RESULTS
Lithologies
Chilga sedimentary strata are primarily composed of claystone and mudstone, indicating that clay and silt-size particles were the primary sediments deposited in this basin. Sandstones are relatively rare and, where present, consist primarily of very fine-grained reworked airfall tuffs, feldspars, reworked siderite nodules, and mud and clay aggregates similar in appearance to sandstones composed of pedogenic mud aggregates that have been described from other basins of various ages (Gierlowski-Kordesch and Gibling 2002 and references therein). These observations suggest that allochthonous sand-sized sediment likely entered the basin as airfall deposits. Nevertheless, there is evidence for reworking and transport of Chilga sediments by fluvial processes and pedogenic alteration on the floodplains.
Stream Channel Deposits. Relatively few stream channels have been recognized in the Chilga strata. The general absence of these features may reflect, in part, the generally very fine-grained nature of the Chilga sediments and our inability to discern channel-like structures within rocks with these grain-size characteristics. However, trough-cross bedding and inclined heterolithic cross stratification, similar to point-bar deposits in meandering streams (e.g., Walker and Cant 1984), were observed within the very fine and fine-grained sandstone deposits of reworked airfall tuffs, suggesting that alluvial processes were an important means of sedimentary transport in the Oligocene sedimentary strata.
Overbank Deposits. Overbank deposits in the Chilga region consist primarily of claystones and mudstones. Very fine sands are locally present as thin tabular beds with sharp, non-scoured bases and are typically structureless, but may exhibit planar laminations and asymmetrical ripple cross-laminations. Individual sandstone beds range in thickness from a few centimeters to ~40 cm. These thin sandstone units are interpreted as crevasse splay and levee deposits or airfall tuffs.
Mudstone and claystone beds consist of
structureless to finely laminated units ranging from a few centimeters up to
5 m thick. However, some claystones and mudstones exhibit ripple
cross-lamination, and many units contain abundant carbonaceous material and
plant fossils that grade upward into organic-rich lignite layers (Figure
3C). Most overbank lithologies of the Chilga sedimentary strata were subject
to varying degrees of pedogenesis. Because of their abundance,
paleosols are an important component of the overbank lithofacies
architecture in the Chilga strata.
Paleosols
Based on the field inspection of over 141 paleosol profiles distributed across the western region of Chilga strata, we have recognized five morphologically distinct paleosol types that represent the majority of the observed variability (e.g., horizonation, structure, fabric, color, mineralogy; Table 1; Figures 3A-F, 4A-F).
Below, we present generalized
descriptions of the characteristics of the five paleosol types. We also
discuss the morphological variability within a given paleosol type, its
stratigraphic and lateral distribution within the study area, and classify
each paleosol type according to the
Mack et al. (1993) paleosol
classification system to indicate the closest estimated soil taxon within
the context of the USDA Soil Classification System (Soil Survey Staff 1975,
1998). In order to avoid genetic terms that relate to the character of the paleosol types, we arbitrarily refer to these five paleosol-types as A
through E. The paleosol types are vertically and laterally distributed
through the Chilga sedimentary strata. Horizon descriptions for each
paleosol type are listed in Table 1.
Type A Paleosols. Description: The only significant pedogenic features in Type A paleosols are rooting structures, obliteration of original depositional features, and very weak development of soil structures, such as slickensides and wedge-shaped aggregate structure with little or no horizonation (e.g., Figure 3A). These paleosols occur as claystone-or mudstone-rich profiles and very fine sandstone-rich profiles that range from ~20 to >150 cm thick. Claystones are generally gray (5Y 4/1) to olive (5Y 5/2), with few, fine and prominent orange (7.5YR 5/8) mottles and common, medium to coarse, faint gray (5Y 5/2) mottles. Very fine sandstone-rich Type A paleosols are light yellow (5Y 7/3) with massive or single-grain (i.e., sand grains) structure that may grade upward into massive, reddish-orange (7.5YR 5/4), Fe-cemented horizons. Rooting structures are typically composed of lignitic organic material or fine-grained silica (Figure 3A). Type A paleosols that are composed of very fine sandstone are associated with channel and crevasse-splay sandstones, ashfall tuffs and fine-grained overbank deposits of the floodplain facies. Type A paleosols have highly variable lateral continuity throughout the Chilga strata; these profiles grade laterally into Type B and Type C paleosol profiles.
