Greg H. Mack

Classification of paleosols

Despite increased interest in paleosols during the past decade, no satisfactory classification is in current use. Presented here is a largely descriptive classification system that utilizes those pedogenic features that have the highest preservation potential in the rock record. Emphasized in the classification are morphological and mineralogical features that are easily recognizable in the field and through the petrographic microscope. The classification is based on evaluation of the relative prominence in a paleosol of six pedogenic features or processes: organic matter content, horizonation, redox conditions, in situ mineral alteration, illuviation of insoluble minerals/compounds, and accumulation of soluble minerals. The most prominent of these six features/processes provides the key to classifying a paleosol into one of nine orders. Four of the order names are borrowed from Soil Taxonomy (Histosol, Spodosol, Oxisol, Vertisol), whereas the other five order names are presented here for the first time (Calcisol, Gypsisol, Gleysol, Argillisol, Protosol). The orders may be preceded by one or more subordinate modifiers that describe other important features of the paleosol. The classification is relatively easy to apply to the rock record and should enhance communication and aid in the standardization of terminology

Exceptions to the relationship between plate tectonics and sandstone composition

ABSTRACT Routine use of QFL, QpLsmLvm, and LmLsLv plate-tectonic provenance diagrams has led to the recognition of error populations. For these sandstones, the interpretation of tectonic setting based on data exclusive of petrography does not coincide with the interpretation of tectonic setting based on their location on compositional diagrams. The anomalous sandstones can be separated into at least four categories: i) sandstones deposited during the transition between tectonic regimes may be derived, in part, from relict source rocks; ii) the relative enrichment of sandstones in detrital quartz by weathering and/or depositional reworking may lead to an inaccurate interpretation of tectonic setting from compositional data; iii) sandstones deposited in tectonic settings as yet unrepresented on he provenance diagrams may plot between the provenance fields or overlap existing fields; iv) the role of detrital carbonate rock fragments directly affects the location of data points on provenance diagrams, and is an unresolved problem. The first two categories represent permanent problems in the use of sandstone composition as an indicator of plate-tectonic setting, whereas the last two may only be temporary problems that can be solved by more data and a consensus on determining detrital modes. Recognition of anomalous sandstones in the rock record will reduce the probability of erroneous interpretations of tectonic setting and enhance the use of sandstone petrology as an indicator of plate-tectonic setting.

Alluvial-fan sedimentation of the Cutler Formation (Permo-Pennsylvanian) near Gateway, Colorado

The Permo-Pennsylvanian Cutler Formation near Gateway, Colorado, is the most complete (1,334 m) proximal section of alluvial-fan sediments deposited along the western flank of the Ancestral Uncompahgre uplift. Cutler facies can be correlated with depositional processes observed on modern “dry” alluvial fans. Proximal Cutler facies include matrix-supported bouldery debris-flow and channel-form streamflood conglomerates. Midfan sedimentation in the Cutler is represented by trough–cross-bedded, granular, braided-stream sandstones, laterally continuous streamflood conglomerates, and sheetflood deposits. Laterally continuous streamflood conglomerate was deposited at the mouth of large channels near the intersection point and consists of a cross-bed set as much as 2 m thick with a basal boulder-cobble lag. Rippled and laminated siltstone with gravel channels represents distal sheetflood sedimentation. Pedogenic features include rhizocretions and calcareous nodules. Vertical changes in facies and maximum clast size delineate three megasequences on the scale of hundreds of metres thick. Each mega-sequence is composed of a coarsening-upward sequence of proximal facies overlain by a fining-upward sequence of more distal facies. Coarsening-upward sequences record periods of tectonic uplift and fan progradation, whereas fining-upward sequences result from tectonic quiescence and weathering-back of the mountain front. Small-scale cycles on the scale of ten metres occur within the larger megasequences and represent changes inherent to the alluvial-fan system (autocyclic). Sedimentologic data on Cutler alluvial-fan sediments at Gateway support previous interpretations of semiarid or arid paleoclimate during Permian time along the western flank of Uncompahgria and may act as a standard of comparison for tests of the role of tectonism on sedimentation trends in the Paradox basin.

The distribution and discrimination of shallow, authigenic carbonate in the Pliocene–Pleistocene Palomas Basin, southern Rio Grande rift

