Peter J. McCabe

Perspectives on the Sequence Stratigraphy of Continental Strata

This report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 NUNA conference in Banff, Canada. To date, sequence stratigraphic concepts have been applied mainly to the marine realm, but unconformity-bounded units have long been recognized in nonmarine strata. Successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply. As with shallow marine environments, relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings. Farther inland, stratigraphic base level, which determines accommodation space, is more complex and takes various forms, such as the graded profile for fluvial strata, groundwater tables for some eolian strata, and lake level for some intermontane sediments. Sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area. The influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments. The fact that the major controls of climate, tectonism, and eustasy are somewhat interdependent, and that a change in one parameter will most likely be reflected in others, is also more readily apparent in continental strata. Although in its infancy, sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture. Of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata, and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits. The application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs.

Predicting facies architecture through sequence stratigraphy—An example from the Kaiparowits Plateau, Utah

Recognition of depositional sequences in Upper Cretaceous strata in the Kaiparowits Plateau allows examination of facies-tract geometries within coeval shoreface, alluvial, and coal-bearing strata. Changes in depositional architecture, sandstone-connectedness, sand-shale ratios, coal-bed geometry, and degree of shoreface and foreshore preservation are related to position within a depositional sequence. Base-level falls produced regionally extensive sequence boundary unconformities and a basinward shift in facies tracts. Slow rates of base-level rise resulted in amalgamated fluvial channel complexes, thin discontinuous coal beds, and progradational shoreface parasequence sets. More rapid rates of base-level rise produced vertically isolated meander-belt sandstones, thick extensive coal beds, and aggradational shoreface parasequence sets. The highest rates of base-level rise resulted in retrogradational shoreface parasequences and tidally influenced fluvial systems at least 60 km inland of coeval shoreface deposits.

Facies studies of coal and coal-bearing strata

Abstract Prior to 1960 most discussions on the depositional environment of coal-bearing strata were related to theories of the origin of cyclothems. Work during the 1960s on modern deltas, particularly the Mississippi, led to the interpretation of the classic cyclothem as deltaic in origin, with coals originating in delta-top swamps. During the 1970s and early 1980s, detailed models of depositional environments were developed for coal-bearing strata. Coals have now been described in association with virtually all types of terrestrial depositional environments. There are surprisingly few published examples of facies analyses of coal-bearing strata. Detailed measured sections and meticulous descriptions of sedimentary structures are necessary for geologists to make independent assessments of published environmental interpretations and to make comparisons between various coal-bearing formations. The coal industry could benefit from more predictive facies models, which will require greater integration of coal and clastic facies studies. In most mires close to areas of active clastic deposition, ash content of peat is too high for them to be precursors of coal. Most coal must have originated as peats in raised mires or in low-lying mires well removed from active clastic environments. In the latter case, a significant hiatus in deposition may be represented by the contact of coal with underlying sediments. Subsurface studies, which allow an understanding of the three-dimensional character of coal-bearing strata, may provide clues as to the type of swamp in which a coal was deposited. Recent studies in the Upper Cretaceous of Alberta, for example, provide clear evidence that coals originated in mires well removed from contemporaneous clastic depositional systems.

Sandstone-Body and Shale-Body Dimensions in a Braided Fluvial System: Salt Wash Sandstone Member (Morrison Formation), Garfield County, Utah

Excellent three-dimensional exposures of the Upper Jurassic Salt Wash Sandstone Member of the Morrison Formation in the Henry Mountains area of southern Utah allow measurement of the thickness and width of fluvial sandstone and shale bodies from extensive photomosaics. The Salt Wash Sandstone Member is composed of fluvial channel fill, abandoned channel fill, and overbank/flood-plain strata that were deposited on a broad alluvial plain of low-sinuosity, sandy, braided streams flowing northeast. A hierarchy of sandstone and shale bodies in the Salt Wash Sandstone Member includes, in ascending order, trough cross-bedding, fining-upward units/mudstone intraclast conglomerates, single-story sandstone bodies/basal conglomerate, abandoned channel fill, multistory sandstone bodies, and overbank/flood-plain heterolithic strata. Trough cross-beds have an average width:thickness ratio (W:T) of 8.5:1 in the lower interval of the Salt Wash Sandstone Member and 10.4:1 in the upper interval. Fining-upward units are 0.5-3.0 m thick and 3-11 m wide. Single-story sandstone bodies in the upper interval are wider and thicker than their counterparts in the lower interval, based on average W:T, linear regression analysis, and cumulative relative frequency graphs. Multistory sandstone bodies are composed of two to eight stories, range up to 30 m thick and over 1500 m wide (W:T > 50:1), and are also larger in the upper interval. Heterolithic units between sandstone bodies include abandoned channel fill (W:T = 33:1) and overbank/flood-plain deposits (W:T = 70:1). Understanding W:T ratios from the component parts of an ancient, sandy, braided stream deposit can be applied in several ways to similar strata in other basins; for example, to (1) determine the width of a unit when only the thickness is known, (2) create correlation guidelines and maximum correlation lengths, (3) aid in interpreting the controls on fluvial architecture, and (4) place additional constraints on input variables to stratigraphic and fluid-flow modeling. The usefulness of these types of data demonstrates the need to develop more data sets from other depositional environments.

Formation of Reactivation Surfaces Within Superimposed Deltas and Bedforms

ABSTRACT Under constant stage, reactivation surfaces can be produced in the cross-stratification of a flume delta with superimposed ripples. When it is washed out over the delta crest, the rate of erosion of a ripples stoss-side increases. As the reattachment flow, causing this erosion, is controlled by the migration of the next ripple upstream, erosion of the delta crest may continue after the first ripple is washed out. The rounding of the delta crest and its subsequent burial by the next ripple produces an inclined erosion surface similar in form to many described reactivation surfaces. In natural environments, this type of reactivation surface probably forms in superimposed bedforms and small-scale deltas. The marked variation in avalanche rates in superimposed bedforms should also produce pronounced foreset lamination.

