Paleowind Directions Research Articles

Hierarchy of bounding surfaces in aeolian sandstones of the Jurassic Tordillo Formation (Neuquén Basin, Argentina)

The Tordillo Formation is a continental clastic unit deposited in the Neuquen Basin during the Late Jurassic. This paper discusses the stratigraphy of the succession outcropping at the Quebrada del Sapo, with emphasis on the origin, dimensions and hierarchy of bounding surfaces of aeolian deposits. Field survey, supported by the measurement of three detailed stratigraphic sections and line drawings of photographic panels allow the identification of four unconformity bounded units within the succession, informally named as T1, T2, T3 and T4. Units T1 and T3 are composed of conglomerates and pebbly sandstones deposited by density flows in a lacustrine environment. Paleocurrents indicate a source area located in the northeast while the presence of angular sandstone blocks suggests resedimentation processes. T2 and T4 units are composed of fine to medium grained sandstones of aeolian origin, characterized by large scale dunes and minor dry interdunes. Both units have sharp bases, and overlie a deflation surface characterized by the presence of ventifacts. Paleocurrents suggest a paleowind direction from the southwest. Internal bounding surfaces show a hierarchy of at least four discrete surfaces which were numbered according to their crescent extension. Type 1 surfaces are related to the normal advance of the dune front. Type 2 are reactivation surfaces within a single dune set. Type 3 surfaces relate to set superposition. Type 4 surfaces are related to extensive deflation of the dune complex, and define at least nine elementary aeolian sequences in the T4 unit.

The regional and temporal significance of primary aeolian magnetic fabrics preserved in Alaskan loess

The anisotropy of magnetic susceptibility (AMS) data presented herein underscore the resolving power magnetic anisotropy has in non-lithified poorly consolidated sediments. We turn our attention to primary aeolian sedimentary magnetic fabrics preserved along two coeval loess profiles in Central Alaska at the Halfway House (HH3) and Gold Hill Steps (GHS2) sites. We first demonstrate that published preliminary results along HH1 and HH2 are reproducible. Moreover, the new sample suite, the first ever at GHS, collected at a higher temporal resolution and statistical confidence permits a better evaluation of the significance of the inferred paleowind directions and their temporal variability. The data demonstrate that the resolvability of the magnetic lineation is independent of both the eccentricity of the AMS ellipsoid and the strength of the magnetic foliation. Furthermore, the orientations and temporal variability of the most significant magnetic lineations are not artifacts of the data filtering process developed to isolate them. We conclude that the temporal variations of inferred paleowind directions along HH3 track paleoclimatic glacial–interglacial cycles as well as provide additional constraints to the geochronology of the loess deposit. Along GHS2, the limited coeval primary magnetic fabrics suggest an influence from a local microclimate not present at the HH site, complicating the interpretation of small-scale variability in paleowind directions. However, the overall average paleowind directions are similar to those inferred along HH3 confirming a regional (spatial) consistency of past surface air circulation.

Early Proterozoic geomagnetic field in western Laurentia: implications for paleolatitudes, local rotations and stratigraphy

