Abstract Terrestrial sediments are difficult to correlate because they are laterally variable and generally lack easily identifiable chronostratigraphic surfaces. However, we have been able to identify systematic variations in petrographic properties of paralic coal that respond to changes in accommodation. These properties enable us to distinguish two types of paralic peat cycle (transgressive and regressive) characterized by wetting-upward and drying-upward behavior linked to variations in the groundwater table. They also enable recognition of a range of terrestrial stratigraphic surfaces that record responses to changing accommodation, including accommodation reversal surfaces, flooding surfaces, hiatal surfaces, paludification surfaces, and terrestrialization surfaces. A combination of these coal parameters, together with the facies characteristics of the surrounding terrestrial and marginal marine sediments, enables recognition of distinctive high-resolution sequence stratigraphic signatures. This in turn provides a previously unavailable ability to correlate stratigraphic units from their down-slope marine position, through the shoreline zone, and into the terrestrial realm. Results show that earlier concepts of parasequences in marine sediments need to be significantly modified in the terrestrial realm. Sharp hiatal parasequence boundaries in the marine realm such as flooding surfaces and wave or tidal ravinement surfaces may correlate up-slope to packages of sediments that pass gradationally from transgressive to regressive units and preserve the transitions between the two. Terrestrial sediments may accumulate during and following shoreline regression, and prior to and during shoreline transgression. The exact style and preservation of the terrestrial stratigraphic package depends on the local balance between accommodation and sediment flux at the time of deposition. Coals occur in both regressive and transgressive styles and may initiate or terminate parasequences. Coals may also occur as compound coals that span more than one parasequence and contain internal discontinuity surfaces. Application of Modern Stratigraphic Techniques: Theory and Case Histories SEPM Special Publication No. 94, Copyright © 2010 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-199-5, p. 201–219.

Abstract A seismic and multibeam sonar (swath) imaging study onboard the RV Southern Surveyor in Hervey Bay, east coast of Australia, revealed an incised valley (up to 50 m deep, 600 m wide, and at least 5 km long) cut into a Pleistocene shelf carbonate platform. The incision occurred in the Late Pleistocene, during the last eustatic sea-level lowstand. The subsequent transgression drowned the incision, but none of the fluvial or estuarine sediments that are typically deposited within incised-valley systems during passage through the coastal zone are evident on seismic. Instead, the seismic shows a low-amplitude fill with few internal impedance contrasts, suggesting a homogeneous, sandy composition. The incision is currently underfilled, and shows as a sinuous impression on the sea floor. Shelf sand dunes can be seen on the surrounding carbonate shelf migrating from the south towards the valley. At one location in the study area, the dunes migrate into the valley and appear to be filling it, because the dune crests can be traced across the valley margin and into the valley. Backscatter data from the multibeam survey also confirm the presence of extensive sand covering the valley floor. Scours can be seen adjacent to the steep valley walls, indicating the presence of strong tidal currents, which could be a mechanism for redistributing sand inside the valley. Implications for existing facies models of incised-valley systems are significant, because the filling of this incision did not occur in the coastal zone during transgression. Filling by shelf sediments could occur at any time during the sea-level cycle when the shelf and valley are not subaerially exposed. Depending on the dominant shelf process responsible for filling the valley, the valley-fill deposits can be anything from homogeneous sand, as in the case described here, to highly variable shelf facies, which would complicate any potential reservoirs in such a valley fill.

