Paul J. McCarthy

Abstracts

Evolution of an ancient coastal plain: palaeosols, interfluves and alluvial architecture in a sequence stratigraphic framework, Cenomanian Dunvegan Formation, NE British Columbia, Canada

Paul J. McCarthy, A. Guy Plint and Ubiratan F. Faccini

To date, discussion of changes in alluvial style, and in the character of palaeosols, in relation to changes in accommodation and sediment supply on floodplains has largely been from a conceptual standpoint: few case studies are available against which to test ideas. We examined 130 m of nonmarine strata of the Dunvegan Formation in 14 closely-spaced sections in the canyon of the Kiskatinaw River, NE British Columbia, Canada. This site was located about 120 km inland from the transgressive limit of the contemporary marine shoreline and represents almost exclusively freshwater environments. Fluvial channels in the Kiskatinaw River section are of two types. Small, single storey, very fine- to fine-grained sandstone ribbons with W/T ratios <30, encased in fine-grained floodplain sediments, are interpreted as anastomosed channels. Fine- to medium-grained, laterally-accreted point-bar deposits forming multi-storey sandbodies with individual W/T ratios >30 are interpreted as the deposits of meandering rivers filling incised valleys. Interchannel facies include the deposits of crevasse channels and splays, lakes, floodplains and palaeosols. Floodplain palaeosols consist of laterally heterogeneous, simple palaeosol profiles and pedocomplexes similar to modern Entisols, Inceptisols and hydromorphic soils. Interfluve, sequence-bounding palaeosols adjacent to incised vallleys are laterally continuous, up to 3 m thick and can be reliably identified using a combination of (1) stratigraphic position, (2) field observations such as thickness, structure, colour, degree of rooting, and (3) micromorphological features such as evidence of bioturbation, clay coatings, ferruginous features, and sphaerosiderite. Interfluve palaeosols are similar to modern Alfisols and Ultisols.

Correlation of the local stratigraphic succession with the regional sequence stratigraphic framework, based on 2340 well logs and 60 outcrop sections, shows that the vertical changes in coastal plain character (more coals and lakes vs. more pedogenesis) can be related to relatively high-frequency base-level cycles (eustatic?) that in downdip areas are expressed as transgressive-regressive marine cycles. Regional isopach maps show that these cycles were progressively overprinted and modified by an increasing rate of tectonic subsidence in the north and west.

The character of palaeosols developed on aggrading floodplains primarily reflects local sediment supply and drainage. In contrast, well-developed interfluve palaeosols record pedogenesis during periods of reduced or negative accommodation (base-level fall). Vertical changes in floodplain palaeoenvironments and palaeosol types reflect changes in accommodation rate. Our detailed micromorphological analysis of interfluve palaeosols represents a powerful application of an under-utilized technique for the recognition of key surfaces in the geological record. This has broad implications for nonmarine sequence stratigraphy.

Sedimentology, 46: (in press)

Floodplain palaeosols of the Cenomanian Dunvegan Formation, Alberta and British Columbia, Canada: Micromorphology, pedogenic processes and palaeoenvironmental implications

Paul J. McCarthy and A. Guy Plint

Floodplains represent both sedimentological and pedological entities that record a unique blend of potential environmental indicators in the rock record. Because floodplains develop in aggradational environments with a high preservation potential, they preserve a detailed record of the local pedosedimentary history. Assemblages of micromorphological features present in floodplain palaeosols record, with a high degree of temporal and spatial resolution, specific pedological processes that operated under particular environmental conditions. Detailed micromorphological analyses of floodplain deposits, in conjunction with their regional distribution, are required for accurate palaeoenvironmental reconstruction of these environments.

Micromorphological features are described from floodplain palaeosols in the Upper Cretaceous (Cenomanian) Dunvegan Formation in northwestern Alberta and northeastern British Columbia, Canada. The palaeosols developed on low-lying and generally poorly drained floodplains during a time when the tectonic subsidence rate was very low. High-frequency fluctuations in base level were superimposed upon this tectonic signature, resulting in at least seven phases of valley cutting and the corresponding development of mature interfluve palaeosols. The major pedogenic processes operating were clay illuviation, redox processes, precipitation of soluble salts (primarily FeCO3) and pedoturbation. Palaeopedological features in the Dunvegan Formation are consistent with features developed in modern cool temperate soils with a non-seasonal precipitation distribution, such as Brunisols and Luvisols. The presence of embedded grain argillans, compound silt-clay coatings and rounded aggregates suggests at least the possibility of limited frost penetration in inland soils. Floodplains of the mid-Cretaceous Dunvegan Formation (palaeolatitude c. 65 N) record evidence for cool temperate conditions that supports other palaeobotanical and sedimentological evidence for cool Cretaceous palaeoclimates at high latitudes.

