Dolores Formation Research Articles

Continental mollusk assemblages from the Quaternary of Uruguay

Only a few studies on Quaternary continental mollusks of Uruguay have been published over the last few decades. Despite the high occurrence of this group in many of the Late Pleistocene outcrops of the country, all of them are better known for their classic Ice Age mammal assemblages. To overcome this situation, 23 localities were sampled from two lithostratigraphic units: the Sopas and Dolores formations. The identified species are included in the families Sphaeriidae, Cyprinidae, Mycetopoda, Acridae (freshwater bivalves); Ampullariidae, Cochliopidae, Tateidae, Chilina, Physidae, Planorbidae (freshwater gastropods); Scolodontidae, Succineidae, Charopidae and Bulimia (terrestrial gastropods). As a result of these new samplings, the Sopas Formation mollusk species count increased from 20 to 29 species, and the Dolores Formation from six to 29 species. Seventeen of these species are new records for the Quaternary of Uruguay, and eight for the Quaternary of South America. Two bivalves, five freshwater gastropods, and a new terrestrial gastropod were added for the Sopas Formation; and for the Dolores Formation, four new bivalves, 14 new freshwater gastropods, and five new terrestrial gastropods. The Dolores Formation has been the most understudied unit until now, increasing here its biodiversity to 23 species. These new records of freshwater and terrestrial gastropods, increase the biodiversity and biogeographical range of the continental mollusks of the Late Pleistocene-early Holocene of the region. In addition, new AMS 14C ages were obtained for both lithostratigraphic units, framing the age of the mollusk assemblages for both formations in the a maximum range of 13,000–48,000 years before the present.

The otter Lontra Gray, 1843 (Mustelidae, Lutrinae) in the late Pleistocene – early Holocene of Uruguay

Within the order Carnivora, mustelids represent the most diverse family worldwide, with a great ecomorphological diversity in terms of diet, size and locomotory adaptations. Among them, the subfamily Lutrinae corresponds to semiaquatic forms, in which the genus Lontra is included. Widely distributed at present day, this genus is represented by three species across South America (Lontra longicaudis, Lontra felina and Lontra provocax), however their fossil record in the continent is very scarce. In the present contribution, new fossil material assigned to the Neotropical otter Lontra cf. L. longicaudis are described from late Pleistocene – early Holocene (Lujanian Stage/Age) deposits of Uruguay (Sopas and Dolores formations). Based on these findings, some aspects related to the knowledge of the group, paleoecology and paleoenvironmental conditions are discussed.

Marine deposits of the Chuy Formation (Late Pleistocene) and isostatic readjustments in the area of Laguna de Rocha (Uruguay)

The Chuy Formation is characterized by up to 135 m of an alternation of green pelites and fine to coarse sandstones with sparse invertebrate fossils. The marine deposits are interbedded with continental, loessic deposits of the Libertad Formation and overlain by the Dolores Formation, made up of mudstones with calcareous concretions. The results of the radiocarbon dating showed calibrated ages of 13.9±0.2, 41.5±1.9 and 50±3 ka BP on bioclasts of marine fossils at an elevation (above sea level, asl) of -2.8, -6.13 and -8 m respectively. Such ages and elevations are in disagreement with the sea-level evolution inferred for the Late Pleistocene. Two possible explanations are: (a) the ages represent minimum ages and deposition took place during the last interglacial at 115-130 ka (Marine Isotope Stage MIS 5e), or (b) they represent depositional ages but the area experimented large isostatic readjustments during and after the last glaciation. Several lines of evidence suggest a Late Pleistocene age for the upper Chuy Formation, including the d18O curve obtained from invertebrate shells, which shows large secular variations consistent with MIS 1 to 7. The proposed scenario envisages significant subsidence between 50-20 ka due to the far-field effects of glacial load in the Andes/Patagonia. At 20 ka the eustatic regression outpaced subsidence, leading to continentalization of the Laguna de Rocha area. Marine conditions returned at 15 ka and into the Holocene, except for continental deposits (Dolores Formation) at ca. 11-10 ka (Younger Dryas). An uplift of 115 m took place between 15 and 9 ka in the area, which is interpreted as post-glacial rebound. In the Holocene, moderate subsidence was further recorded. A regional trend is observed, with uplift of marine deposits increasing towards the W-SW, which is consistent with an explanation as post-glacial isostatic rebound.