Abstract The east-west trending, post-orogenic subvertical Cleaver Dykes from the Great Bear Magmatic Arc are dated by the U–Pb (baddeleyite) method to be 1740 +5/−4 Ma. They have a mean direction of magnetization of D, I=136.5 and 57.4°, k=64, α95=4.5°, based on samples from 17 diabase dykes, a mean location 65.7°N, 118.03°W, and a corresponding paleopole at 19.4°N, 83.3°W, A95=6.1°. Studies of baked contacts, country rocks distant from dykes, and later intrusions that cut them, show that the dykes became magnetized soon after intrusion. The paleopole is not significantly different from that obtained from the 1759±1 Ma post-orogenic Jan Lake Granite from the Hudsonian Orogen in southeast Saskatchewan. These paleopoles fall among a cluster previously determined from post-orogenic overprints in a wide variety of rocks and geological settings across Laurentia. Their mean is 20.8°N, 94.5°W, A95=5.2°. Collectively they record a widespread remagnetization episode following the Hudsonian Orogeny, during which the polarity of the field remained constant. This single polarity interval (the Cleaver Superchron) has a mean age of approximately 1747 Ma, and its duration is at least 13 million years but is probably much longer. With further refinement, it could become a global statigraphical marker. There are 18 paleopoles from red beds and igneous rocks which record the paleofield in western Laurentia mainly over an extended interval we estimate to be at least 100 million years between about 1960 and 1830 Ma during the Hudsonian Orogeny. Over this interval, western Laurentia remained at a low and essentially constant latitude. Reversals of the geomagnetic field occurred opposed magnetizations being of approximately equal frequency; there are numerous magnetochrons which could, in future, assist in regional stratigraphic correlations. On the basis of six studies, we estimate that the reference paleopole for this interval is at 1.1°S, 71.7°W, A95=12.1° (N=6). Paleopoles from the other 12 studies are spread over a 100° arc indicating large variable rotations about local vertical axes which probably resulted from transcurrent faulting caused by indentation of the bordering Archean Slave and Superior cratons during the Hudsonian Orogeny. These reference paleopoles show that during the Hudsonian Orogeny, western Laurentia was in paleolatitudes of 15–30°. During post-Hudsonian time (ca. 1750 Ma) paleolatitudes were about 20° higher. Paleowind directions inferred by others, indicate that Laurentia during these intervals was in the northern hemisphere.

Paleowind directions from the magnetic fabric of loess profiles in central Alaska

Two parallel profiles of alternating loess and paleosol horizons from the Halfway House site west of Fairbanks, in central Alaska, were evaluated in terms of their potential for preserving paleowind directions. Combined, the profiles span SPECMAP marine oxygen isotope stages 1–6, approximately the last 150 ka. Our analytical tools consisted of measuring the sample’s anisotropy of magnetic susceptibility and characterizing the magnetic mineralogy by multiple approaches. Both primary eolian and secondary reworked magnetic fabrics are recognized along certain intervals of both profiles. Primary eolian fabrics display a horizontal magnetic foliation and a well defined magnetic lineation, corresponding to the paleodirection of sediment transport and deposition. A systematic change in transport direction is observed where long-term prevailing winds appear to shift from a NW–SE to N–S direction during glacial and interglacial periods, respectively.

Environmental conditions and paleowind directions at the end of the Weichselian Late Pleniglacial recorded in aeolian sediments and geomorphology (Twente, Eastern Netherlands)

AbstractThe Late weichselian Pleniglacial wind regime in the eastern Netherlands is reconstructed by means of landform and sedimentological analysis. This analysis involves aeolian and fluvial landforms in the Dinkel river valley in the Twente region. The aeolian deposits considered here date from the Last Glacial Maximum (approximately 22 ka) to the start of the Belling Interstadial at 14.7 ka.A major event in this period is the formation of a cryoturbation level caused by permafrost degradation, overlain by an erosional hiatus dated between 21 and 17 ka. Both features are attributed to a period of warmer and moister climate, causing permafrost degradation and erosion by surficial runoff. Thereafter aeolian activity prevailed under relatively arid conditions. A deflation surface was formed, the Beuningen Gravel Bed. This deflation surface is present in many Weichselian sections in the Netherlands and the adjacent parts of Belgium and Germany. The deflation occurred concurrently with deposition of coversand at other places.The morphology of the coversand-landscape in the Dinkel valley was controlled by the relief of the pre-existing floodplain and the wind pattern. Coversand ridges consisting of low dunes accumulated near the margins of the active channel belt. Relatively thick sand sheets occur in the leesides of the ridges, thin sand sheets are found at greater distance.Mainly westerly sand-transporting winds operated during winter and summer. In winter aeolian deposition occurred by frequent and strong easterly winds also. On the smallest, local scale, the pattern of deposition was determined by the topography and moisture of the receiving surface.Coversand deposition came to an end with the formation of a sand sheet under relatively warm and less arid conditions. Coversand deposition continued into the Belling Interstadial; colonization of the coversand surface by vegetation probably has been delayed by nutrient-poor conditions.