Interpretation: Paleosols with these characteristics are classified as Protosols, which exhibit weak development of pedogenically altered horizons (Mack et al. 1993). Specifically, profiles composed of grayish-green claystones are Gleyed Protosols (Mack et al. 1993), a category that roughly covers the range of characteristics observed in the USDA Soil Taxonomy soil suborders Aquents and Aquepts. Profiles composed of very fine sandstone with Fe-cemented horizons are Ferric Protosols (Mack et al. 1993), which correspond to the USDA soil great group Petraquept (Soil Survey Staff 1998). The presence of protosols in the Chilga strata indicates that sedimentation within the basin (i.e., upon the floodplains and along stream channels) ceased long enough for colonization by terrestrial flora in poorly drained environments (Buol et al. 1997).
Type B Paleosols. Description : Type B paleosols have two primary components, a lower mineral layer of fine-grained siliciclastic material and an upper layer composed mainly of organic material (Figure 3C). The mineral layers consist of primarily drab (5Y 5/1), structureless to very coarse, angular, blocky kaolinitic claystone or mudstone with mm-scale spherulitic siderite nodules (e.g., Figure 4E). These pedogenically altered horizons grade upward from laminated to thinly bedded sediment. The organic layers are laminated to thinly bedded lignites that range from 10 mm to 700 mm in thickness. The lignite beds are (almost entirely) composed of monocot leaf and stem compressions, and parting lineations within these layers commonly exhibit jarosite and gypsum.
Type B paleosols are distributed throughout the Oligocene strata of the Chilga basin within laminated to thin-bedded claystones and mudstones of overbank depositional environments. The organic-rich horizons define regional “lenticular structures” that can be traced over a few 10's of meters to over 2 km, where they pinch-out and change laterally to Type A and Type C paleosols.
Interpretation: Paleosol Type B is a Histosol based on an inferred surficial accumulation of organic material (Mack et al. 1993; Soil Survey Staff 1998). Modern, laterally discontinuous Histosols form in low-lying, waterlogged regions characterized by anoxic pore waters that promote in situ accumulation of organic horizons (O horizons). Laterally discontinuous, organic accumulations in Type B paleosols are interpreted as O horizons (Soil Survey Staff 1975, 1998) that formed by in situ accumulation of plant material. The abundance of fossil root traces that exhibit shallow and tabular morphology and base-depleted kaolinitic layers immediately underlying O horizons may correspond to eluvial A-horizons that formed as a result of intense hydrolysis or acidolysis due to seasonally poor drainage (cf. van Breeman and Harmsen 1975). However, the presence of spherulitic siderite nodules in horizons below the inferred surface of these paleosol profiles indicates a relatively low partial pressure of O2, but high concentration of CO32- at depth in these soils (e.g., Mozley 1993). The shallow, tabular distribution of root traces further reflects the poorly drained conditions under which Type B paleosols formed (cf. Retallack 1988, 1990).
Type C Paleosols. Description: Type C paleosols consist of massive to medium to coarse, angular blocky, gray (5Y 5/3) to yellow-green (5Y 5/2) claystones and mudstones with fine to medium reddish-orange (7.5YR 6/4) mottles. These pedogenically altered layers grade upward from finely laminated to massive sediment or occur superimposed upon older paleosols. These paleosols range from ~450 mm to >2 m thick and contain abundant mm-scale spherulitic siderite nodules and finely disseminated micritic siderite cements that indurate entire horizons (Figure 3B, Figure 4E-F). In addition, the upper 10 to 100 mm of Type C paleosol profiles may consist of organic-rich lignitic horizons. Type C paleosols change laterally into Type A, B, and D paleosol profiles.
Interpretation: Paleosols with these characteristics are interpreted as Gleysols, which exhibit morphological characteristics indicative of formation in poorly drained areas near the interface with the local or regional groundwater table (Mack et al. 1993). Gleysols roughly correspond to the USDA soil great group Aquepts (Soil Survey Staff 1998). The drab greenish-gray paleosol matrix colors, reddish mottling, subsurface accumulation of spherulitic siderite nodules, and in situ accumulation of surficial organic matter collectively indicate that Type C paleosol profiles developed in areas characterized by high water tables and poor soil drainage across the Oligocene floodplains.