Abstract Modern estuaries and incised valleys are important depositional settings that have widespread significance for human land use. The deposits of these environments are economically important for hydrocarbon exploration and production. Estuaries and incised valleys are a complex and possibly unique environmental grouping, inasmuch as they represent creation of depositional space by one process (mainly fluvial erosion) and fill of that space by a range of other processes (fluvial, estuarine, and marine deposition). Early investigations of valleys began slowly in Greek and Roman times, but increased in the nineteenth century, when they were used to develop ideas on the age of the earth in uniformitarian debates. Gradual progress was made throughout the nineteenth and twentieth centuries with the introduction of ideas on river grade, fluvial equilibrium profiles, and base level, followed by the development of fluvial facies models in the 1960s. Studies on estuaries began in earnest much later than those on valleys, and major advances were not made until the mid-twentieth century, with development of the first comprehensive facies model in the 1990s. Research on estuaries and incised valleys was energized in the 1980s by the concept of sequence stratigraphy, and work in the field has mushroomed since then. Indeed, the currently used facies models for estuaries and incised valleys were among the first to explicitly take into account the external control on the creation of accommodation and to be presented in a sequence-stratigraphic framework. In line with other sedimentary environments, the facies models for estuary and incised-valley environments have also proliferated, leading to the need for fundamental advances in how facies models are conceived. Estuaries, as defined geologically here, are transgressive in nature. They receive sediment from both fluvial and marine sources, commonly occupy the seaward portion of a drowned valley, contain facies influenced by tide, wave, and fluvial processes, and are considered to extend from the landward limit of tidal facies at their heads to the seaward limit of coastal facies at their mouths. Estuaries can be divided, on the basis of the relative power of wave and tidal processes, into two main types, wave-dominated estuaries and tide-dominated estuaries. Estuarine facies models exhibit generally retrogradational stacking of facies and a tripartite zonation reflecting the interaction of marine and fluvial processes. All estuaries and incised valleys have a fluvial input by definition, but estuarine facies models reflect the balance between wave and tidal processes. Valleys form because the transport capacity of a river exceeds its sediment supply. An incised-valley system is defined as a fluvially eroded, elongate topographic low that is characteristically larger than a single channel, and is marked by an abrupt seaward shift of depositional facies across a regionally mappable sequence boundary at its base. The fill typically begins to accumulate during the next baselevel rise, and it may contain deposits of the following highstand and subsequent sea-level cycles if the accommodation is not filled during the first sea-level cycle. Incised valleys may be formed by either a piedmont or a coastal-plain river and can exhibit a simple or compound fill. The erosion that creates many incised valleys is thought to be linked to relative sea-level fall, although climatically produced changes in discharge and/or sediment supply may independently cause incision, even in areas far removed from the coast. In the case of valleys in coastal areas, fluvial deposition typically begins at the mouth of the incised-valley system when sea level is at its lowest point and expands progressively farther up the valley as the transgression proceeds, producing depositional onlap in the valley. Based on the longitudinal distribution of broad depositional environments, the length of an incised valley can be divided into three segments. Ideally, the fill of the seaward portion of the incised-valley (segment 1) is characterized by backstepping (lowstand to transgressive) fluvial and estuarine deposits, overlain by transgressive marine deposits. The middle reach of the incised valley (segment 2) consists of the drowned-valley estuarine complex that existed at the time of maximum transgression, overlying a lowstand to transgressive succession of fluvial and estuarine deposits similar to those present in segment 1. The innermost reach of the incised valley (segment 3) is developed headward of the transgressive estuarine-marine limit and extends to the point where relative sea-level changes no longer controlled fluvial style (i.e., to the landward limit of sea-level-controlled incision). This segment contains only fluvial deposits; however, the fluvial style changes systematically due to changes in the rate of change of base level. The effect of base-level change decreases inland until eventually climatic, tectonic, and sediment-supply factors become the dominant controls on the fluvial system. In valleys far removed from the sea, the fill consists entirely of terrestrial deposits, but shows changes in fluvial style that are similar to those in segment 3, even though the stacking patterns are controlled more by local tectonics and climate. Recent and future development of estuarine and incised-valley facies models has emphasized the use of ichnology to recognize brackish-water deposits and the ability to subdivide compound valley fills on the basis of sediment composition. Imaging the valley and its fill has been greatly improved with 3D and 4D seismic techniques. Seabed mapping of modern estuaries has enabled detailed distributions of facies and morphology to be compiled, enhancing the ability to predict these features in ancient rocks. Our current set of facies models represents the early classification stage in the development of depositional models. The appropriate way forward appears to be a transformation from qualitative approaches to empirical and quantitative computer-based models with predictive capability, based on a thorough understanding of the dominant processes operating in each environment.

Abstract Incised-Valley Systems: Origin and Sedimentary Sequences - Incised valleys were not widely recognized prior to the 1980?s. Most early workers forced the isolated, incised-valley deposits along an uncomformity into a single continuous unit, ignored them by including them within larger stratigraphic units, or interpreted them as deltaic distributaries or non-incised fluvial channels. In the last decade, intense interest in the influence that changes in accommodation space have on stratigraphic organization has focused attention on incised-valley systems, because they are one of the most visible records of major decreases in accommodation. In practical terms, they are also a significant key to the identification of sequence-bounding uncomformities. As a result, many successions have been re-examined and incised-valley fills are being found in rapidly growing numbers. This volume is an outgrowth of this widespread interest in incised-valley sedimentation. Many of the papers were initially presented at the Special Session on ?Recognition and Facies of Incised Valley Fills? held at the AAPG-SEPM Annual Meeting (Calgary) in June, 1992.