Magnetic properties and paleoclimatic implications of loess-paleosol sequences of Czech Republic

Detailed rock magnetic investigations and X-ray diffraction (XRD) were carried out on loess-paleosol sequences of the last interglacial-glacial at Znojmo section in Czech Republic. The results indicate that pedogenesis causes susceptibility enhancement in the paleosols, which is similar to that observed in the Chinese Loess Plateau. k -T curves, IRM, and XRD show that magnetite is the dominant magnetic mineral in the loess-paleosol sequences at the Znojmo section, while maghemite, hematite, and pyrite/pyrrhotite are minor minerals. Measurements of anisotropy of magnetic susceptibility (AMS) indicate that the magnetic lineation is smaller than the foliation. The susceptibility ellipsoids are oblate and the directions of the maximum principal axes (K max ) are distributed randomly, and cannot be used to determine the paleo-wind direction.

Paleowind (11,000 BP) directions derived from lake spits in Northern Michigan

We report on the characteristics of several, previously unstudied, late Pleistocene spits in northern lower Michigan. Each spit developed as strong waves eroded a headland or island within the Main and/or later stages of glacial Lake Algonquin. Most have ESE to WNW orientations with prominent recurved hooks at their ends. Some are as long as 20 km. Sedimentologically, the spits contain thick (several meters or more) sequences of sand and well-rounded gravel. We suggest that summertime winds during the spits' formation (11,800 to ≈10,500 BP) were both very strong and dominantly from the E and SE, based on the following data: (1) their ESE (source, or head, of the spit) to WNW orientation, (2) the presence of steep, wave-cut bluffs on the east sides of the islands and headlands, and (3) spit locations (on the northeastern shore of Michigan's lower peninsula, coupled with their absence on the northwestern shore, just 100 km away). Wind direction data provided by these spits agree with paleoclimatic models that suggest that the Laurentide Ice Sheet had developed a strong anticyclonic circulation. The near lack of archeological sites from this time period in the northern lower peninsula, which was surrounded by Lake Algonquin at that time, supports the notion that the climate of northern lower Michigan was inhospitably cold and very windy in the late Pleistocene.

Late Quaternary Inland Dunes of Southern Louisiana and Arid Climate Phases in the Gulf Coast Region

Thirty-five sand hills that form six scattered groups rise abruptly from the flat late Pleistocene coastal plain in southeastern Louisiana. New studies confirm their eolian origin. For the first time, several late Wisconsin to early Holocene episodes of arid climate conditions have been recognized and dated in this currently humid warm-temperate subtropical region. Periods of dune formation and reactivation (28,800 to 7900 yr B.P.) were determined by the thermoluminescence method. The onset of the current climate in this Gulf coastal region postdates early Holocene time. The textural and structural homogeneity of the ridge lithosomes, good sorting of their sand fraction, and the dominantly orange hues of the dune sediments contrast with the underlying yellowish–brown to light-brown sandy silts and the well-stratified, occasionally gravelly sands of the underlying alluvial Prairie Formation. Sharply defined, unconformable ridge bases; symmetrical, oval, occasionally parabolic mound shapes; and steep slopes confirm the dune origins. The dominant orientations of ridges and ridge chains clearly reflect paleowind directions. Age comparison with dunes of the lower Mississippi Valley, the northeastern–eastern Gulf of Mexico coast, and south Atlantic coastal areas confirms the existence of at least seasonally dry climate conditions from early Wisconsin to middle Holocene times. The onset of the modern humid-subtropical climate phase in this region thus dates back only to the middle Holocene.

Regularity of paleowind directions of the Early Cretaceous Desert in Ordos Basin and climatic significance

By the measurement of the wind directional indicator of the Cretaceous desert in Ordos Basin, the regularity of the paleoprevailing wind directions and pattern of the paleowind belts are revealed. It is considered that the desert was controlled by a planetary wind system. In the early stage, the subtropical high pressure zone drifted south and northwards with short cycles, resulting in the alternation of westerlies and northeast trades; in the late stage, the subtropical high pressure zone drifted southwards with a long cycle, placing the desert under the westerly belt and making the desert completely controlled by the westerly belt. The reconstruction of the paleowind belts has provided the evidence of the general circulation of the atmosphere for the pattern of the circulation before the uplift of the Tibetan Plateau.