Una Fauna Local Lujanense (Pleistoceno Superior–Holoceno Inferior) del sur de Uruguay: Santa Regina, Departamento de Colonia

A Lujanian Local Fauna (Upper Pleistocene–Lower Holocene) from southern Uruguay: Santa Regina, Colonia Department. On the southwestern coast of Uruguay, in the Colonia and San José departments, sedimentary rocks that correspond to the Neogene and Quaternary are found. Herein we describe the fossiliferous locality of Santa Regina, located on the east coast of the department of Colonia. The stratigraphic profile shows at the base grayish-green sandy rocks belonging to the Camacho Formation (Upper Miocene). Overlying this unit, there is a tough yellowish-greenish siltstone in net contact, in which several pieces of a Notiomastodon platensis specimen were found. Towards the upper part, a thicker greenish-brownish bed is observed, ranging upwards from conglomerates to siltstones. The lower part of this bed includes disarticulated and fragmentary remains of Lestodonarmatus, Glyptodon reticulatus, Pampatheriidae indet., Toxodon cf. T. platensis, Morenelaphus sp., and Lycalopex gymnocercus, and to the top large portions of carapaces of Glyptodon sp. and Panochthus sp. The aforementioned mastofaunistic assemblage corresponds to the Lujanian Mammal Age or regional Stage (Upper Pleistocene-Greenlandian). The lithostratigraphy allows us to refer the two upper levels to the Dolores Formation, a characteristic unit of the Quaternary of southern Uruguay, and not to the Raigón Formation, as indicated in some previous reports for this locality. The presence of Lestodon armatus, Glyptodon reticulatus, Notiomastodon platensis, Lycalopexgymnocercus, and the probable presence of Toxodon platensis, allow us to infer landscapes of open or semi-open areas, with a predominance of grasslands and shrubs during the genesis of the deposits. Keywords: LatePleistocene–Early Holocene, land mammals, stratigraphy, Uruguay.

Bioerosive traces in a Pleistocene Anatid bone from Uruguay

We report the first avian bone collected in the Late Pleistocene Dolores Formation, which crops out near Colonia city. The fossil was identified as a femur belonging to an Anatidae that represents the second Anseriform from Uruguay. Two bioerosive traces found on this fossil consist of borings of different sizes and depths were probably occasioned by larvae insect. These traces might have been produced during a brief period of subaerial exposure, in a continental paleoenvironment with dry episodes.

An extremely large saber-tooth cat skull from Uruguay (late Pleistocene–early Holocene, Dolores Formation): body size and paleobiological implications

Among the three recognized species of Smilodon, S. populator is the largest in size and has the widest distribution across South America. The present contribution describes an almost complete skull assigned to the aforementioned felid. The material was recovered from sediments of the Dolores Formation (Lujanian Stage/Age) from the southern part of Uruguay. This specimen is remarkable for its unusual craniodental measurements, indicating that it is one of the largest known specimens of the genus. Estimates of body mass indicate that this individual weighed over 400 kg. In addition, maximum prey size estimation greatly surpasses 1 t and approach nearly 3 t. Based on this, aspects of the paleobiology and paleoecology of S. populator are discussed. Undoubtedly, this kind of carnivorous mammal was at the top of the food chain, with clear adaptations for feeding upon the largest available herbivores, which potentially includes several megafaunal mammal species found in South America during the late Pleistocene.Aldo Manzuetti [aldomanzuetti@adinet.com.uy], Facultad de Ciencias (UdelaR), Iguá 4225, CP 11400, Montevideo, Uruguay; Daniel Perea [perea@fcien.edu.uy], Facultad de Ciencias (UdelaR), Iguá 4225, CP 11400, Montevideo, Uruguay; Washington Jones [wawijo@yahoo.com.ar], Museo Nacional de Historia Natural, 25 de Mayo 582, CP 11000, Montevideo, Uruguay; Martín Ubilla [ubilla@fcien.edu.uy], Facultad de Ciencias (UdelaR), Iguá 4225, CP 11400, Montevideo, Uruguay; Andrés Rinderknecht [apaleorinder@yahoo.com], Museo Nacional de Historia Natural, 25 de Mayo 582, CP 11000, Montevideo, Uruguay.

