Proposal for a Systematic Study of Tidal-Bar Deposits · 2005-11-13 · By contrast, at the seaward end of these systems where the channels are very broad, the tidal bars are elongate

Proposal for a Systematic Study of Tidal-Bar Deposits: Bar Recognition, 3D Geometry and Variability

By Bob Dalrymple, Ron Steel and Murray Gingras

Summary Tidal bars are the fundamental building block of the deposits formed in almost all tidal environments (Figure 1). The study proposed here will investigate systematically the range of characteristics of these bars, in order to provide:

• criteria for their recognition; • insight into the factors that control their variability; • documentation of their preserved 3D geometry and size; and • criteria by which the location in which each type formed can be inferred.

Our approach will involve three inter-related components:

1. A review of the published literature on tidal bars in the full range of settings; 2. A study of the spatial variability of the facies characteristics of tidal bars in the

Ogeechee River, Georgia (a classic area studied by Howard and Frey); and 3. An investigation of the geometry and facies characteristics of preserved tidal-bar

deposits in selected ancient examples (e.g., Mutti’s classic tidal-bar deposits in the Baronia Formation, Ager Basin, Spain; the Drumheller Formation, Alberta; and other locations as may be suggested).

Our overall purpose will be to provide information on the nature of tidal-bar deposits that can be used to make predictions about production characteristics in tidal successions.

Figure 1: Tidal channel-bar deposits in the McMurray Formation (Athabasca Tar Sands), northern Alberta.

The Problem and a Solution “Tidal bars” are widely recognized as being the distinctive morphological feature of tide-influenced to tide-dominated sedimentary environments, and they are typically considered to be elongate features that are oriented more or less parallel to the tidal currents. Although tidal bars have been described from a number of modern settings, the most widely used model for their deposits is based on Eocene tidal deposits in the Baronia Formation, northern Spain (Mutti et al. 1985; Figure 2). This model has serious problems, however, in that very few modern tidal bars show the upward-coarsening succession depicted in the model. Furthermore, the model illustrates forward accretion (i.e., migration of the larger feature in the same direction as the smaller bedforms) whereas all known modern examples appear to show lateral accretion. In addition, the existing database on tidal bars does not provide a solid basis for understanding the nature and significance of facies variability within and between tidal-bar occurrences. As a result, it is difficult to identify and interpret tidal bars in petroleum reservoirs. The problem is compounded by the fact that tidal-bar deposits are commonly very heterolithic and hence provide significant challenges for efficient reservoir development.

We suggest that it is possible to make significant advances in our practical understanding of tidal bars by means of a systematic, focused study of carefully selected modern and ancient examples that span the spectrum of possible tidal-bar occurrences. We will take a very broad definition of “tidal bar” in order to ensure that the full spectrum of possible deposit types is described.

Figure 2. Mutti’s (1985) model for a tidal-bar deposit, based on examples from the

Baronia Fm, Spain. Such upward-coarsening tidal bars appear to be fairly unusual in other locations, except where the bars represent a bayhead delta. We plan to look at the

3D geometry and spatial the variability of these bars (see Figure 8 below).

Background

Tidal systems are inherently channelized (Figures 3 and 4), except in the case of shelfal environments. The nature of these channels changes systematically through the fluvial-marine transition. All tidal channels widen seaward because of the seaward increase in the tidal flux. Estuarine systems display a “straight-meandering-straight” pattern of sinuosity changes (Figure 3), whereas deltaic channels become progressively less sinuous and straighter in a seaward direction (Figure 4).

Figure 3: Fitzroy River estuary, Australia, showing the typical estuarine channel pattern

termed “straight-meandering-straight” by Dalrymple et al. (1992). The tightly meandering section occurs at the location of the label “Fitzroy River”. Note that all of

these channels also show the funnel shape that typifies all tidal channels.

Figure 4: Indus River delta, showing extensive development of tidal channels, especially in the “inactive” parts of the delta plain, where fluvial input is not presently occurring.

Note how the tidal channels become wider and less sinuous in a seaward direction.