Energy resources-cornucopia or empty barrel

Over the last 25 yr, considerable debate has continued about the future supply of fossil fuel. On one side are those who believe we are rapidly depleting resources and that the resulting shortages will have a profound impact on society. On the other side are those who see no impending crisis because long-term trends are for cheaper prices despite rising production. The concepts of resources and reserves have historically created considerable misunderstanding in the minds of many nongeologists. Hubbert-type predictions of energy production assume that there is a finite supply of energy that is measurable; however, estimates of resources and reserves are inventories of the amounts of a fossil fuel perceived to be available over some future period of time. As those resources/reserves are depleted over time, additional amounts of fossil fuels are inventoried. Throughout most of this century, for example, crude oil reserves in the United States have represented a 10-14-yr supply. For the last 50 yr, resource crude oil estimates have represented about a 60-70-yr supply for the United States. Division of reserve or resource estimates by current or projected annual consumption therefore is circular in reasoning and can lead to highly erroneous conclusions. Production histories of fossil fuels are driven more by demand than by the geologic abundance of the resource. Examination of some energy resources with well-documented histories leads to two conceptual models that relate production to price. The closed-market model assumes that there is only one source of energy available. Although the price initially may fall because of economies of scale long term, prices rise as the energy source is depleted and it becomes progressively more expensive to extract. By contrast, the open-market model assumes that there is a variety of available energy sources and that competition among them leads to long-term stable or falling prices. At the moment, the United States and the world approximate the open-market model, but in the long run the supply of fossil fuel is finite, and prices inevitably will rise unless alternate energy sources substitute for fossil energy supplies; however, there appears little reason to suspect that long-term price trends will rise significantly over the next few decades.

Organic control on shoreface stacking patterns: Bogged down in the mire

In ever-wet climates, raised mires that are elevated several metres above flood levels can cover significant portions of coastal plains. Because peat accumulation may keep pace with moderate rates of base-level rise, the development of raised mires may reduced the areal extent of marine transgressions. Thick, low-ash coals are present immediately landward of many vertically stacked shoreface parasequences in Cretaceous state of the Western Interior of North America. We suggest that these coals formed in raised mires that stabilized shorelines for long periods of time. In such settings, the rate of sediment supply (including peat accumulation) to the coastal environment is partly a function of the rate of change in base level.

Genetic features of petroleum systems in rift basins of eastern China

Abstract Most oil-bearing basins in eastern China are Mesozoic-Cenozoic continental rifts which have played a habitat for oil and gas in China. Investigation of the petroleum systems may give a better understanding of the oil and gas habitats in these basins. Of the essential elements of the petroleum system, the source rock is the most important in rift basins. However, rift tectonic evolution controls all the essential elements and processes necessary for a petroleum system. A four stage evolution model is suggested for the controls in the rift basin. A rift basin may consist of sub-basins, depressions, and sub-depressions, and major, moderate, and minor uplifts. A depression or sub-depression has its own depocentre (mainly occupied by source rock) and all kinds of lacustrine sediments, and thus has all the essential elements of a petroleum system. However, only those depressions or sub-depressions which are rich in organic matter and deeply buried to generate oil and gas form petroleum systems. Immature oil, another characteristic, complicates the petroleum system in the rift basins. Three types of oil and gas habitats are described as a result of this analysis of the petroleum systems of the 26 largest oil and gas fields discovered in eastern China rift basins: uplifts between oil source centres are the most prospective areas for oil and gas accumulations, slopes connecting oil source centres and uplifts are the second, and the third type is subtle traps in the oil source centre.

Coals Associated with Tidal Sediments in the Wilcox Group (Paleogene), South Texas

ABSTRACT The several coal seams in 50 m of Wilcox strata on the Chacon Creek East lignite property in South Texas are closely associated with tidal sediments. The coal-bearing sequence is underlain by greenish, bioturbated sandstone deposited in open marine water. The sediments between the coals and cutting the coals consist of coarsening- and fining-upward sequences of interbedded sandstone and mudstone with lenticular, wavy and flaser bedding and ripple-laminated and cross-bedded sandstone. The sediments contain marine dinoflagellates and palynomorphs of an inland flora. Most of the sediment accumulated in tidal channels. Each coal is underlain by rooted strata, usually an underclay. Palynomorphs in the lignite suggest the peat accumulated in low-lying swamps dominated by tupelo trees. These swamps lay along the landward margin of a tidal channel complex. The channel complex probably extended 10 to 30 km inland from the main coastline. Repeated regressive and transgressive episodes with corresponding expansion and contraction of the peat swamps within the estuarine complex would explain the coal-bearing sequence at Chacon Creek East.

Erosion Surfaces Within Giant Fluvial Cross-Beds of the Carboniferous in Northern England

ABSTRACT Giant cross-beds within Carboniferous fluvial channels in northern England contain three types of internal erosion surfaces: a) multiple convex upward surfaces, which may have been produced under steady state conditions by erosion in front of smaller superimposed bedforms; b) isolated convex upward surfaces with fine-grained drapes which are interpreted as a product of falling stage erosion with low stage deposition of fine-grained sediment; and c) concave upward surfaces, some of which may result from erosion of the slip face due to changes in the geometry of the leeside flow, but in others the large amount of sediment removed indicates substantial erosion which may have resulted from a shift in the position of the thalweg during falling or low stage.