Abstract The study of unconformities has a long and distinguished history, and incised valleys have been recognized for more than 70 years. Early descriptions of incised-valley deposits lacked detail, with fluvial and deltaic interpretations predominating. Estuarine deposits were largely unrecognised until advances in our understanding of estuarine sedimentation permitted more sophisticated treatment of the fluvial-marine transition. Interest in incised-valley systems has increased dramatically in the last decade due to widespread application of sequence-stratigraphic concepts. Following standard definitions, we urge that the term "incised valley" be restricted to fluvially eroded features that are larger than a single channel. A loss of accommodation space and the resulting formation of incised valleys may occur in response to factors unrelated to changes in relative sea level; however, all but one of the examples described in the volume are believed to be associated with a drop of relative sea level. Thus, the model proposed by Zaitlin and others (this volume) for this type of incised-valley system is used to group the papers according to which portion of an incised-valley system the deposits represent: segment 1 —the portion between the mouth of the valley and the initial highstand shoreline, which is transgressed and overlain by marine deposits; segment 2 —the region occupied by the drowned-valley estuary at the time of maximum transgression; and segment 3 —the incised valley landward of the limit of marine/estuarine facies, which contains and is overlain exclusively by fluvial deposits. Each segment displays a predictable succession of environments and stratigraphic surfaces, but differences exist between the examples due to the poorly understood influence of such factors as the rate of sediment input and the magnitude and duration of the relative sea-level fall and rise.

Abstract Valleys are formed by both erosional and tectonic forces, although the former is the most common. Most valleys form by channel incision, and they follow an evolutionary sequence of deepening and widening that is controlled by the lithologic and structural character of the valley perimeter and the erosive power of the river that is eroding the valley. Usually valleys are single features that are contained by valley walls, but on alluvial plains a complex anastomosing network of multiple valleys can develop as a result of channel avulsion. In general, valley dimensions increase down valley, and older valleys are larger than younger valleys, but the main characteristic is great variability both in cross section and longitudinally. Unlike stream channels formed in relatively homogeneous alluvium that reflect hydrologic and hydraulic variables, valley morphology also reflects lithologic and structural controls. Valley-fill deposits should also reflect this variability to a lesser extent, as channel morphology varies in response to variations of valley slope and to tributary influences.

Abstract The response of naturally occurring river systems to changes in regional controls such as base level or climate involves a complex response by many factors. These include changes at different reaches within the drainage network of: (1) river vvidth, depth, slope, and type (e.g., braided vs. meandering); (2) bedload to suspended load ratio; (3) bed friction factors; (4) vegetation; (5) bed and bank grain-size distribution; (6) evaporation and runoff potential; (7) discharge flashiness; (8) drainage system plan form and density; (9) avulsion frequency; (10) meander migration speed and wavelength; (11) average drainage-basin size for each order of stream; and (12) valley broadening. The complexities of an evolving river system are somewhat simplified by considering a self-similar drainage network of tributaries such that tributaries themselves have tributaries. In such a self-similar network, the various factors considered above change in a theoretically predictable fashion such that the lowest-order (alluvial-valley) streams are dominantly erosional while the highest-order (coastal-plain) streams are dominantly depositional. Primary among changes from the alluvial valley to coastal plain are changes in discharge flashiness, flood-stage frequency, the rate of overbank deposition, and sediment delivery ratio. These changes are theoretically derived from an advective-diffusive-wave equation for flood-wave travel in a tributary network obeying Horton's Laws. Base-level, tectonic, and climatic changes can act as perturbations to this homeostatic, self-similar drainage system. A qualitative response model is proposed based on the above theoretical work that links these forcing factors to their depositional response. Stratigraphic examples of incised-valley formation and fill are used to test the implications of this model. Both this theoretical model and field observations suggest that: (1) broad valley erosion occurs with a seaward shift of the dominantly erosional alluvial valley, which makes up the upper reaches of the drainage network; (2) incised alluvial valleys formed during periods of slow sea-level fall preferentially incise into lower coastal-plain sediments and, thus, the expected fill of these valleys is transitional, estuarine, or marine sediment; (3) entrenched channels formed during periods of slow sea-level fall preferentially incise into lower alluvial-valley sediments and, thus, the expected fill of these channels is fluvial rather than transitional or marine sediment; and (4) entrenched channels formed during periods of rapid sea-level fall preferentially incise into lower coastal-plain sediments and, thus, the expected fill of these channels is transitional, estuarine, or marine sediment.