In: Marriott, S., Alexander, J. & Hey, R. (eds). 1999. Floodplains: Interdisciplinary Approaches. Geological Society, London, Special Publ. 163: 289-310.

Pedogenic and diagenetic influences on void coating formation in Lower Cretaceous paleosols of the Mill Creek Formation, southwestern Alberta, Canada

Paul J. McCarthy, I. Peter Martini and Dale A. Leckie

Pedogenic and diagenetic influences on void coating formation were studied in two paleosols from the Lower Cretaceous (Albian) Mill Creek Formation in southwestern Alberta, Canada, using detailed micromorphological, mineralogical and geochemical methods. The paleosols consist of alternating decimetre-scale beds of red, grey-green and variegated mudstone. Intermediate horizons (red and grey-green variegated mudstones) contain associations of pale yellow clay coatings, dark red clay coatings, Fe-depletion hypo-coatings and Fe-oxide quasi-coatings. SEM-EDX-ray analyses indicate that the dark reddish coatings are dominated by Si, Al and Fe, with subordinate amounts of K, Na, Ca, Mg and Ti. Pale yellow clay coatings have a higher Si content, and lower Al, K, Na, Ca and Fe contents relative to reddish coatings. Within the paleosol profiles, the distribution of clay minerals indicates: (1) an increase in discrete clay minerals downward, (2) a decrease in mixed-layer illite-smectite downward and (3) an increase in chlorite upward. X-ray diffraction indicates that the pale yellow and dark red clay void coatings are polymineralic. Pale yellow clay coatings have a higher proportion of mixed layer illite-smectite and kaolinite, less chlorite and are less crystalline than dark reddish coatings. Despite a diagenetic overprint, including development of diffusion gleyans, degradation of organic matter, burial reddening, and probably some smectite illitization, both compositional and morphological gradients exist in the paleosol profiles and void coatings. These gradients are most likely related to depositional variations and to pedogenic development in the original soils under variable Eh-pH conditions in the presence of organic matter. Therefore, although the void coatings are altered (diagenetic features) their morphology, mineralogy and perhaps even some of their microchemistry is attributable to the presence of pedogenic precursors initially formed in soils similar to modern temperate Alfisols and glossic soils.

Geoderma, 87: 209-237.

Recognition of interfluve sequence boundaries: Integrating paleopedology and sequence stratigraphy

Paul J. McCarthy and A. Guy Plint

In shallow-marine and coastal-plain strata, sequence boundaries are easily recognized at the base of incised valley fills. Between valleys, however, it is more difficult to recognize sequence boundaries where they are expressed as interfluve paleosols. We suggest that interfluve paleosols can be most reliably identified using a combination of (1) stratigraphic position; (2) field observations such as thickness, structure, color, and degree of rooting; and (3) micromorphological features such as bioturbation fabric, clay coatings, ferruginous features, and siderite and barite. Only micromorphology permits recognition of temporal changes in drainage, surface stability, and protracted pauses in sedimentation that typify these surfaces. This three-part approach has been successfully employed to identify interfluve sequence-bounding paleosols in the Cenomanian Dunvegan Formation in Alberta and British Columbia, Canada.

Geology, 26: 387-390.

Use of micromorphology for palaeoenvironmental interpretation of complex alluvial palaeosols: an example from the Mill Creek Formation (Albian), southwestern Alberta, Canada

Paul J. McCarthy, I. Peter Martini and Dale A. Leckie

Field observations are often not sufficient for process-based interpretations of palaeosols, particularly where they form parts of thick aggradational pedocomplexes within alluvial successions. Under such conditions micromorphology provides genetic, temporal and spatial information on soil-forming processes that is critical to an understanding of past environmental conditions. Thick alluvial successions of the Albian Mill Creek Formation contain abundant evidence of pedogenesis, but few well-developed palaeosol profiles, and therefore, provide an ideal case study in which to demonstrate the usefulness of micromorphological data for palaeoenvironmental interpretation. The micromorphological features of greatest interpretive value are types of clay coatings and ferruginous segregations, structure and fabric. Papules, evidence of bioactivity and ferruginous concretions provide information on geomorphic surface stability and assist in reconstructing temporal changes in drainage conditions. While individual features can provide some palaeoenvironmental information, the relationships of features to one another and assemblages of features provides additional information when analysed hierarchically to establish a sequence of sedimentologic and pedogenic events. A common, recurring sequence of palaeoenvironmental events, subject to local variations, can be recognized throughout the Mill Creek Formation. The presence of illuvial clay requires that water percolated through the soil and that the soil periodically dried out so that the translocated clay was retained. Dark reddish clay coatings indicate clay illuviation under freely drained conditions, while pale-yellow and silty clay coatings suggest that phases of free drainage alternated with phases of poorly-drained or saturated soil conditions. The presence of iron depletion coatings, iron nodules and quasiferrans indicates that these units were at least periodically saturated, and the occurrence of multiple, overlapping phases within single thin sections demonstrates that redox conditions fluctuated, strongly suggesting development in the vadose zone. Recent soils containing similar assemblages of features develop under warm temperate seasonal climates. Alternating phases of well-drained and saturated conditions on the Mill Creek floodplains are attributed to changing sediment supply and local palaeogeomorphology rather than to any major regional climate change. This type of process-based, micromorphological analysis should have broad application in other complex, pedogenically modified alluvial successions and similar studies would lead to a more detailed understanding of ancient palaeoenvironments.