Late Pleistocene Eolian Sediments Related to Pyroclastic Eruptions of Mauna Kea Volcano, Hawaii

Loess and dune sands that mantle volcanic rocks on the northwest flank of Mauna Kea volcano consist predominantly of fine-grained pyroclasts of the alkalic Laupahoehoe Volcanics produced by explosive eruptions. The loess is divided into lower and upper units, separated by a well-developed paleosol, while older and younger dune sands are separated by loess. Four interstratified tephra marker horizons aid in regional stratigraphic correlation. Radiocarbon ages of charcoal fragments within the loess, U-series ages of rhizoliths in the dune sand, and K/Ar ages and relative stratigraphic positions of lava flows provide a stratigraphic and temporal framework. The lower loess overlies lava flows less than 103,000 ± 10,000 K/Ar yr old, and14C dates from the paleosol developed at its top average ca. 48,000 yr. Loess separating the dune sand units ranges from ca. 38,000 to 25,00014C yr old; the youngest ages from the upper loess are 17,000–18,00014C yr B.P. Dips of sand-dune foreset strata, isopachs on the upper loess, and reconstructed isopachs representing cumulative thickness of tephra associated with late-Pleistocene pyroclastic eruptions suggest that vents upslope (upwind) from the sand dunes were the primary source of the eolian sediments. Average paleowind directions during the eruptive interval (ca. 50,000–15,000 yr B.P.), inferred from cinder-cone asymmetry, distribution of tephra units, orientation of dune foreset strata, and the regional pattern of loess isopachs, suggest that Mauna Kea has remained within the trade-wind belt since before the last glaciation.

Jurassic-Cretaceous Paleogeography, Paleoclimate and Upwelling of the Northern Margin of Tethys: ABSTRACT

The Jurassic and Cretaceous global paleogeographic reconstructions illustrate the changing configuration of mountains, land, shallow seas and deep ocean basins. Active plate boundaries, such as spreading centers and subduction zones, are also shown. The Pliensbachian, Toarcian, Bathonian, Oxfordian-Kimmeridgian, Tithonian-Berriasian, Valanginian, Albian, Turonian and Maastrichtian maps were generated The outlines of paleogeography are used as input for paleoclimatic modeling. The PALEOCLIMATE program models global atmospheric pressure, derive paleo-wind directions and estimate the likelihood of coastal upwelling. The program is based on the paleoclimatic methods first developed by Judith Parrish, adopted by C. R. Scotese and modified by M. I. Ross. The maps depict air pressure, wind directions, humid zones and areas favorable for upwelling conditions plotted on the paleogeographic background. Paleoclimate modeling suggests that prevailing Jurassic-Cretaceous wind directions in the northern Tethys area were from north-northeast. These winds were parallel to the axis of Czorsztyn ridge. The ridge was uplifted between Magura and Pieniny basins as the result of extension during Jurassic supercontinent breakup. The upwelling may have been induced at the southeastern margin of the ridge. The model is consistent with rock records, especially from the upper part of ammonitico rosso type Czorsztyn formation. Mass occurrence of Tithonian and Berriasianmore » brachiopods was probably controlled by upwelling-induced trophic relationships which is resulted in the intense growth of benthic organisms on the ridge. This is additionally supported by the presence of phosphorites at localities which corresponded to the continental shelf/slope transition.« less