Type D Paleosols. Description: Type D paleosols are composed of smectite-rich claystones and silty claystones that grade upward from laminated fine-grained sediments or partially overprint underlying paleosols, such as Type C paleosols. Type D paleosols generally exhibit weakly developed slickenplanes and wedge-shaped aggregate structures that range from 50 to 200 mm across, with secondary coarse angular blocky structure and abundant clay pressure faces at depth (Buol et al. 1997). The lower horizons also typically contain mm-scale spherulitic siderite nodules. The upper boundaries of these horizons are distinct and wavy (Figure 3D), grading upward to greenish-gray (5Y 3/5) horizons with well-defined, wedge-shaped aggregate structure (Fig. 4E), slickenplanes and fine to medium, prominent, reddish-orange (7.5YR 5/4) mottles. Internally, the microfabric of the paleosol matrix in these horizons exhibits striated highly oriented, birefringent clays oriented in a “boxwork” (lattisepic) to continuous, highly oriented, birefringent clays oriented in concentric “spheroids” (Oosepic; Brewer and Sleeman 1976). (Figure 4C-D), micromophology that is typical of modern, clay-rich soils characterized by seasonal wetting and drying (Brewer and Sleeman 1988). The uppermost horizons generally exhibit coarse prismatic structure with secondary medium angular blocky structure. Smectite clays dominate the mineralogy (Table 1), indicating that these soils were not deeply weathered. These paleosols are broadly distributed throughout the stratigraphic succession and may be traced laterally in excess of 1 km across the Chilga strata. Type D paleosol profiles are observed to change laterally to Type C, Type B, and Type E paleosols.
Interpretation: Type D paleosols are Vertisols, based on their high clay content (>35%), presence of slickensides, and v-shaped desiccation cracks, which together indicate seasonal soil wetting and drying (Mack et al. 1993; Soil Survey Staff 1998). Hematitic redox concentrations and low-chroma matrix colors throughout much of the profile support a more specific paleosol classification as gleyed Vertisols (Mack et al. 1993) or the soil suborder Aquerts (Soil Survey Staff 1998). Modern Vertisols typically form on flat terrain with strongly contrasted, wet and dry seasonal or monsoonal climates (Buol et al. 1997). Seasonal wetting and drying of expandable 2:1 phyllosilicate minerals (e.g., smetctite) leads to shearing of plastic soil materials, slickenside formation and development of Vertisol morphology. Vertisols require a period of soil-moisture deficit and sparse vegetation to maintain a high concentration of basic cations, thereby preserving expansible clay minerals in the soil matrix (Retallack 1990; Duchaufour 1982). We interpret Type D paleosols to have formed under wet conditions, given the presence of gley colors and redoximorphic features, with relatively short periods of soil-moisture deficit. Furthermore, Type D paleosols developed in clay-rich overbank deposits of the Chilga floodplains likely characterized by a short period of soil drying.
Type E Paleosols. Description: Type E paleosols are comprised of several well- developed mudstone and claystone horizons. The uppermost horizon is dusky-red (7.5YR 3/2) mudstone, underlain by several yellowish-gray (5Y 5/2) claystone horizons that grade upward from thinly laminated to ripple-cross laminated claystones and mudstones. The lowest of the claystone horizons contains slickensides with wedge-shaped aggregate structure and secondary, medium to coarse, angular blocky structure with thin, discontinuous clay skins (argillans; Figure 4A-B; Brewer 1976). Overlying horizons contain medium to fine, angular blocky structure with thick, continuous clayskins and ferrans (2.5YR 5/5) upon ped surfaces and in soil pores (Figure 3E). Mixtures of kaolinite and smectite dominate the <2 µm fraction in the claystone layers. In addition, abundant root halos (sensu Retallack 1990) are present in the upper horizons of Type E paleosols. Type E paleosols are found only in the upper ~40 meters of the Chilga sequence, and they change laterally to Type C and Type D paleosols.
Interpretation: The clay-enriched horizons with abundant clayskins upon ped surfaces likely correspond to an argillic horizon (Retallack 1990; Soil Survey Staff 1998). In this regard, Type E paleosols are Argillisols (Mack et al. 1993), which roughly corresponds to the soil orders Alfisols and Ultisols (Soil Survey Staff 1998). The dominant pedogenic process in soils with these characteristics is translocation of clay via leaching of Ca2+ from clay-exchange sites in the soil profile (Franzmeier et al. 1985). Significantly, argillic horizons form only in seasonal climates, upon well-drained, stable portions of the landscape, indicating that these Argillisols must have formed upon drier parts of the paleolandscape. Mixtures of kaolinite and smectite clays suggest that these soils underwent more significant weathering, possibly attributed to better drainage and/or duration of pedogenesis, than all other paleosol types discussed here. However, the presence of gley colors in Type E paleosols likely corresponds to sufficiently long periods of soil saturation and anoxic conditions to facilitate leaching of ferric Fe from the profiles. Vertic features in these paleosols indicate seasonal wet-dry cycles, suggesting seasonal precipitation in this region of tropical Africa.