Palaeogeography, Palaeoclimatology, Palaeoecology, 143: 87-110.

Anatomy and evolution of a Lower Cretaceous alluvial plain: sedimentology and palaeosols in the upper Blairmore Group, south-western Alberta, Canada

Paul J. McCarthy, I. Peter Martini and Dale A. Leckie

The Lower Cretaceous (Albian) upper Blairmore Group is part of a thick clastic wedge that formed adjacent to the rising Cordillera in south-western Alberta. Regional transgressive intervals are superimposed on the overall regressive succession. Alluvial conglomerates, sandstones and mudstones were deposited in east-north-eastward draining fluvial systems, oriented transverse to the basin axis. Five facies associations have been identified: igneous pebble conglomerate, thick sandstone, interbedded lenticular sandstone and mudstone, thick mudstone with thin sandstone interlayers, and fossiliferous sandstone and mudstone. The facies associations are interpreted as gravelly fluvial channels, sandy fluvial channels, sand-dominated floodplains, mud-dominated floodplains and marine shoreline deposits, respectively.

Five types of palaeosols are recognized in the upper Blairmore Group based on lithology, the presence of pedogenic features (clay coatings, root traces, ferruginous nodules, slickensides, carbonate nodules) and degree of horizonization. The regional distribution of the various types of palaeosols enables a refinement of the palaeoenvironmental reconstruction permitting an assessment of the controls on floodplain evolution. In source-proximal areas, palaeosol development was inhibited by high rates of sedimentation. In source-distal locations, poor drainage resulting from high watertables, low topography and lower rates of sedimentation also inhibited palaeosol development. The best-developed palaeosols (containing Bt horizons) occur in intermediate alluvial plain positions (tectonic hinge zone) where the floodplains were most stable due to a balance between sedimentation, erosion and subsidence rates. Extrapolating from the upper Blairmore Group suggests that the tectonic hinge zone of continental foreland basins can be established by palaeosol analysis. At the hinge zone, soil development is controlled primarily by climate and tectonics and their effect on sediment supply, whereas closer to the palaeoshoreline, relative sea level fluctuations, resulting in poor drainage, may have a more significant influence.

Sedimentology, 44: 197-220.

Pedosedimentary history and floodplain dynamics of the Lower Cretaceous upper Blairmore Group, southwestern Alberta, Canada

Paul J. McCarthy, I. Peter Martini and Dale A. Leckie

Thick alluvial successions in the upper Blairmore Group in the Foothills of southwestern Alberta contain few well-developed paleosol profiles, but abundant evidence of pedogenic modification. The pedogenically modified successions are characterized by vertical root traces, compound illuvial clay coatings, and ferruginous coatings and nodules. Five representative paleosol successions, composed of nine microfacies, are analyzed. The paleosol successions indicate soil development on more or less continuously aggrading floodplains; however, locally, rates of sedimentation and pedogenesis were highly variable. The paleosol successions contain features similar to modern alluvial soils, Brunisols, and Luvisols. Vertical trends within individual paleosol successions preserve the changing paleoenvironmental record at each site. The dynamic nature of the floodplain topography is reflected in alternating drainage conditions, represented by quasi-regular colour banding, compound clay coatings, and the type and degree of pedogenic development upwards within the paleosol successions. Overall, the paleosols indicate soil development under a warm, temperate paleoclimate with seasonal precipitation. Although representing minor diastems, the paleosols formed contemporaneously with sedimentation and are an integral part of the alluvial successions. Local variations in paleosol development are attributed to variations in sediment supply and water-table conditions related to the overall floodplain geomorphology.

Canadian Journal of Earth Sciences, 34: 598-617.

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