Provenance of dust in the Pacific Ocean

Eolian dust preserved in deep-sea sediment cores provides a valuable indicator of past atmospheric circulation and continental paleoclimate. In order to identify the provenance of eolian dust, Nd and Sr isotopic compositions and Rb, Sr and rare earth element (REE) concentrations have been determined for the silicate fractions of deep-sea sediments from the north and central Pacific Ocean. Different regions of the Pacific Ocean are characterized by distinct air-borne inputs, producing a large range in ε Nd (−10 to +1), 87Sr/ 86Sr (0.705–0.721), La/Yb (5–15), Eu N/Eu N * (0.6–1.0) and Sr/Nd (4–33). The average Nd isotopic composition of Pacific deep-sea sediments ( ε Nd = −6), is more radiogenic than the average from the Atlantic ( ε Nd = −8). In contrast, the average 147Sm/ 144Nd ratio for Pacific sediments (0.114) is identical to that of Atlantic sediments and to that of global average riverine suspended material. The values of ε Nd and 147Sm/ 144Nd are positively correlated for the Pacific samples but negatively correlated for Atlantic samples, reflecting a fundamental difference between the dominant components in the end members with radiogenic Nd (island-arc components in the Pacific and LREE-enriched intraplate ocean island components in the Atlantic). Samples from the north central Pacific have distinctive unradiogenic ε Nd values of −10, 87Sr/ 86Sr > 0.715, high La/Yb (> 12), and low Eu N/Eu N * (0.6) and Sr/Nd (3–6). These data are virtually identical to the values for loess from Asia and endorse the use of these sediments as indicators of Asian paleoclimate and paleowind directions. Island-arc contributions appear to dominate in the northwest Pacific, resulting in higher ε Nd (−1 to +1) and lower 87Sr/ 86Sr (≈ 0.705) and La/Yb (≈ 5). Sediments from the eastern Pacific tend to have intermediate Sr and Nd isotopic compositions but regionally variable Sr/Nd and REE patterns; they appear to be derived from the west margin of the North and South American continents, rather than from Asia. Our results confirm that dust provenance can be constrained by isotopic and geochemical analyses, which will facilitate reconstructions of past atmospheric circulation and continental paleoclimate.

Paleomagnetic Orientation of Fractures and Bedding in Rotliegende and Zechstein Cores from the Southern Permian Basin, North Sea

Fractures and bedding in Rotliegende sandstone and Zechstein dolomite cores of the southern Permian Basin can be accurately oriented using our standard paleomagnetic core-orientation technique. In recent project involving vertical, deviated, and horizontal wells, we have paleomagnetically oriented 1874 ft of Rotliegende and Zechstein cores from 113 intervals ranging in length from 8 in. to 119 ft. Accuracy of our paleomagnetic core-orientations can be demonstrated by (1) consistency in fracture patterns derived from vertical and horizontal cores near the same reservoir location, (2) consistency in Rotliegende paleowind directions derived from paleomagnetically oriented cores with paleowind directions derived from mechanically oriented cores from the southern Permian Basin, and (3) agreement between structural dip determined from paleomagnetically oriented interdune bedding planes with structural dip determined from paleomagnetically oriented strike-0parallel, strike-perpendicular, and bedding-plane fractures. Paleomagnetic orientation of Rotliegende cores can be especially cost effect. Excellent core recovery and use of long core barrels in the Rotliegende means that [open quotes]continuous intervals[close quotes] (defined as the maximum lengths of core that can be reliably reconstructed by fitting adjacent core pieces) of more than 100 ft long can be achieved by following our recommended core-handling procedures. We statistically average the same number of paleomagnetic plugmore » samples regardless of the length of a [open quotes]continuous interval.[close quotes] The paleomagnetic signals in Rotliegende sandstone and Zechstein dolomite are sufficiently stable that fractures and bedding can be paleomagnetically oriented even in slabbed cores drilled decades ago.« less

Lee-face airflow, surface processes, and stratification types: Their significance for refining the use of eolian cross-strata as paleocurrent indicators