We interpret Type E paleosols to have developed upon the more stable and well-drained areas of the Chilga region. This interpretation is based on the fine-grained nature of the strata associated with the profiles and pedologic features, such as argillic horizons, that formed upon older, more stable portions of the landscape (Soil Survey Staff 1975; Buol et al. 1997).
Paleobotany
A field survey of the Chilga basin in 2001
documented abundant and widespread plant fossil localities, which have now
produced over 1000 leaf, fruit, seed, flower, and wood specimens. These
specimens allow us to fill a large temporal gap in the tropical African
plant fossil record, and to address Paleogene plant evolution, biogeography,
and paleoclimate. The plant fossils occur in a variety of depositional
settings, the majority of which fall into the following five categories: (1)
overbank or pond deposits, which preserve autochthonous organic litter
consisting of leaf, flower, insect, and twig compressions associated with
pollen and in situ carbonized trees (associated with paleosol Type A,
Protosols; Figure 5A, C); (2) airfall tuffs, which preserve leaf and seed
compressions and impressions, in many cases associated with in situ
silicified tree stumps (Figure 5D); (3) tuffaceous ironstones, which
preserve fruit and seed casts associated with leaf impressions and
vertebrate fossils (associated with paleosol Type A, Protosols;
Figure 5E);
(4) lignites, which grade from compressed peat containing leaves and stems
to low-grade undifferentiated coal (associated with paleosol Type B,
Histosols); and (5) silicified in situ forests in a variety of interfluvial
depositional settings (Figure 5B).
In the following sections, we summarize what is known from the plant fossils found among these five preservational settings, and then integrate our results with those from geological and vertebrate paleontological research on the same deposits.
Overbank or Pond Deposits. These organic-rich deposits are to date the most productive in terms of density of plant fossils and quality of preservation. One of us (A. Pan) is focusing on the study of a single depositional unit (~30 cm thick) from which four sublocalities have been collected along a lateral exposure of approximately 60 m. Macrofossil compressions of at least 30 taxa among 533 specimens have been documented along this exposure, and include palm leaflets, petioles, and flowers representing the subfamilies Calamoideae, Coryphoideae, and Arecoideae; leaflets of cf. Sorindeia (Anacardiaceae), Dioscorea (Dioscoreaceae, section Lasiophyton), and Fabaceae (legumes); a flower of cf. Rubiaceae, leaves provisionally referred to the families Sapotaceae and Malvaceae s.l.; and one insect wing. Leaf cuticle and morphological features, such as hairs and glands, have been instrumental in providing key characters necessary for identification. The concentration of palm fossils and presence of at least three of the five extant subfamilies together at this site are unusual compared with the limited diversity of palms usually found in living African forests (Moore 1973). On the other hand, the presence of Dioscorea and Fabaceae species is more typical of living African forest communities.Table 2 allows a comparison of preliminary taxonomic lists among the four sublocalities, which range from 11 to 37 m apart from one another. Between 18% and 75% of the taxa recorded (to date) at each sublocality are unique to that sublocality. Furthermore, neighboring sublocalities share very few, if any, taxa with each other. The limited local distribution of taxa in the small sampling area documents heterogeneity of composition within the original plant community (cf. Burnham 1993). As our study has progressed, both diversity and heterogeneity have increased, and while the numbers shown in Table 2 may change somewhat with further work, we consider this variability of local plant composition comparable to that of modern forests of tropical West Africa (Richards 1996).
Plant community physiognomic information is provided by the presence of two charcoalified and partially compressed tree trunks lying prone at the base of the leaf bed (Sublocality 2). Leaf fossils were found draped over the trunks, but not below them. Thus, a forest gap was created by tree falls, perhaps associated with a fire, at a time prior to leaf-bed deposition, but not long enough before to allow for decay of the fallen trees. Upright, in situ, charcoalified tree stumps also occur along this outcrop (Sublocalities 1, 2, and 4), but appear to be rooted in the paleosol below the leaf bed. Thus, leaves and other forest debris fell around the bases of the in situ trees following a fire (presumably the same fire responsible for the tree falls).Airfall Tuffs. Five ash horizons can be traced laterally within the basin (for ≤ 5 km), and the least weathered of these (ash IV) was fossiliferous at all locations visited. While these deposits show some sedimentary structures associated with transport by water, there is evidence of at least periodic direct airfall consistent with the preservation of delicate fern fronds, which indicate rapid burial without transport more than a short distance. One fern taxon, Acrostichum, is found in ash IV at several locations. This genus grows today in edaphic settings associated with high salt content or poorly drained, swampy conditions (Mabberly 1993). Other plants from ash IV include a small pinnatifid fern, legume fruits and leaflets, fan palm leaves, broad-leaved dicot impressions, and in situ silicified tree trunks including palms. An Ar40/Ar39 age of 27.36+/-0.11 Ma on ash IV glass shards provides precise time control for the plant fossils (Figure 2; Kappelman et al. 2003).