The nature of the “tidal bars” also changes systematically through the fluvial-marine transition because of the changes in channel characteristics (Figure 5; Dalrymple and Choi, 2003). In the narrow channels in the inland parts of the transition, the “tidal bars” consist of tidally influenced to tidally dominated points bars or bank-attached bars. By contrast, at the seaward end of these systems where the channels are very broad, the tidal bars are elongate and essentially flow parallel, and subdivide the larger channel into a series of sub-channels (Figure 5C, D).

These two types of bars are commonly considered to be genetically distinct forms. However, observations in many modern systems show that the parts of the system that lie between areas with these two bar types have bars that have hybrid characteristics (i.e., that combine elements of point bars and elongate tidal bars). In these channels with an intermediate width and sinuosity, the bank-attached bars become partially detached from the bank by the development of a flood barb, which is a headward-terminating flood-dominated channel that lies between the bar crest and the inner bank at the downstream end of the bar (Figure 5A). Thus, the landward end of the bar has the characteristics of a point bar, whereas the seaward end is morphologically similar to an elongate tidal bar.

Figure 5: Proximal-distal change (A D) in tidal-bar morphology in the Fly River delta. (A) Tidal point bar from the tidal-fluvial zone, showing the development of a short flood barb at the downstream end, that is separated from the main ebb channel by a short “elongate tidal bar” (dashed white line). (B) Straight channel in the delta plain showing

a straight-crested elongate tidal bar (dashed line) that attaches to the southern bank a short distance off the left edge of the image. (C) Two straight-crested elongate bars that

join to form a U-shaped form that lies in the middle of a wide channel. (D) Series of straight tidal bars (dashed lines) in the relatively unconfined mouth-bar portion of the

delta. From Dalrymple et al. (2003).

Furthermore, the small amount of data that exists on the architecture of the deposits formed by these various bar types indicates that they all generate (predominantly) lateral-accretion deposits. (It should be noted that this observation is at odds with the foreward-accretion geometry shown in the so-called “tidal bar” model popularized by Mutti et al. (1985).) Thus, from a pragmatic point of view, it may be difficult to distinguish between curved tidal point bars of inland locations and the straighter elongate tidal bars of more seaward areas, although sinuosity and facies variations are to be expected through the fluvial-marine transition, as outlined in a schematic manner in the “proximal-distal” report prepared and provided to the sponsoring companies in previous research. Proposed Research We believe that advancing our understanding of tidal bars (geometry, facies and architecture) requires a three-pronged approach, with a first phase from May 1, 2006 to April 30, 2008. Task 1. Literature review: There are a number of previous studies of tidal bars of various types, but these studies have never been reviewed and synthesized in a systematic manner. Dalrymple will undertake to synthesize the existing data on the geometry, facies and architecture of tidal bars in the context of the concepts laid out in the “proximal-distal” report produced in earlier research. This synthesis will be used to guide observations in the other two aspects of the research, and will provide a larger database on which to construct generalizations. It will also investigate what is known about the behaviour of meander bends (both fluvial and tidal), in order to develop ideas about which parts of meander bends are preserved (e.g., to determine the relative importance of bend translation versus bend expansion). The existing data should also help to determine whether lateral accretion or forward accretion is the dominant architectural element within tidal bars. Task 2. Field studies in the Ogeechee River, Georgia: There is a marked lack of systematic studies in the modern of how facies vary through the tidal-fluvial transition zone. To help alleviate this lack, we propose to undertake such a study using the Ogeechee River, Georgia. This work will be done by Murray Gingras (University of Alberta), with the assistance of Bob Dalrymple. The Ogeechee River (Figures 6 and 7) is a classic area for the study of sedimentation in the fluvial-marine transition because it was the area used by Howard, Frey and others for their pioneering studies of the ichnology of such environments (Dörjes and Howard 1995; Greer 1975; Howard et al., 1975). The Ogeechee River is a piedmont system that drains the Appalachian Mountains and carries a substantial amount of sand. It enters the Atlantic Ocean via a nearly filled, mixed-energy estuary. The tidal range is mesotidal, but wave energy is only moderate, yielding a coastal system that consists of short barrier islands, breached by tidal inlets that pass almost directly into the tidal-fluvial channel. This channel shows the “straight-meandering-straight” pattern (Figures 6 and 7) that is typical of estuaries, thereby providing a diverse set of meander geometries for examination. A substantial advantage of using this area for our study is the availability to us of the original data and core X-radiographs, all of which is already

in Gingras’ possession. The existence of a nearby marine research station will make the conduct of the research logistically simple and relatively inexpensive. The field work would be carried out over the summers of 2006 and 2007 by a Masters student whom Murray has already recruited. This will allow the synthesis of the data by the end of the second year of funding for this Phase. She is the top student in her graduating class and is almost assured of obtaining a government scholarship, thereby reducing the cost of this component of the overall project.

