Mary J. Kraus

Paleosols in clastic sedimentary rocks: their geologic applications

Interest in pre-Quaternary paleosols has increased over the past decade, in large part, because they have proved to be beneficial in solving diverse geological problems. The majority of paleosols are described from continental deposits, most commonly from alluvial strata. Criteria for recognizing these paleosols have been extensively described; however, classifying them has proved more complicated. Pre-Quaternary paleosols are generally classified according to one or more modern soil classification systems, although one new classification has been proposed exclusively for paleosols to avoid problems using the modern soil classifications. In addition to taxonomic classification, paleosols can be categorized according to the interplay among deposition, erosion, and the rate of pedogenesis when they formed. Paleosols can be solitary if they formed during a period of landscape stability following the development of an unconformity. Such paleosols are commonly thick and extremely well developed. More commonly, paleosols are vertically stacked or multistory because they formed in sedimentary systems undergoing net aggradation. If erosion was insignificant and sedimentation was rapid and unsteady, compound paleosols generally formed. If the rate of pedogenesis exceeded the rate of deposition, composite paleosols developed. Thick, cumulative paleosols indicate that erosion was insignificant and that sedimentation was relatively steady. Both autogenic and allogenic processes can influence depositional and erosion patterns and, thus, affect the kinds of soils that form. Consequently, paleosols can help to interpret the history of sediment deposition and the autogenic and allogenic processes that influenced a sedimentary basin. Paleosols are also helpful in stratigraphic studies, including sequence stratigraphic analyses. They are used for stratigraphic correlations at the local and basinal scale, and some workers have calculated sediment accumulation rates based on the degree of paleosol development. In addition to their stratigraphic applications, paleosols can be used to interpret landscapes of the past by analyzing paleosol-landscape associations at different spatial scales, ranging from local to basin-wide in scope. At the local scale, lateral changes in paleosol properties are largely the result of variations in grain size and topography. At the scale of the sedimentary basin, paleosols in different locations differ because of basinal variations in topography, grain size, climate, and subsidence rate. Paleosols are used to . reconstruct ancient climates, even to estimate ancient mean annual precipitation MAP and mean annual temperature .MAT . Ancient climatic conditions can be interpreted from modern soil analogs or by identifying particular pedogenic properties that modern studies show to have climatic significance. Stable carbon and oxygen isotopes are also used to interpret ancient climate, and some effort has been made to estimate MAT from isotopic composition. On the basis of

Integration of channel and floodplain suites; I, Developmental sequence and lateral relations of alluvial Paleosols

ABSTRACT The lower Eocene Willwood Formation of the Bighorn Basin, northwest Wyoming, consists of about 770 m of alluvial rocks that exhibit extensive mechanical and geochemical modifications resulting from Eocene pedogenesis. Willwood paleosols vary considerably in their relative degrees of maturity; maturity is defined as stage of development as a function of the amount of time required to form. Five arbitrary stages are proposed to distinguish these soils of different maturities in the Willwood Formation. Stage 1 soils, the least mature, are entisols; stage 2 and stage 3 soils are intermediate in maturity and are probably alfisols; and stage 4 and stage 5 soils, the most mature, are spodosols. These stages are not only time-progressive elements of an in situ maturation sequence for W llwood soil formation, but, in the lateral dimension, they are also usually distributed sequentially. Study of Willwood paleosols indicates that an inverse relationship exists between soil maturity and short-term sediment accumulation rate. The least mature Willwood paleosols formed in areas of relatively high net rates of sediment accumulation on 1) channel, levee, and crevasse-splay sediments of the proximal alluvial ridge, and 2) deposits filling large and small paleovalleys formed by major episodes of gullying (lowered baselevels). In contrast, the fine-grained sediments of the distal floodplain, where net sediment accumulation rates were relatively low, experienced development of much more mature soils. Soils of intermediate maturities occur in the order of their stage on intervening proximal floodplain and distal alluvial ridge sediments. Adjacent bodies of sedimentary rock that differ in their ancient soil properties because of distance from areas of relatively high sediment accumulation are denoted by the new term pedofacies. The remarkable sequence of paleosols in the Willwood Formation clearly illustrates several important principles of soil-sediment interrelationships in aggrading alluvial systems that have broad application to other deposits. This is especially true in view of the widespread distribution of paleosols in nearly all ancient fluvial rocks. Further study of Willwood paleosols will not only enable precise lateral correlation of coeval alluvial sediments, and thereby fluvial sedimentary events, from the distal to the proximal realms of the floodplain but will also contribute to increasingly informative evaluations of the nature, tempo, and mode of alluvial succession.