Tuffaceous Ironstone. Fruit and seed casts are abundant in these deposits, which have also produced the majority of vertebrate fossils from Chilga. These sediments can be recognized everywhere in the basin by their red and black color derived from iron and manganese oxides. Organic plant material (i.e., cuticle) has not been found, but some leaf impressions show detailed venation and the fruit and seed casts preserve diagnostic surface features. Preliminary identifications include Arecaceae (palm) fruits, Fabaceae (legume) fruits and leaflets, and the seeds of at least three morphologically distinct species of Anonaspermum (Annonaceae).
Lignite. Macrofossils preserved in the lignites consist of robust stems and shoots. These have not yet been studied, but fieldwork in December 2004 documented the rare occurrence of cuticle in some lignite deposits. Thus, there is potential for further work on plant cuticle from the macrofossils and for the assessment of these deposits palynologically.
In Situ Silicified Forests. Trunk diameter measurements from 72 trees at two localities (34 and 38 trees, respectively) provide tree height estimates (Appendix) using the regression formulae of Rich et al. (1986) and Niklas (1994). The two sets of estimates are remarkably similar; however, estimates based on the formula of Rich et al. (1986) may be more accurate because the predictive formula was derived from dicotyledonous trees of modern tropical wet forest (Costa Rica), while the formula of Niklas (1994) was derived from a combination of woody gymnosperms and angiosperm dicots (not exclusively tropical). Both sets of estimates indicate that the trees in these forests stood between about 20 and 35 m tall (Appendix). The distribution of trees mapped at one of these localities, using GPS measurements with cm-scale accuracy, documents a density of one tree every 3 m, a minimum considering that not every tree would have been preserved. Identification of the specimens is in progress, but analysis of thin sections to date indicates forest assemblages with richness of ≥ 20 tree species over 6400 m2.Vertebrate Fossils
One of the most interesting and long-standing observations of Afro-Arabian vertebrate evolution concerns the dramatic turnover between the endemic archaic faunas of the Paleogene and the more modern largely immigrant faunas of the Neogene (Maglio 1978). Prior to the discovery of the Chilga vertebrates, the general absence of fossil localities dating from between 32-24 Ma offered little evidence as to the tempo and mode of this continental-scale turnover event. Discoveries in the Chilga region have helped to better establish the circumstances of this turnover and point out new research directions.Vertebrate localities occur through the entire section at Chilga (Figure 2) but are also more concentrated in tuffaceous ironstones associated with Paleosol Type A (Protosols). The remains are usually fragmentary, most commonly represented by teeth and jaws, although limb elements are not rare, and generally represent medium- to large-sized herbivores. The same depositional and preservational characteristics of the sediments that together act to produce such a superb paleobotanical record (see above) apparently work to bias the vertebrate record; mammals smaller than about 20 kg are not represented in our collections and their absence, along with the absence of birds, fish, or other small vertebrates, may be due to diagenetic leaching of bone prior to fossilization (Kappelman et al. 2003).
The Chilga vertebrate fauna (Table 3) is composed of large paenungulate herbivores that are well represented in older Paleogene Afro-Arabian localities (Sanders et al. 2004). The most distinctive form is a new species of Arsinoitherium (Order Embrithopoda), A. giganteum, larger in size than earlier forms, and representing the youngest known occurrence for this group. The hyracoids are a second group showing moderate diversity at Chilga, with two new species of the common but conservative genera Pachyhyrax and Megalohyrax, being well represented. A third species is larger in size and is probably related to Pachyhyrax, while a fourth is closely related to Bunohyrax. Proboscidea are the most diverse order of mammals and are represented by three families and five new species. Three of these new proboscideans belong to the family Palaeomastodontidae (Phiomia major, aff. Palaeomastodon sp. nov. A, and aff. Palaeomastodon sp. nov. B), with the Chilga occurrences representing the youngest known for this group. Deinotheres are also represented by a new genus and species, Chilgatherium harrisi, and extend back into time the earliest occurrence of this family by 7 Myr. The third family of Proboscidea is the Gomphotheriidae, with the Chilga occurrence of a new species (cf. Gomphotherium sp. nov.) again representing the oldest known for this group (Sanders et al. 2004).