New Canid Remains from Dolores Formation, late Pleistocene-early Holocene, Uruguay

The fossil record of carnivorous mammals in Uruguay is scarce and fragmentary, but informative. In the present contribution, two new records of canids allocated in sediments of the Dolores Formation (late Pleistocene-early Holocene) are described. These records, based on their anatomical-comparative study and multivariate analysis, correspond to two foxes: one of medium size, Cerdocyon thous, conforms to the first record of this taxon in the country, meanwhile the other one, of larger size, is referred to Dusicyon avus and is the first fossil record of this animal in the south of the territory and the second record in the whole country. Until now, the only carnivorous mammals registered in this formation were the hunters of large herbivores (Arctotherium sp. and Smilodon populator). In this way, these discoveries complement and expand the set of placental mammals with a carnivorous diet for this unit, particularly with the capacity to predate over small- and medium-size mammals.

Lagostomus maximus (Desmarest) (Rodentia, Chinchillidae), the extant plains vizcacha in the Late Pleistocene of Uruguay

Ubilla, M. & Rinderknecht, A., April 2016. Lagostomus maximus (Desmarest) (Rodentia, Chinchillidae), the extant plains vizcacha in the Late Pleistocene of Uruguay. Alcheringa 40, xxx–xxx. ISSN 0311-5518The extant plains vizcacha, Lagostomus maximus, is described from the Late Pleistocene (Dolores Formation) of Uruguay based on an almost complete articulated skeleton. It is compared with the nominally extinct Pleistocene species of the genus. An AMS 14C taxon-age is determined for L. maximus at 11 879 ± 95 years BP (cal. BP 13 898–13 941). Lagostomus maximus is absent from modern mammal communities in Uruguay, and no Holocene evidence is available. Because L. maximus exhibits remarkable sexual and ontogenetic dimorphism, we examined a range of juvenile, sub-adult and adult male and female specimens. It is not possible to differentiate the articulated Pleistocene fossil from sub-adult specimens of L. maximus based on this sample. Moreover, the skull characteristics, including a broad vertical ramus of the zygomatic arch, semi-circular temporal crest and short robust sagittal crest, most closely resemble extant female individuals. Molar size is non-predictive for ontogenetic stage or body mass because it reaches stability during early adulthood. Other fragmentary skull remains are herein assigned to L. sp. cf. L. maximus. Lagostomus cavifrons from the Pleistocene of Argentina is considered synonymous with L. maximus. Its character states concur with the range of intraspecific variability and it has been established on a young adult or adult male individual. Likewise, the other Argentinean Pleistocene species based on incomplete mandibles, L. angustidens, L. striatus, L. heterogenidens, L. egenus, L. minimus and L. debilis, concur with ontogenetic morphs and are here assigned to Lagostomus sp. The ecological preferences of extant L. maximus infer open arid or semi-arid landscapes for the latest Pleistocene of southern Uruguay. This hypothesis is reinforced by the coeval presence of Microcavia, Galea and small camelids in the same strata. The Last Glacial Maximum likely promoted this environmental setting. Latest Pleistocene or early Holocene climatic change might have facilitated local extinctions and/or range shifts among this mammal fauna.Martín Ubilla [martinubilla@gmail.com], Facultad de Ciencias, Universidad de la Republica, Iguá 4225, 11400, Montevideo, Uruguay; Andrés Rinderknecht [apaleorinder@yahoo.com], Museo Nacional de Historia Natural, Paleontología, 11000, Montevideo, Uruguay.

Los gonfotéridos (Mammalia, Proboscidea) de Uruguay: taxonomía, estratigrafía y cronología

A large sample composed of relatively fragmentary remains of Gomphotheriidae was found in Uruguay. Belonging to this family there are materials assigned to Camacho Formation (Late Miocene), and to Pleistocene to Early Holocene units (Sopas, Libertad and Dolores formations). The Camacho and Sopas formations contain remains of indeterminate Gomphotheriidae, and the species Stegomastodon platensis is identified for the Libertad and Dolores formations. We think taxonomically suitable to support the opinion that all gomphotherids inhabitants of South American plains pertain to the genus Stegomastodon and to consider that species as the only representative of the genus in South America.