The data to be collected would include: process measurements (e.g., tidal-current speeds, salinity, suspended-sediment concentrations) at selected sites through the fluvial-marine transition; detailed bathymetric data to record the 3D geometry of the tidal bars; and direct observations and box cores to determine the physical and biological structures, including the orientation of any bedforms relative to the local bar surface. If possible, high-resolution seismic (i.e. sub-bottom profiling) data will also be collected. Such data will be collected at selected bars through the fluvial-marine transition, AND at several transects along the length of the selected bars, in order to document the local spatial variability of the facies. Work will give special attention to the subtidal portions of the bars because this is the part with the highest preservation attention. Gingras has considerable experience in such settings, having studied Willapa Bay, Tillamook Bay and the Shepody portion of the Bay of Fundy for many years. He brings a significant body of previous work on the tidal-fluvial transition to this project.

Figure 6: Map of the Ogeechee River estuary, Georgia(top), showing the relatively open-mouthed nature of the system, and the occurrence of the “straight-meandering-straight” channel pattern. The tidal limit lies inland of the left-hand edge of the map. The lower

two panels show the correspondence between salinity and the abundance and diversity of organisms through the salt-water—fresh-water transition (After Howard et al., 1975).

Figure 7: Map of the Ogeechee River ( Tom Saunders) showing the longitudinal distribution of species diversity and number of individuals (graph in lower right corner—

see also Figure 6) and the animal and burrow assemblages that occur through the fluvial-marine transition based on the classic work of Dörjes and Howard (1995), Greer

(1975) and Howard et al. (1975).

Task 3. Studies of ancient tidal bars: The third important component of the project will be an examination of the facies and architectural variability within and between identifiable tidal-bar macroforms in a selected set of ancient tidal successions. This portion of the study will focus on documenting the 3D geometry of the preserved components of tidal bars, to understand the nature of physical and ichnological facies changes along the length of such bodies, as well as their orientation and dimensions. The selection of units to be studied will be guided by the desire to have examples that occur in a wide range of depositional conditions, including particularly those spanning the fluvial-marine transition..

One attractive study area is the Baronia Formation in the Ager Basin of northern Spain. This is the area where Emiliano Mutti developed his “tidal-bar” model (Mutti et al., 1985; Figures 2 and 8): by revisiting this area, we can determine what it was precisely that he was describing and bring that earlier work into the context of our current understanding. A further significant advantage for this area is the existence of an apparently robust sequence-stratigraphic framework that has been correlated along the

Figure 8: Outcrop of the “type example” of a tidal-bar deposit (sensu Mutti et al. 1985; see Figure 2 above) in the Ager Basin, northern Spain.

length of the basin (Mutti et al., 1985; Dreyer and Fält, 1993). This provides a temporal framework that will allow us to examine tidal bars within a single time slice over a spectrum of proximal-distal locations, something that would be extremely difficult or impossible almost anywhere else. Exposure in the Ager Basin is extremely good, so we should have the potential to examine the 3D architecture of the tidal-bar bodies.

Another possible locality for studies of this type include the Cretaceous tidal-bar deposits that outcrop near Drumheller, Alberta (Figure 9). These bars, which appear to have formed in the tidal-fluvial transition zone, have been described in very general terms by Ray Rahmani (Rahmani, 1988) and Bruce Ainsworth and Roger Walker (1994). Detailed 3D reconstructions are possible in selected areas, but have not yet been undertaken. As time and resources permit, we will also explore the possibility of adding further examples, to expand the database, in particular to locate and study tidal bars that formed in an unrestricted open-marine setting. Certain examples in the Cretaceous of the US Western Interior Seaway (e.g., the transgressive upper part of the Hygiene Sandstone near Denver; Izzet et al. 1971) may be of this type. Gingras may also be able to bring previous observations of tidal-bar deposits from exposures of the Athabasca Tar Sands, and we already have observations of tidal bars from the Sego Sandstone at our disposal. Overall, we believe that we can build a substantial database from a variety of settings.