Nine types of authigenic carbonate are present in the Pliocene–Pleistocene alluvial-fan and axial-fluvial sediment of the Palomas half graben in the southern Rio Grande rift. Pedogenic and other vadose carbonate includes (1) pedogenic carbonate of stage II and stage III morphology underlying Bw and Bt horizons, (2) mudstones in which the carbonate nodules may be pedogenic or the result of shallow groundwater invasion of the vadose zone, (3) pedogenic calcic nodules and tubules in eolian sand, and (4) gully-bed cement of proximal-fan conglomerates formed by infiltration and evaporation of surface runoff. Shallow groundwater carbonate exists as (5) thin (30–50 cm), massive beds with an upper fringe of nodules and tubules precipitated at the water table and in the capillary fringe, (6) thick (1.5–3 m), massive beds deposited by lateral flow of groundwater or at springs, and (7) thin (30 cm), calcified root mats associated with near-surface, water-saturated sediment or springs. Phreatic spar cements (8) occupy the interstices of conglomerates and sandstones and locally exist as (9) oriented concretions. Groundwater carbonates are best developed near the toes of the large, hanging-wall–derived alluvial fans, whereas phreatic cement preferentially exists in footwall-derived, alluvial-fan conglomerates. Pedogenic carbonate is distinguished from groundwater carbonate by the association with other diagnostic paleosol horizons, a predominantly vertical arrangement of root traces, peds, desiccation cracks, and calcic tubules, and by gradational contacts. The δ 13 C and δ 18 O values are similar among pedogenic and shallow groundwater carbonate, although locally nodules in eolian sand and gully-bed cement have higher δ 18 O values, perhaps due to the effects of evaporation. Some phreatic cements may be distinguished from pedogenic and shallow groundwater carbonate by lower values of δ 13 C and δ 18 O. Authigenic carbonate in footwall-derived, alluvial-fan sediment has consistently higher values of δ 13 C than that in hanging-wall–derived sediment, which may reflect differences in vegetative type and/or density on either side of the basin.

Lateral erosion (‘toe-cutting’) of alluvial fans by axial rivers: implications for basin analysis and architecture

Abstract: We document the neglected phenomenon of lateral erosion (‘toe-cutting’) of alluvial fans by non-incising axial river channels. Field examples from the Holocene of the Big Lost River basin, Idaho and the Plio-Pleistocene of the Rio Grande Rift, New Mexico help to establish architectural models with more general application to basin analysis. The process of toe-cutting can lead to complete fan destruction and may be a response to climate change, tectonic tilting, fault propagation or a combination of these variables. It gives rise to: near horizontal erosion surfaces cut in fan sediment; steep fan-margin scarps; progressive up-fan incision from the scarp by a network of channels; soil formation up-fan away from the incised channel network; a deposit of axial alluvium that overlies the erosion surface and onlaps the scarp. Once avulsion occurs to take the axial channel away from the bajada margin, distinctive ‘healing-wedges’ of fan alluvium prograde across abandoned axial river channel and floodplain deposits, gradually onlapping the eroded scarp and its upstream network of incised channels. Toe-cutting has important stratigraphic basin analysis and economic consequences: bajada deposits subject to the process exhibit appreciable extra groundwater and petroleum reservoir potential in the intercalations of more porous and permeable axial fluvial sediments.

Tectonic control on facies distribution of the Camp Rice and Palomas Formations (Pliocene-Pleistocene) in the southern Rio Grande rift

The Pliocene-Pleistocene Camp Rice and Palomas Formations in the Rio Grande rift of southern New Mexico provide an excellent test of the role of basin symmetry in the distribution of piedmont and axial-fluvial facies. In the asymmetrical Palomas and northern Mesilla basins, the axial-fluvial facies is characterized by multistory channel sands and sandstones and is concentrated near the locus of maximum subsidence within a few kilometers of the footwall scarp. Fanglomerate derived from the footwall uplift extends only a few kilometers or less from the footwall scarp, whereas alluvial-fan and alluvial-flat conglomerate, sand, and mudstone deposited on the hanging-wall dip slope occupy a much wider outcrop belt. In the symmetrical Hatch-Rincon basin, the axial-fluvial facies extends to within a few kilometers of both the northern and southern basin margins and has a much higher percentage of fine-grained overbank deposits, some of which contain calcareous paleosols. In all three basins, a tongue of fanglomerate as much as 30 m thick prograded over the axial-fluvial facies near the end of Camp Rice and Palomas deposition. The distribution of facies of the Camp Rice and Palomas Formations supports previously published models that predict a two-stage history of asymmetrically subsiding basins. During tectonically active periods, axial-river channels preferentially avulse into the topographically lowest area of the alluvial plain, which is directly above the axis of maximum subsidence. During the postorogenic stage, when erosion rate exceeds subsidence rate, coarse, transverse-dispersed sediment progrades toward the center of the basin.

FIRST QUANTITATIVE TEST OF ALLUVIAL STRATIGRAPHIC MODELS : SOUTHERN RIO GRANDE RIFT, NEW MEXICO

Since 1978 the results of computational architectural models have been widely used to aid interpretation of ancient alluvial successions: here we present the first quantitative test of such models. We parameterize variables from field and magnetostratigraphic data collected from the well-exposed Pliocene-Pleistocene Camp Rice and Palomas Formations of the Rio Grande rift in south-central New Mexico. Computational runs establish that the LAB (Leeder, Allen, and Bridge) model correctly predicts the gross architectural patterns of ancestral axial Rio Grande half grabens and full grabens. Convergence of tectonic subsidence rate and mean sedimentation rate over the studied interval suggests that the dynamic basis of the models is correct; i.e., it is the tectonic “drawdown” of axially supplied sediment that controls the net preservation potential of alluvial successions.