Catonyx cuvieri (Xenarthra, Mylodontidae, Scelidotheriinae) from the late Pleistocene of Uruguay, with comments regarding the systematics of the subfamily

ABSTRACT Increased field collecting over the last few years, combined with the examination of historical collections in Uruguay, has resulted in the discovery of a great number of specimens of the Scelidotheriinae, indicating that this subfamily is better represented in Uruguay's Pleistocene fauna than previously thought. Because much of this new material is diagnostic, in this work we provide a fuller description of some specimens (a skull with associated mandible and the manus, another almost-complete skull, and two partial dentaries) from the late Pleistocene of Uruguay for which only preliminary descriptions have been previously made, with tentatively assignments to Catonyx. A discriminant analysis was performed using 48 adult specimens including Scelidotherium, Catonyx, and Proscelidodon and supports the contention that Catonyx is a valid genus and the inclusion of these new specimens within this genus. This analysis also allows us to identify those cranial characters that better differentiate the genera. The variables involved in the discriminant analysis are those related to the length of the skull, whereas variables related to the width have been excluded from the models or when included have had little significance to the analysis. We provide new optically stimulated luminescence (OSL) dates in the range of 16–32 ka that confirms the late Pleistocene age of the Dolores Formation, the source of these specimens. The described remains represent the first record of Catonyx cuvieri in Uruguay and the first evidence for the presence of the species outside Brazil. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Lungfish burrows in the Upper Triassic Chinle and Dolores formations, Colorado Plateau; new evidence suggests origin by a burrowing decapod crustacean; discussion and reply

Lungfish burrows in the Upper Triassic Chinle and Dolores formations, Colorado Plateau; new evidence suggests origin by a burrowing decapod crustacean; discussion and reply

Lungfish Burrows in the Upper Triassic Chinle and Dolores Formations, Colorado Plateau: REPLY

Lungfish Burrows in the Upper Triassic Chinle and Dolores Formations, Colorado Plateau: REPLY

Synthesis of late Paleozoic and Mesozoic eolian deposits of the Western Interior of the United States

Late Paleozoic and Mesozoic eolian deposits include rock units that were deposited in ergs (eolian sand seas), erg margins and dune fields. They form an important part of Middle Pennsylvanian through Upper Jurassic sedimentary rocks across the Western Interior of the United States. These sedimentary rock units comprise approximately three dozen major eolian-bearing sequences and several smaller ones. Isopach and facies maps and accompanying cross sections indicate that most eolian units display varied geometry and complex facies relations to adjacent non-eolian rocks. Paleozoic erg deposits are widespread from Montana to Arizona and include Pennsylvanian formations (Weber, Tensleep, Casper and Quadrant Sandstones) chiefly in the Northern and Central Rocky Mountains with some deposits (Hermosa and Supai Groups) on the Colorado Plateau. Lower Permian (Wolfcampian) erg deposits (Weber, Tensleep, Casper, Minnelusa, Ingleside, Cedar Mesa, Elephant Canyon, Queantoweap and Esplanade Formations) are more widespread and thicken into the central Colorado Plateau. Middle Permian (Leonardian I) erg deposits (De Chelly and Schnebly Hill Formations) are distributed across the southern Colorado Plateau on the north edge of the Holbrook basin. Leonardian II erg deposits (Coconino and Glorieta Sandstones) are slightly more widespread on the southern Colorado Plateau. Leonardian III erg deposits formed adjacent to the Toroweap-Kaibab sea in Utah and Arizona (Coconino and White Rim Sandstones) and in north-central Colorado (Lyons Sandstone). Recognized Triassic eolian deposits include major erg deposits in the Jelm Formation of central Colorado-Wyoming and smaller eolian deposits in the Rock Point Member of the Wingate Sandstone and upper Dolores Formation, both of the Four Corners region. None of these have as yet received a modern or thorough study. Jurassic deposits of eolian origin extend from the Black Hills to the southern Cordilleran arc terrain. Lower Jurassic intervals include the Jurassic part of the Wingate Sandstone and the Navajo-Aztec-Nugget complex and coeval deposits in the arc terrain to the south and west of the Colorado Plateau. Major Middle Jurassic deposits include the Page Sandstone on the Colorado Plateau and the widespread Entrada Sandstone, Sundance Formation, and coeval deposits. Less extensive eolian deposits occur in the Carmel Formation, Temple Cap Sandstone, Romana Sandstone and Moab Tongue of the Entrada Sandstone, mostly on the central and western Colorado Plateau. Upper Jurassic eolian deposits include the Bluff Sandstone Member and Recapture Member of the Morrison Formation and Junction Creek Sandstone, all of the Four Corners region, and smaller eolian deposits in the Morrison Formation of central Wyoming and apparently coeval Unkpapa Sandstone of the Black Hills. Late Paleozoic and Mesozoic eolian deposits responded to changing climatic, tectonic and eustatic controls that are documented elsewhere in this volume. All of the eolian deposits are intricately interbedded with non-eolian deposits, including units of fluvial, lacustrine and shallow-marine origin, clearly dispelling the myth that eolian sandstones are simple sheet-like bodies. Rather, these units form some of the most complex bodies in the stratigraphic record.

