in the Paleocene where the trend is decidedly toward the east. There is no generally satisfactory explanation for this Paleocene distribution, although it is consistent with the trend in Upper Cretaceous and across the Cretaceous-Paleocene boundary. The opening of the Atlantic in Upper Cretaceous may have influenced sedimentation, although most Paleocene sediments are far to the west.
Mesozoic paleocurrents express a gradual shift from the easterly directed currents of the upper portion through the widely scattered mid Mesozoic currents to the generally westerly directed Triassic currents. Below Upper Cretaceous the Mesozoic appears not to be under the influence of any prevailing continent wide vectorial forces. The degree of scatter manifest in the Mesozoic is in sharp contrast to the paleocurrents of the immediately underlying Paleozoic.
Paleozoic paleocurrents indicate the influence of directional forces on a grand scale over an extended period. Various authors have attributed the directionality to such things as "regional slopes," but it is difficult to see how this could apply to deposits of such diverse origins over so wide an area. The lack of strong directionality in the underlying Precambrian sustains the need to seek understanding of what makes the Paleozoic style of sedimentation unique with respect to directional indicators.