Abstract The most common form of incised-valley system develops during a lowering in base level associated with a fall in relative sea level. This form of incised-valley system provides the most complete, and at times, the only evidence of lowstand to early-transgressive deposition in shelf and/or shallow ramp depositional settings. Incised-valley systems of this type are characterized by a flu-vially-eroded, elongate paleotopographic low, generally larger than a single channel, which displays an abrupt basinward shift of facies at its base. The valley fill typically begins to accumulate during base-level rise, and may contain deposits of the following highstand and subsequent sea-level cycles. Two major varieties of incised valley occur during a lowering of sea level: (i) incised-valley systems that have their headwaters in a (mountainous) hinterland and cross a "fall line" (or knickpoint) are here considered to be piedmont incised-valley systems , and (ii) incised-valley systems that are localized within low-gradient coastal plains and that do not cross a "fall line" are here termed coastal-plain incised-valley systems . An incised-valley system that is filled during one depositional sequence is termed a simple fill , whereas a compound fill records multiple cycles of incision and deposition. The fill of an incised-valley system that forms in response to a lowering of base level is divisible into three segments: (i) the seaward reaches of the incised valley (SEGMENT 1) is characterized by backstepping (lowstand to transgressive) fluvial and estuarine deposits, overlain by transgressive marine sands and shelf muds; (ii) the middle reach of the incised valley (SEGMENT 2) consists of the drowned-valley estuarine complex that is developed at the time of maximum transgression, overlying a lowstand to transgressive succession of fluvial and estuarine deposits like those in segment 1; and (iii) the innermost reach of the incised valley (SEGMENT 3) lies headward of the transgressive estuarine limit, and extends to the point where changes in relative sea level no longer control fluvial style. Segment 3 is characterized by fluvial deposits throughout its depositional history; however, the fluvial style may change systematically due to changes in base level and the rate of creation of accommodation space. The stratigraphic organization of these incised-valley systems is characterized by a number of stratigraphically-significant surfaces that differ greatly in their origin, geographic extent, and chronostratigraphic significance. Filling of the valley may begin during the lowstand, but typically continues through the succeeding transgression. Thus, the transgressive surface (i.e., the flooding surface separating the Lowstand Systems Tract and the Transgressive Systems Tract) should be present in the lower portion of the fill. It may occur within fluvial deposits or at the fluvial-estuarine contact in segments 1 and 2, and at a correlative change in fluvial depositional style in segment 3. Erosion by tidal currents in tidal inlets or other tidal channels creates a tidal ravinement surface which is confined to the incised valley in segment 1 and the seaward part of segment 2. More regional erosion by waves at the retreating shoreface produces a wave ravinement surface that separates fluvial and/or estuarine sediments from overlying marine deposits in segment 1. Both of these surfaces are diachronous, and could become amalgamated with the sequence boundary. In the idealized case, a maximum flooding surface may extend throughout the incised-valley fill, passing from its typical position within marine shales in segment 1, through the center of the estuarine deposits in segment 2, into fluvial sediments in segment 3. However, rapid relative sea-level fall after the end of the transgression, or renewed sea-level rise after valley filling (but before the onset of significant progradation), may prevent development of the maximum flooding surface. Compound valley fills may contain multiple sets of these surfaces.

Abstract Over 1,000 km of high-resolution seismic profiles and nearly 200 cores were interpreted to document the evolution of the Trinity/Sabine incised-valley system, which lies offshore of Galveston Bay and Sabine Pass. Long-term sea-level fluctuations, related to Wisconsinan ice-sheet growth, caused multiple cycles of incision and valley-filling that are preserved as terraced fluvial deposits. The oldest incision began approximately 110 ka (δ 18 O stage 5d). Age estimates are possible because the unconformities which bound the fluvial terraces are traceable laterally and bound offlapping sequences (probably deltaic) on the adjacent interfluves. The sequence stratigraphy of the interfluves suggests each offlapping sequence correlates to an isotope substage (20 ka in duration). The terraced fluvial deposits represent the lowstand and transgressive phase of each 20 ka cycle, and the offiapping sequences represent the highstand phase. Short-term changes in the rate of sea-level rise during the Holocene transgression affected the facies architecture of the incised-valley fill. There are three parasequences recognized, and they step landward within the incised valley, thus the distribution of valley-fill facies is discontinuous. Parasequences formed during sea-level stillstands and consist of paired upper-bay and tidal inlet facies. Periods of rapid rise are manifested as flooding surfaces that bound each parasequence. Large shelf sand banks situated adjacent to the incised valley are associated with each stillstand parasequence but were extensively reworked and isolated by subsequent sea-level rises. The younger parasequences show decreased preservation of lower bay facies, attributed to the slowing of sea-level rise (therefore, decreasing accommodation) about 6 ka.