Plio-pleistocene pumice floods in the ancestral Rio Grande, southern Rio Grande rift, USA

Abstract At least four times during the late Pliocene and early Pleistocene pyroclastic eruptions in the Jemez volcanic field, northern Rio Grande rift, flooded the ancestral Rio Grande with gravel-sized pumice. Following as much as 400 km of fluvial transport, the pumice was deposited in beds 0.2 to 2.0 m thick in the Camp Rice Formation of the southern Rio Grande rift. A combination of reversal magnetostratigraphy and single-crystal sanidine 40 Ar/ 39 Ar dating constrains the ages of pumice-clast conglomerates at 3.1, ∼2.0, 1.6, and 1.3 Ma. The coarsest pumice beds (cobbles, boulders) were deposited as antidune-like bedforms in a fluvial channel and as a crevasse-splay sheet. Granule and pebble-sized pumice was deposited as dune bedforms in fluvial channels and as ripple bedforms on the floodplain. The abundance of pumice clasts in the gravel fraction (60–100%) suggests very rapid transport downriver, probably in a few days or weeks. The two older pumice-clast conglomerates correlate with the Puye Formation in the Jemez volcanic field, whereas the younger two are coeval to the Lower Bandelier Tuff and Cerro Toledo Rhyolite.

Cyclic Sedimentation in the Mixed Siliciclastic-Carbonate Abo-Hueco Transitional Zone (Lower Permian), Southwestern New Mexico

ABSTRACT In southwestern New Mexico, Lower Permian (Wolfcampian) rocks grade southward from nonmarine siliciclastics (Abo and Earp Formations) to marine carbonates (Hueco and Horquilla Formations). A transitional zone between siliciclastic and carbonate facies trends east-northeast across southwestern New Mexico and consists of 64 to 186 m of cyclically interbedded siliciclastic and carbonate rocks, which were deposited in tidal-flat and shallow-marine environments. Shallow-marine facies include fossiliferous limestone and olive-gray shale. Tidal-flat facies consist of 1) ripple-laminated sandstone, which was deposited on intertidal sandflats near mean low tide, 2) mixed sandstone-shale, which was deposited on an intertidal flat shoreward of the ripple-laminated sandstone facies, and 3) nodula shale, which is characterized by pedogenic calcareous nodules and was deposited in a supratidal setting. The intertidal facies are truncated by or grade laterally into rare channel sandstones, which represent tidal-creek or estuarine facies. In addition to siliciclastic tidal-flat deposits, a few beds of laminated carbonate also were deposited in the intertidal zone. Vertical sequence analysis aids in delineating three types of depositional cycles. Asymmetrical cycles display the vertical sequence: basal fossiliferous limestone--olive-gray shale--ripple-laminated sandstone--mixed sandstone-shale--nodular shale, and record shoreline progradation. The asymmetrical cycle is always overlain by fossiliferous limestone, which indicates a major transgression that inhibited siliciclastic sedimentation. A common symmetrical cycle consists of fossiliferous limestone--olive-gray shale--ripple-laminated sandstone--olive-gray shale--fossiliferous limestone, and indicates systematic seaward and landward migration of facies zones associated with small-scale sea-level changes. A less common symmetrical cycle involves laminated carbonate--fossiliferous limestone-- aminated carbonate. Cyclic sedimentation in Abo-Hueco transitional strata is most likely the result of glacial eustatic sea-level fluctuations.

Calcic paleosols of the Plio-Pleistocene Camp Rice and Palomas Formations, southern Rio Grande rift, USA

Abstract Seventy-four separate paleosols are recognized in the Plio-Pleistocene Camp Rice and Palomas Formations of the southern Rio Grande rift. Seven types of profiles are present: A-Bw-Bk-C, A-Bt-Bk-C, Bw-Bk-C, Bt-Bk-C, Bt-K-C, Bk-C, and Ak. Calcic and petrocalcic horizons (Bk and K) commonly overlain by argillic horizons (Bt) dominate most paleosol profiles. Greater pore size and enhanced permeability, typical of sandy and silty parent material, favored the development of argillic horizons. However, micrite is the dominant form of calcium carbonate in Bk and K horizons regardless of parent material grain size. Non-calcified A horizons are typically lighter in color than adjacent subsurface pedogenic intervals due to their oxidized nature or lower clay content. They are only rarely preserved, probably due to erosion prior to final burial. Calcified root mats (Ak) are not directly associated with other pedogenic horizons. Abundant horizontal roots suggest root mats formed under high water table conditions. Restriction of Ak horizons to the distal piedmont lithofacies supports their development as spring or pond deposits near the toes of alluvial fans. Individual pedogenic horizons can often be traced hundreds of meters to kilometers normal and parallel to a basins axis. Paleosol maturity and preservation potential are greatest for symmetrical basins, especially where pedogenesis was far removed from active channels within the axial fluvial system or within distal piedmont settings of asymmetrical basins.