Abstract A basin-scale pedostratigraphic model that focuses on paleosols and their pedostratigraphic relationships has been established for the Cenomanian Dunvegan Formation, a unit that represents a large delta complex. A detailed sequence stratigraphic and paleogeographic framework permits analysis of paleosol development with respect to distance from marine shorelines and coeval valleys. Paleosols that bracket sequence boundaries vary depending upon their paleo-landscape position. The sequence-bounding package of paleosols can be partitioned into three spatial zones based upon both the degree of development and the architecture of the paleosols. Zone 1 occurs in seaward localities near the maximum regressive shoreline and is characterized by hydromorphic, weakly developed paleosols typical of a poorly drained, progradational, and aggradational coastal plain. Zone 2 occurs in an intermediate location and is characterized by well-developed Alfisol-like welded paleosols that record a complex architecture indicating (i) an aggradational phase; (ii) a subsequent static and/or degradational phase related to valley incision, nondeposition, and soil thickening; and (iii) a final aggradational phase related to valley filling and renewed sedimentation across the coastal plain. Zone 3 occurs in more up-dip settings and is characterized by compound and complex Inceptisol-like paleosols that developed as the result of a reduced aggradation rate when valleys were being incised further down-dip. Because accommodation, sediment supply, and hydrological conditions vary in both dip and strike directions, the three zones represent lateral soil facies equivalents. The soil-forming interval bracketing the sequence boundary comprises a geosol composed of welded paleosols that subdivide both up-dip and down-dip into more weakly developed aggradational paleosol complexes. Above the sequence boundary, a high accommodation phase (equivalent to the Transgressive Systems Tract) is represented by widespread lacustrine and poorly drained floodplain facies and weakly developed hydromorphic paleosols. As accommodation rate decreases (late Highstand Systems Tract time) the alluvial succession becomes paleosol dominated, comprising floodplain pedocomplexes that record a regional decrease in the accommodation/sediment supply ratio. Up-dip variability along the sequence boundary and within sequences is controlled primarily by variations in the accommodation/sediment supply ratio, by hydrological variations associated with floodplain incision during valley formation, and by tectonic subsidence rates that vary in space and in time.

Abstract The Dunvegan Formation is a mid-Cretaceous alluvial plain-deltaic deposit exposed along the Rocky Mountain Foothills and Peace River Valley of Alberta and British Columbia, Canada. A multiproxy approach, combining paleosol micromorphology, geochemistry, and mineralogy with palynology, is used to reconstruct the climatic, pedogenic, and depositional history of this high-latitude setting during a greenhouse climate regime. Intrinsic features of paleosols within the Dunvegan Formation suggest a warm to cool temperate paleoclimate. These paleosols experienced multiple depositional phases superimposed on pedogenic phases that resulted in complicated compound, complex, and welded paleosol profiles. Well-preserved palynomorph assemblages within the paleosols are composed primarily of fern spores, with small percentages of gymnosperm pollen. The palynomorphs suggest a humid paleoclimate ranging from cool temperate to subtropical. The abundance of fern spores in all of the paleosol profiles suggests early successional colonization of the floodplain. Better-developed interfluve paleosols contain greater percentages of tree pollen, indicating the presence of nearby forests. Within interfluve paleosols, intervals barren of pollen coincide with sequence boundaries identified on the basis of micromorphology and geochemistry. Our combined paleopedological and palynological data sets, together with macrofloral and geochemical paleoclimate indicators, suggest that the Dunvegan alluvial-coastal plain complex probably formed under a humid, warm to cool temperate paleoclimate with a mean annual temperature (MAT) between 12 and 14° C and mean annual precipitation (MAP) between 1200 and 1300 mm yr -1 . These integrated data sets also provide a better understanding of the stratigraphic development of the coastal plains.