Lungfish burrows in the Upper Triassic Chinle and Dolores formations, Colorado Plateau; comments on the recognition criteria of fossil lungfish burrows; discussion and reply

Lungfish burrows in the Upper Triassic Chinle and Dolores formations, Colorado Plateau; comments on the recognition criteria of fossil lungfish burrows; discussion and reply

Lungfish Burrows in the Upper Triassic Chinle and Dolores Formations, Colorado Plateau: REPLY

Lungfish Burrows in the Upper Triassic Chinle and Dolores Formations, Colorado Plateau: REPLY

Lungfish Burrows in the Upper Triassic Chinle and Dolores Formations, Colorado Plateau

ABSTRACT Vertical-to-inclined, cylindrical trace fossils that occur in the Upper Triassic Chinle and Dolores Formations on the Colorado Plateau are interpreted to be the casts of lungfish burrows. The casts, which are as much as 11 cm in diameter and as much as 1.6 m long, were formed by passive silicilastic and carbonate sedimentation into apparently abandoned lungfish burrows. Locally, the burrow fillings are overwhelmingly abundant, and many intersect and have destroyed former burrow fillings. Superposition of bioturbation episodes has obliterated most primary sedimentary structures. This bioturbation has contributed to the mottled coloration and the knobby-weathering texture of the rocks. The burrow-fillings occur ubiquitously in three lithofacies, comprising 1) purple- and white-mottled, ilicified sandstone and siltstone, 2) red and brown siltstone and mudstone, and 3) pink and green limestone. These strata were deposited in a continental environment that included fluvial channels and floodplains, sand sheets and playa mudflats, and lacustrine basins, marshes, and deltas. The identification of the trace fossils as the positive casts of lungfish burrows is based on their morphologic similarity to previously identified lungfish burrows and to available hand specimens. The widespread occurrence of the lungfish burrows in the Chinle and Dolores Formations attests to the extensive habitat that supported lungfish in the Late Triassic and to conditions favorable for burrow preservation. Analogy with the environments that support modern lungfish populations suggests that the Late Triassic climate in the study area provided sufficient moisture to support large populations of lungfish and that this climate was probably punctuated by seasonally dry periods.

Upper Cretaceous Stratigraphy and Tectonic History of the Ridgway Area, Northwestern San Juan Mountains, Colorado

Detailed mapping and stratigraphic studies in the Ridgway area, along the northwestern flank of the San Juan Mountains in Colorado, indicate that the lower Mancos Formation (Upper Cretaceous) can be subdivided into four members. From oldest to youngest, they are the Mancos 1 , Juana Lopez, Mancos , and Niobrara Members. These members have lithologic and paleontologic content similar to their equivalents in the Denver and San Juan basins, Colorado, and Black Hills, Wyoming. Subsurface and outcrop data to the south indicate that the study area straddles the southwestern margin of the Pennsylvanian Uncompahgre uplift. Structural evolution has been controlled largely by three east-west-trending Precambrian fault blocks- from north to south, the Orvis block, the Ouray graben, and the Uncompahgre block. Recurrent movement of these fault blocks from the Paleozoic through the Cenozoic affected both sediment thickness and facies distribution. A cross section restored to the base of the Dolores Formation (Upper Triassic) indicates that the Permian Cutler Formation increases 1,050 feet in thickness across the Orvis fault, which had down-to-the-south movement, into the Ouray graben. The north-dipping monocline now present at the surface suggests that significant reversal in movement took place during the Laramide orogeny.

Upper Cretaceous Stratigraphy and Tectonic History of Ridgway Area, Northern San Juan Mountains, Colorado: ABSTRACT