Research Article| November 01, 1992 Lee-face airflow, surface processes, and stratification types: Their significance for refining the use of eolian cross-strata as paleocurrent indicators M. L. SWEET M. L. SWEET 1BP-Exploration, P.O. Box 4587, Houston, Texas 77210 Search for other works by this author on: GSW Google Scholar Author and Article Information M. L. SWEET 1BP-Exploration, P.O. Box 4587, Houston, Texas 77210 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (11): 1528–1538. https://doi.org/10.1130/0016-7606(1992)104<1528:LFASPA>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. L. SWEET; Lee-face airflow, surface processes, and stratification types: Their significance for refining the use of eolian cross-strata as paleocurrent indicators. GSA Bulletin 1992;; 104 (11): 1528–1538. doi: https://doi.org/10.1130/0016-7606(1992)104<1528:LFASPA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Observations of dune migration, wind velocity (measured near the dune surface on relatively short time scales and near the dune field over a period of several years), and dune surface processes (that is, wind-ripple migration, grainfall, avalanching, and superposed dune migration) were made on a complex-crescentic dune in the Algodones dune field, California, to document the link between eolian dunes, their deposits, and the wind regimes in which they form. The observations suggest that typically there is not a simple link between the orientation of eolian cross-strata and the primary, or regional, paleowind direction. Leeward of dunes with near-angle-of-repose lee faces, an alongslope-directed secondary airflow develops in response to primary winds that are oblique to the dune. This secondary airflow drives the migration of lee-side superposed dunes and wind ripples. The deposits of these superposed bedforms may significantly differ in dip direction from any one component of the primary wind regime, or the vector sum of all of its components. A record of the short-term fluctuations in primary wind direction is recorded by the interlamination of eolian stratification types, primarily grainflow and wind-ripple deposits.Insights gained from this study of modern dunes, their deposits, and wind regimes were applied to two well-documented ancient eolian deposits, the Jurassic Page and Navajo Sandstones of the southwestern United States. Both units contain alternating wind-ripple and grainflow deposits. These cyclic stratification patterns are interpreted to have resulted from a seasonally varying wind regime with a component or components of flow oblique to the dune, responsible for wind-ripple deposition, and another component of flow perpendicular to the dune, responsible for deposition by grainflow. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Rain-impact Microtopography (RIM): An Experimental Analogue for Fossil Examples from the Maroon Formation, Colorado

ABSTRACT Small-scale sedimentary features in medium- to coarse-grained siltstones (loessites) of the Maroon Formation (Middle Pennsylvanian to Early Permian) of northwestern Colorado are interpreted to be of rain-impact origin. These features, which have been experimentally produced by simulated oblique rain impact, are herein termed rain-impact microtopography, or RIM. Experiments indicate that RIM forms only in sediments ranging from fine silts to medium very fine sands (7 to 3.5 ). Probable fossil (ancient) examples of RIM superficially resemble numerous other sedimentary and biogenic structures, but a variety of observations and experimental procedures demonstrate their uniqueness. ecognition of RIM can be valuable as an indicator of subaerial exposure and paleowind direction.

Late pleistocene paleowind directions, Atlantic Coastal Plain, U.S.A

Parabolic dunes, transverse dunes and fields of irregular dunes are common along the left banks, the north or east sides, of streams of the Atlantic Coastal Plain from Georgia to Delaware and probably beyond. Similar dunes occur in the Appalachian Piedmont, but are less common. The features have been eroded, to some extent, and are normally recognizable on topographic maps only as areas of irregular topography with wet and dry depressions. The larger parabolic dunes can be recognized on aerial photographs and satellite images because the associated soils are poor and the vegetative cover relatively sparse. The dunes are of eolian origin and late Pleistocene in age. It is thought that most of the dunes are late Wisconsin in age and contemporary with the Carolina Bays. A small sample of paleowind directions based on average directions of parabolic dune axes indicates that late Wisconsin winds in Georgia were westerly and grain sizes suggest that the winds were stronger than present-day coastal winds of the area. Paleowind directions shift progressively toward the south across the Carolinas and are southwesterly in North Carolina. They change abruptly to the northwest in the Delmarva Peninsula. Following the theory that Carolina Bays are former lakes that are elongated perpendicular to the effective wind direction, the fact that paleowind directions derived from Carolina Bay orientations are virtually identical to those from parabolic dune orientations, strongly supports the wind orientation theory of Carolina Bay origin and confirms the validity of the small sample of directions obtained from parabolic dunes. By analogy with dunes of the eastern shore of Lake Michigan and the Atlantic and Pacific Coasts, the Atlantic Coast Plain dunes do not necessarily indicate arid conditions. The sediment load of some streams may have been sandier than at present and streams with sandy, braided channels may have been common.