Eocene Hydromorphic Paleosols: Significance for Interpreting Ancient Floodplain Processes

ABSTRACT Pervasive mottling, iron-oxide nodules, and slickensides indicate that alluvial paleosols of the lower Eocene Willwood Formation in the Elk Creek area of the Bighorn Basin, Wyoming were hydromorphic. Two basic groups of hydromorphic paleosols are present: cumulative, which show evidence for concurrent deposition and pedogenesis, and simple (noncumulative), which show evidence for a single episode of deposition and pedogenesis. Lithologic packages consisting of vertically stacked simple paleosols and crevasse-splay sandstones alternate with cumulative paleosols. The simple paleosols are weakly developed, indicating rapid deposition with relatively little time for pedogenesis. Cumulative paleosols show greater pedogenic development, indicating significantly longer periods of pedogenic m dification and slower, more episodic sediment accumulation. Differences between the simple and cumulative paleosols reflect differences in the deposition of their parent materials. The packages of simple paleosols are similar to avulsion-belt deposits described from the Saskatchewan River, and suggest that avulsion was significant in floodplain construction. The upper parts of cumulative paleosols were formed on true overbank deposits, which accumulated gradually after avulsion and establishment of a new channel. The lower parts of many cumulative paleosols resulted from a prior episode of avulsion deposition and simple soil formation. Because hydromorphy is associated with texture, these different depositional processes influenced lateral variations in hydromorphy of the cumulative paleosols. Paleosols in the Elk Creek area provide a simple means of identifying avulsions. Sediment accumulation rates suggest that avulsions occurred with a regular period of about 20,000 years and may have been induced by climatic cycles.

The Fayum Primate Forest Revisited

In Oligocene times, the Fayum area of northern Egypt was a subtropical to tropical lowland coastal plain with damp soils and seasonal rainfall that supported an abundance and variety of vegetation, including lianes (large vines), tall trees, and possibly mangroves, and a large and varied vertebrate fauna. The Oligocene marine strandline was close by and principal Jebel Qatrani Formation streams were probably brackish several kilometers inland due to tidal incursions. Sediments of the Jebel Qatrani Formation were deposited by several large meandering streams, associated with minor but sometimes extensive floodbasin ponds. These rocks provide no evidence for the former existence, in early Tertiary time, of a “Proto-Nile” River. Large accumulations of silicified fossil logs in the Jebel Qatrani Formation are autochthonous and the logs were transported only a short distance before burial. The Oligocene higher primates Aegyptopithecus, Propliopithecus, Parapithecus , and Apidium lived in this paleoenvironment and postcranial remains of Aegyptopithecus and Apidium demonstrate that these animals were arboreal. This scenario for the paleoenvironment of the Fayum area in Oligocene times differs greatly from the nearly treeless, sparsely vegetated, semiarid sahelien Oligocene Fayum paleoenvironment populated by terrestrial primates that was recently proposed by Kortlandt (1980) .

Integration of Channel and Floodplain Suites, II. Vertical Relations of Alluvial Paleosols

ABSTRACT Extrachannel sediments of the lower Eocene Willwood Formation, which were subdivided into different pedofacies or developmental stages, exhibit two well-developed orders of pedofacies sequences, over which a grosser scale cyclicity is superposed. Simple sequences, with thicknesses measured in meters, are comprised of one or more soil profiles that are bounded above and below by pedogenically unmodified crevasse-splay deposits. On a larger scale, compound pedofacies sequences are tens of meters thick and consist of several simple sequences sandwiched between channel sandstones. Between these sandbodies, pedogenic maturity of the multistorey paleosols progressively increases and then decreases upwards in response to episodic avulsion. Formation of both compound and simple pedofacies seq ences was largely controlled by local patterns of deposition and erosion that produced vertical variability in both rate of sediment accumulation and parent material. Because paleosol maturity also reflects the effects of extrabasinal factors, a pedofacies megasequence, which is hundreds of meters thick, developed in response to changes in tectonic activity through Willwood time. Comparison between Willwood deposits in the northern and central parts of the Bighorn Basin also reveals that areally differing sediment accumulation rates and thereby basin subsidence rates can be interpreted from large-scale pedofacies sequences. Consequently, paleosols are useful in determining changes in relative rates of sediment accumulation that are controlled by both intrabasinal and extrabasinal factors.

Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum

Warming and Shrinking In most mammals, individual body sizes tend to be smaller in warmer regions and larger in cooler regions. Secord et al. (p. 959; see the Perspective by Smith) examined a high-resolution 175,000-year record of equid fossils deposited over a past climate shift—the Paleocene-Eocene Thermal Maximum—for changes in body size. Using oxygen isotopes collected from the teeth of co-occurring mammal species to track prevailing environmental temperature, a clear decrease in equid body size was seen during 130,000 years of warming, followed by a distinct increase as the climate cooled at the end of the period. These results indicate that temperature directly influenced body size in the past and may continue to have an influence as our current climate changes. Oxygen isotope measurements of fossil teeth show that the body size of the horse Sifrhippus decreased as temperature increased. Body size plays a critical role in mammalian ecology and physiology. Previous research has shown that many mammals became smaller during the Paleocene-Eocene Thermal Maximum (PETM), but the timing and magnitude of that change relative to climate change have been unclear. A high-resolution record of continental climate and equid body size change shows a directional size decrease of ~30% over the first ~130,000 years of the PETM, followed by a ~76% increase in the recovery phase of the PETM. These size changes are negatively correlated with temperature inferred from oxygen isotopes in mammal teeth and were probably driven by shifts in temperature and possibly high atmospheric CO2 concentrations. These findings could be important for understanding mammalian evolutionary responses to future global warming.

Facies and facies architecture of Paleogene floodplain deposits, Willwood Formation, Bighorn Basin, Wyoming, USA

Abstract Paleogene deposits of the Willwood Formation were analyzed in two areas of the Bighorn Basin to provide a better understanding of the facies and facies arrangement of floodplain deposits and the various processes that influence floodplain construction and facies variability. Despite similar facies and facies organization, floodplain deposits in the two areas differ in grain size, hydromorphy and maturity of the paleosols, and scale and organization of ribbon sandstones in the avulsion deposits. These differences appear to be controlled by basin position of the study areas and differences in avulsion between the areas. Two kinds of cumulative floodplain paleosols, red and purple paleosols, formed on overbank deposits. The red paleosols were better drained and formed on less clay-rich parent material. Intense mottling and iron-oxide nodules indicate that the purple paleosols, which formed on more clay-rich parent material, were poorly drained. The grain size differences reflect basin position, in particular, distance from a local sediment source. Avulsion deposits are volumetrically important, comprising about half of the floodplain deposits in both areas. The avulsion deposits resemble the Stage III splays described by Smith et al. (1989) [Smith, N.D., Cross, IA., Dufficy, J.P., Clough, S.R., 1989. Anatomy of an avulsion. Sedimentology 36, 1–24.] in terms of characteristics of their ribbon sandstones and the presence of thin, sheet sandstones or siltstones. Paleosols that formed on the fine-grained facies show morphologic differences interpreted as developmental differences and attributed to differing rates of avulsion in the two study areas. This study shows that avulsion-belt paleosols provide a key to understanding avulsion rate in the stratigraphic record. Other differences between the Willwood avulsion deposits in the two areas, and between these ancient deposits and the modern avulsion deposits described by Smith et al. may reflect: (1) problems in clearly recognizing ancient avulsion deposits in the field, and (2) sampling bias resulting from the areally restricted view provided by the stratigraphic record of an areally extensive modern depositional feature. At the same time, because exposures are three-dimensional, the Willwood strata reveal aspects of the facies and facies arrangement of avulsion belts not readily observed in the modern ones.

Basin-Scale Changes in Floodplain Paleosols: Implications for Interpreting Alluvial Architecture