Abstract: The extreme sensitivity of rivers to subtle changes of slope resulting from tectonic tilting provides a basis for the interpretation of four Cretaceous paleovalley systems mapped on four successive sequence-bounding surfaces in the mid-Cenomanian Dunvegan Formation, deposited in the foredeep of the Western Canada Foreland Basin. The northern part of the study area overlies the Peace River Arch, a long-lived structural entity that is transected by faults trending dominantly NW–SE and NE–SW. The Dunvegan Fm. comprises a series of at least ten deltaic sequences labeled J–A in ascending order, and each spans ~ 100–200 ky. Valleys mapped on the top of sequences F, G, and H have many straight reaches that tend to parallel the rectilinear fault pattern in the underlying Paleozoic rocks, and many reaches are colinear with deep-seated faults. There is no evidence for deflection of rivers along the axis of the foreland basin. Isopach maps show that sequences H, G, and F do not thicken appreciably into the foredeep, suggesting that the contemporaneous rate of plate flexure was low. In-plane stress level is also inferred to have been low. Under these conditions, the forebulge was suppressed and long-lived faults in the Peace River Arch area were “unlocked”, permitting individual blocks to undergo vertical isostatic adjustment on a scale of meters. The low-gradient coastal-plain rivers were strongly influenced by this tectonic topography. In contrast, sequences E–A show marked westward thickening into a newly developed foredeep. Paleovalleys on sequence E do not have a preferred rectilinear pattern, instead being dominantly dendritic; distally, valleys are abruptly deflected to flow parallel to the basin axis. This new paleodrainage pattern is interpreted to reflect subtle uplift of a forebulge in response to renewed plate flexure, causing river deflection to the SE. The level of in-plane stress is inferred to have increased at this time, “locking” faults over the Arch and causing the land surface to deform as a regionally homogeneous plane, favoring dendritic drainage. The inferred position of the forebulge approximates the position of a long-lived Cretaceous paleodrainage divide and also coincides with a terrane suture in the Precambrian basement. The suture is inferred to have been a lithospheric weak zone, repeatedly serving to localize the forebulge. Although vertical displacement on both faults and forebulge cannot conclusively be recognized in cross sections or isopach patterns, it appears that rivers were readily diverted by these extremely subtle topographic features.

Abstract Until recently, sequence stratigraphic models have attributed systems tracts to periods of relative sea-level rise, highstand and lowstand. Recognition of a discrete phase of deposition during relative sea-level fall has been limited to a few studies, both in clastic and carbonate systems. Our work in siliciclastic ramp settings suggests that deposition during relative sea-level fall produces a distinctive falling stage systems tract (FSST), and that this is the logical counterpart to the transgressive systems tract. The FSST lies above and basinward of the highstand systems tract, and is overlain by the lowstand systems tract. The FSST is characterized by stratal offlap, although this is likely to be difficult or impossible to recognize because of subsequent subaerial or transgressive ravinement erosion. The most practical diagnostic criteria of the FSST is the presence of erosive-based shoreface sandbodies in nearshore areas. The erosion results from wave scouring during relative sea-level fall, and the stratigraphically lowest surface defines the base of the FSST. Further offshore, shoaling-upward successions may be abruptly capped by gutter casts filled with HCS sandstone, reflecting increased wave scour on the shelf during both FSST and LST time. The top of the FSST is defined by a subaerial surface of erosion which corresponds to the sequence boundary. This surface becomes a correlative submarine conformity seaward of the shoreline, where it forms the base of the lowstand systems tract. Differentiation of the FSST and LST may be difficult, but the LST is expected to contain gradationally-based shoreface successions because it was deposited when relative sea level was rising. Internally, the FSST may be an undifferentiated body of sediment or it may be punctuated by internal regressive surfaces of marine erosion and ravinement surfaces which record higher-frequency sea-level falls and rises superimposed on a lower-frequency sea-level fall. The corresponding higher-order sequences are the building blocks of lower-order sequences. The addition of a falling stage systems tract results in a significant reduction in the proportion of strata within a sequence that are assigned to the classical highstand and lowstand systems tracts. Many outcrop and subsurface cross-sections use an overlying ravinement, or maximum flooding surface as datum. Those surfaces might be flat, but they are not horizontal. Both dip seaward at slopes that generally are steeper than the fluvial system responsible for creating the sequence boundary. When a section is restored to such a datum, the falling stage systems tract will appear to record stratigraphic climb, whereas in fact it does not.