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

Abstract

To date, discussion of changes in alluvial style and in the character of palaeosols in relation to changes in accommodation and sediment supply on oodplains has primarily been from a conceptual standpoint: few case studies are available against which to test ideas. One hundred and thirty metres of nonmarine strata of the Dunvegan Formation were examined 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 ®ne-to ®ne-grained sandstone ribbons with W/T ratios <30, encased in ®ne-grained¯oodplain sediments are interpreted as anastomosed channels. Fine-to medium-grained, laterally accreted point-bar deposits forming multistorey sand bodies with individual W/T ratios >30 are interpreted as the deposits of meandering rivers ®lling incised valleys. Interchannel facies include the deposits of crevasse channels and splays, lakes, oodplains and palaeosols. Floodplain palaeosols consist of laterally heterogeneous, simple palaeosol pro®les and pedocomplexes similar to modern Entisols, Inceptisols and hydromorphic soils. Inter¯uve, sequencebounding palaeosols adjacent to incised valleys are laterally continuous, up to 3 m thick and can be reliably identi®ed using a combination of (1) stratigraphic position; (2) ®eld observations, such as thickness, structure, colour, degree of rooting; and (3) micromorphological features, such as evidence of bioturbation, clay coatings, ferruginous features and sphaerosiderite. Inter¯uve palaeosols are similar to modern Al®sols 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 are expressed as transgressive±regressive marine cycles in downdip areas. Regional isopach maps show that these cycles were progressively overprinted and modi®ed by an increasing rate of tectonic subsidence in the north and west. The character of palaeosols developed on aggrading¯oodplains primarily Present addresses: Sedimentology (1999) 46, 861±891 Ó 1999 International Association of Sedimentologists 861re¯ects local sediment supply and drainage. In contrast, well-developed inter¯uve palaeosols record pedogenesis during periods of reduced or negative accommodation (base level fall). Vertical changes in¯oodplain palaeoenvironments and palaeosol types re¯ect changes in accommodation rate. The detailed micromorphological analysis of inter¯uve palaeosols represents a powerful application of an under-used technique for the recognition of key surfaces in the geological record. This has broad implications for non-marine sequence stra...

Alluvial facies and paleosols in the Cretaceous Nanushuk formation, Kanayut River, North Slope, Alaska, Preliminary results from the 2001 field season

“…This process requires the soil to wet sufficiently so that colloidal clay is physically washed down through the soil in suspension, but the soil must also dry out enough so that the clays are retained on ped surfaces . The presence of orange mottles, however, suggests iron migration under variable to poorly-drained conditions, and the presence of sphaerosiderite and grey-brown colors indicate conditions that prevented decomposition of organic matter (McCarthy et al, 1999). These features are best explained by pedogenic development in an area where water table fluctuations were a frequent occurrence.…”

“…Interpretation The high organic content, papery structure and black color are indicative of organic soils or Histosols (Leckie et al, 1989;McCarthy et al, 1999). Coal is indicative of a wet or waterlogged environment of low clastic input where the accumulation rate of organic matter is equal to the rate of subsidence.…”

“…Interpretation The pale grey to grey coloring, siderite nodules and plant debris are indicative of a reducing, organic-rich environment (Retallack, 2001;McCarthy et al, 1999). Organic matter accumulates under anaerobic conditions which prevent decomposition.…”

Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record

“…This model may be considered a fI rst approach to the establishment of detailed sequence stratigraphy in terrestrial basins, but has to be im proved by taking into account the position of the system tracts in the basin, and testing it in areas where there are coeval marine and nonmarine strata (Shanley and McCabe, 1994) . A realistic application of palae osols to the understanding of sequence stratigraphy and fl oodplain development is the McCarthy et al (1999) study of the Cenomanian of British Columbia. Here, the higher frequency sequence boundaries are represented by valleys and interfl uves.…”

Reconstructing a Mid-Cretaceous Landscape from Paleosols in Western Canada

“…The abundance of clay coatings and infillings is evidence for illuviation of fine-grained sediments through the soil profile, and illuviation occurred when the soils were well drained (FitzPatrick 1984;McCarthy and Plint 1998;. The thickness and abundance of clay coatings indicate that the soil was well drained for a long period of time (McCarthy et al 1999b). The deformation and reworking of clay coatings into the matrix suggests prolonged landscape stability.…”