ABSTRACT Floodplain paleosols in time-equivalent strata of the Willwood For mation differ in terms of paleodrainage and degree of pedogenic development. Ancient soils were more poorly drained and more strongly developed in one area compared to a second area about 45 km away. This difference in drainage is attributed to parent material. The more poorly drained paleosols formed in an area with less permeable, clay-rich floodplain deposits. In the stratigraphic interval that accumulated more slowly, paleosols are more strongly developed, less numerous, and thicker than in the more rapidly accumulating stratigraphic interval. These differences are attributed not only to different sediment accumulation rates, which are commonly considered to control the degree of ped ogenic development, but also to differences in avulsion frequency. A simple model is offered to show how paleosols change with varying basin subsidence rates and the relationship between avulsion frequency and accumulation rate. When avulsion frequency increases more rapidly than sediment accumulation rate, an increase in subsidence rate causes strongly developed paleosols to be overlain by a stratigraphic interval with paleosols that are less well developed, thinner, and more densely spaced. When avulsion frequency is not related to accumulation rate, as early models of alluvial architecture assumed, an increase in subsidence rate produces a stratigraphic section in which strongly developed paleosols are overlain by paleosols that are less well developed, thicker, and more widely spaced. Study of the thickness and degree of development of floodplain paleosols can be used together with more traditional studies of alluvial architecture to provide a clearer understanding of the factors that influenced the stratigraphic architecture in a particular alluvial basin.

Lower Eocene alluvial paleosols: Pedogenic development, stratigraphic relationships, and paleosol/landscape associations

Abstract Two kinds of cumulative floodplain paleosols, red and grey paleosols, formed on overbank deposits of the Willwood Formation in the Sand Coulee area of the Bighorn Basin, Wyoming. Although both kinds experienced down-profile iron movement, due to seasonal saturation of a clay-rich A horizon, the red paleosols were better drained and their B horizons were rubified. Various redoximorphic features indicate that the grey paleosols were poorly drained. The red paleosols show progressive increases in the degree of profile development away from an associated channel sandstone, a paleosol/landscape relationship termed pedofacies by Bown and Kraus (1987). Although the grey paleosols show relatively systematic changes in hydromorphy, consistent pedofacies changes were not recognized. Furthermore, no lateral relationship between the red and grey paleosols was observed. These features suggest that, because of the retarding effects of poor drainage on soil weathering, poorly drained soils are not amenable to pedofacies modelling and that the landscape associations of well drained and poorly drained soils may be difficult to document without unusually extensive exposures. Results of this study also show that the pedofacies model is limited by sediment accumulation rates and the kind of fine-grained facies on which paleosols developed. Although readily observable in Sand Coulee, pedofacies are difficult to recognize in the Elk Creek area, where accumulation rates were approximately half as rapid as in Sand Coulee. The attainment of steady-state conditions is believed to have obscured pedofacies in the Elk Creek area. Finally, pedofacies are only developed on true overbank deposits, which, in the Willwood Formation and probably many other ancient alluvial sequences, make up only a fraction of the fine-grained deposits. Only immature soils formed on the remainder of the fine-grained facies because they were deposited very rapidly, as a result of channel avulsion.

Naktodemasis bowni: New Ichnogenus and Ichnospecies for Adhesive Meniscate Burrows (AMB), and Paleoenvironmental Implications, Paleogene Willwood Formation, Bighorn Basin, Wyoming

Abstract Adhesive meniscate burrows (AMB) are common in alluvial paleosols of the Paleogene Willwood Formation, Bighorn Basin, Wyoming. AMB are sinuous, variably oriented burrows composed of a nested series of distinct, ellipsoidal packets containing thin, tightly spaced menisci subparallel to the bounding packet. Menisci are non-pelleted and texturally homogeneous with each other and the surrounding matrix. AMB were constructed most likely by burrower bugs (Hemiptera: Cydnidae), cicada nymphs (Hemiptera: Cicadae), and less likely by scarabaeid (Coleoptera: Scarabaeidae) or carabid beetles (Coleoptera: Carabidae), based on burrow morphology and comparison to similar structures produced by these organisms in modern soils. Extant burrowing insects excavate backfilled burrows in well-rooted A and upper B horizons of soils generally below field capacity depending on soil type. This study demonstrates that AMB are distinct morphologically from such previously described ichnofossils as Beaconites, Laminites, Scoyenia, Taenidium, and Ancorichnus. Naktodemasis bowni, a new ichnogenus and ichnospecies, represents burrows composed of nested ellipsoidal packets backfilled with thin, tightly spaced, menisci subparallel to the bounding packet. The presence of N. bowni indicate periods of subaerial exposure associated with pedogenic modification under moderately to well-drained soil conditions, or during periods of better drainage in imperfectly drained soils. N. bowni, therefore, can differentiate alluvial paleoenvironments from marine and lacustrine paleoenvironments, as well as periods of subaerial exposure of sediments deposited in aquatic settings.