Paleogeographic Reconstructions Using Interactive Three-Dimensional Computer Graphics: ABSTRACT

A computer program has been written (MEGADRIFTER) that permits users to interactively produce plate tectonic, paleoclimatic, and paleogeographic reconstructions. This program has been developed on an Evans and Sutherland PS390 and uses digital data compiled by the PALEOMAP Project, Working Group 2B, of the International Lithosphere Program. The digital data base consists of information describing a variety of geographic and geologic features, such as political boundaries, topography, bathymetry, faults, fracture zones, sutures, magnetic isochrons, volcanic edifices, lithofacies patterns, paleoisobars, and paleowind directions. MEGADRIFTER displays this geographic information in three dimensions on the surface of the globe. The user can produce plate tectonic reconstructions by interactively manipulating dials and function buttons to visually determine best-fitting rotation parameters. MEGADRIFTER is part of an integrated geographic information system that (1) digitally converts map data from Cartesian coordinates (X, Y) to geographic coordinates (latitude/longitude), (2) attaches meaningful attributes to the geographic records, (3) searches and sorts through the geographic attributes using a rudimentary geographic data base manager, and (4) plots the data in map form using standard cartographic techniques (GCTP).

Bestimmung der Paläowindrichtung mittels Runzelmarken am Beispiel einer Platte aus der oligozänen Baid-Formation (SW-Saudi-Arabien)

Bestimmung der Paläowindrichtung mittels Runzelmarken am Beispiel einer Platte aus der oligozänen Baid-Formation (SW-Saudi-Arabien)

Advances in Diplog® Data Processing for Stratigraphic Analysis

Detailed log-derived stratigraphic interpretation requires the use of high vertical resolution logging devices such as a dipmeter. The Dresser Atlas Strata Dip® program, originally intro­ duced in the early 1980's, is a computer program designed to give a detailed point-wise estimate of the orientation and depth of very fme formation laminae. Numerous improvements in the computational algorithms coupled with features such as the removal of the effects of structural dip and intelligent handling of floating pad data have resulted in a second generation product which is described in this paper. The accuracy of the program's results was tested using synthetic data generated by a dip simu­ lation program. The repeatability of the program (in fact of the entire data acquisition and processing system) is demonstrated by separate analyses of three independent runs over the same well interval. Two field examples demonstrate the value of this new program as an aid to stratigraphic analysis. Since its introduction in the early 1980s, numerous improvements in the program have resulted in a second generation product. A description of the program logic is given so that others may gain a greater understanding of the correlation technique employed. While this technique is an intuitively valid approach to detailed dip .analysis, experiments were conducted to verify its application in practice. The first of these experiments demonstrates the accuracy of the program in estimating the location and orientation of bedding planes traversed by the borehole. The second experiment demonstrates the repeatability of the entire Diplog® system which comprises both data acquisition and Strata Dip processing. The usefulness of the program as an aid to stratigraphic analysis is shown via two field examples. The first of these examples is an eolian dune analysis in which the program clearly indicates the individual dune sets and the paleo-wind directions. The second example is an analysis of a sandstone formation comprising sediments deposited in a fluvial environment. This formation is interpreted as a point bar sequence consisting of channel lag, trough cross set, and ripple cross laminae deposits.

Wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western United States—A comparison

Wind directions for Middle Pennsylvanian through Jurassic time are predicted from global circulation models for the western United States. These predictions are compared with paleowind directions interpreted from eolian sandstones of Middle Pennsylvanian through Jurassic age. Predicted regional wind directions correspond with at least three-quarters of the paleowind data from the sandstones; the rest of the data may indicate problems with correlation, local effects of paleogeography on winds, and lack of resolution of the circulation models. The data and predictions suggest the following paleoclimatic developments through the time interval studied: predominance of winter subtropical high-pressure circulation in the Late Pennsylvanian; predominance of summer subtropical high-pressure circulation in the Permian; predominance of summer monsoonal circulation in the Triassic and earliest Jurassic; and, during the remainder of the Jurassic, influence of both summer subtropical and summer monsoonal circulation, with the boundary between the two systems over the western United States. This sequence of climatic changes is largely owing to paleogeographic changes, which influenced the buildup and breakdown of the monsoonal circulation, and possibly owing partly to a decrease in the global temperature gradient, which might have lessened the influence of the subtropical high-pressure circulation. The atypical humidity of Triassic time probably resulted from the monsoonal circulation created by the geography of Pangaea. This circulation is predicted to have been at a maximum in the Triassic and was likely to have been powerful enough to draw moisture along the equator from the ocean to the west.