Detailed mapping and stratigraphic studies in the Ridgway area, along the northern flank of the San Juan Mountains in Colorado, indicate that the lower Mancos Group (Upper Cretaceous) can be subdivided into four formations. From oldest to youngest, they are the Benton Formation, Juana Lopez, Sage Breaks Shale, and Niobrara Formation. These formations have lithologic and paleontologic content similar to their equivalents in the Denver and San Juan basins and Black Hills. The underlying Benton Formation consists of 470 ft (143 m) of black shale that includes the Greenhorn Limestone Member (12 ft or 3.5 m thick) lying 300 ft (91 m) above the base. The overlying Juana Lopez is 40 ft (12 m) thick and consists of alternating layers of calcareous siltstone and black shale. The succeeding Sage Breaks is a 155-ft (47 m) calcareous shale unit. The overlying Niobrara Formation is an 85-ft (26 m) resistant, cliff-forming, calcareous shale. Subsurface and outcrop data in the south indicate that the study area straddles the margin of the Pennsylvanian Uncompahgre uplift. Structural evolution has been controlled largely by three east-west trending Precambrian fault blocks--the Ouray graben, the Orvis block, and the Uncompahgre block. Recurrent movement of these fault blocks from the Paleozoic through the Cenozoic affected both sediment thickness and facies distribution. A cross section restored to the base of the Dolores Formation (Upper Triassic) indicates that the Permian End_Page 572------------------------------ Cutler Formation increases 1,050 ft (320 m) in thickness across the Orvis fault, which had down-to-the-south movement. During the Laramide orogeny, a significant reversal in movement occurred along the fault to cause the north-dipping monocline now present at the surface. End_of_Article - Last_Page 573------------

Clastic Lacustrine Sedimentation in Triassic of Southwestern Colorado: ABSTRACT

Very fine sandstone and mudstone facies of the Upper Triassic Dolores Formation of southwestern Colorado provide evidence for shallow lacustrine deposition coeval with clastic lacustrine systems of the Dockum Group in Texas. Dolores and Dockum lakes had frequent water-level fluctuations; however, Dolores lakes were filled primarily by shoreline sequences, in contrast to the delta-filled Dockum lakes. Typical Dolores shoreline sequences fine upward, are laterally continuous over 3 km, are 5 to 15 m thick, have sharp nonerosional planar bases, and grade upward from very fine sandstones into mudstones. The very fine sandstones contain wavy, 1-cm thick bedding; low-angle trough and planar tabular cross-bedding; and isolated symmetrical channels. The overlying silty mudstones are commonly intensely bioturbated. These fine-grained shoreline deposits suggest that weak longshore currents distributed sand away from distributary mouths, and/or that sediment was transported by flow across the low-gradient, lake-margin plain. Frequent subaerial exposure of these shoreline sequences is documented by abundant desiccation-cracked and rain-textured mudstone drapes, and by well-developed caliche profiles. During low stages of Dolores lakes, distributary channels locally prograded across and sometimes incised into the shoreline sequences. These symmetrical channels, 8 to over 50 m wide, commonly contain basal mudstone-clast/caliche-pebble conglomerates and were abandoned episodically as indicated by alternating beds of very fine sandstone and mudstone. Lake-edge distributary channels contain fillings of wave-reworked, wavy-bedded to rippled very fine sandstone. End_of_Article - Last_Page 904------------

Triassic Paleocaliche in Red Beds of Dolores Formation, Southwestern Colorado: ABSTRACT

Early diagenesis of Upper Triassic lacustrine mudrocks and sandstones in southwestern Colorado includes pedogenic accumulation of calcium carbonate. Petrocalcic horizons, 0.4 to 1.5 m thick, are dominated by unlaminated micritic caliche concretions which range from pellets 1 mm in diameter to nodules 30 mm in diameter. Evidence for in-situ origin of these limestone concretions includes: irregular shape, floating texture, inverse grading, and truncation of back-filled burrows. Mature paleocaliche profiles are capped by calcium carbonate plugged horizons which contain rootlets 1 to 2 mm in diameter. These plugged horizons also contain paleofracture systems filled with reddish brown mudrock. Locally paleofracture morphology suggests plant-root control in its development. In very fine sandstones, micritic caliche nodules coalesce in upward coarsening, sinuous vertical stacks that suggest a rhizo-concretion origin. A maximum of nine petroclastic profiles were observed over a vertical interval of 56 m in one outcrop north of Durango, Colorado. The most mature profile observed exhibited the Stage III development of Gile and others. Caliche nodules and pellets occur throughout the Dolores Formation in southwestern Colorado either in pedogenic profiles or as transported clasts in fluvial, lacustrine deltaic distributary, or fan delta deposits. Literature descriptions suggest that caliche nodules are also present in the correlative Chinle Formation of New Mexico, Arizona, Utah, and Colorado. Based on the areal distribution of modern caliche, a semiarid climate is suggested for the Late Triassic in the Four Corners region of the western United States. End_of_Article - Last_Page 678------------