This portion of the research will be overseen by Ron Steel, with the field observations being undertaken by Cornel Olariu. Murray Gingras and/or James MacEachern will provide ichnological assistance. Although Shuji Yoshida has now moved on to Japan, we will ask him if he is willing to bring his completed work on tidal bars in the Sego Sandstone into the context of the new project.

Figure 9: The large-scale inclined heterolithic stratification in the lower part of these cliffs near Drumheller, Alberta, are interpreted to have formed on tidal bars located in

the inner to middle part of the tidal-fluvial transition zone. Photo courtesy of Ray Rahmani.

Budget The following budget summary has been formulated on an annual basis. All figures are in US dollars.

1. COSTS FOR DALRYMPLE (general involvement in all phases of the research, linkage of modern and ancient components of the project, and preparation of review of tidal-bar deposits—Task 1): Partial salary for research associate, travel to all study sites (Georgia, Spain, Alberta, etc.), travel to Steering Committee meetings, overhead .……………. $35,000

2. COSTS FOR GINGRAS (conduct of Ogeechee River study—Task 2, assistance with fieldwork on ancient deposits—Task 3, and liaison with Steel and Dalrymple): Boat hire, purchase of field equipment, travel to and accommodation in the field, travel and accommodation in Spain, travel to Steering Committee meeting……………………... $48,000

3. COSTS FOR STEEL (conduct of work on

ancient tidal bars—Task 3, and liaison with Gingras and Dalrymple): Partial salary for post-doctoral fellow, support for graduate student, travel and field-work costs for research in Spain, Alberta, etc., travel to Georgia and to Steering Committee meetings, overhead ………………………….$115,000

TOTAL PROJECT COST (Per Year)…………………….….……..…$198,000

COST PER COMPANY (assuming 6 sponsors as at present)………... $33,000 However, because the research year (May-April) does not correspond to the financial year (January-December), sponsoring companies will be invoiced in three parts:

• May-December, 2005: 75% of the annual amount ($24,750/company) • January-December, 2006: 100% of the annual amount ($33,000/company) • January-April, 2007: 25% of the annual amount ($8,250/company)

Concluding comments We believe that the proposed study of tidal bars is the strongest and most focused proposal that we have put to you. It addresses, in a uniquely focused manner, a topic of fundamental importance to the development of hydrocarbon reserves from tidal deposits, because it provides information on the basic architectural elements that govern fluid flow. This project also has the benefit of being undertaken by three recognized experts in the field, who have a track record of working collaboratively; all three of us will have direct input into all three phases of the research, even though we are each primarily responsible for one part of it. We are able to provide a practical integration of knowledge of physical and biologic processes and stratigraphic principles that is unlikely to exist in any other research proposal. Two of the main study areas that we have proposed, the Ogeechee River, Georgia, and the Ager Basin of Spain, are both attractive areas for field trips, that would build on the experience that company representatives will have from our trips to Utah, the Bay of Fundy, France and Korea (the trip to this last area is to be run next May, during the first month of this new project). Furthermore, funding of this new project ensures that we can continue to build on the work we have done over the last four years (e.g., Dalrymple and Choi, 2003; Yoshida et al., in prep.). The large-scale issues that we have undertaken to examine, such as the nature of facies changes through the fluvial-marine transition and as a function of systems tract, will continue to underlie the new research. As a result, we will be able to provide progressively better insight to these fundamental issues. It might be noted also that the proposal for the current project stated that the completion of the Korean work would not occur until 2006-2007, because of the extra time needed to undertake laboratory analyses of the samples. The synthesis and interpretation of the Korean dataset will take place during the first year of the new project: no extra funding is requested for this. In addition, all three of the lead researchers are undertaking complementary research on related topics, with funding from other (non-restricted) sources. Although the data obtained in such studies (e.g., Porebski and Steel 2005; Yang et al. 2005, in press, in review) is not explicitly a part of this project, it will inevitably find its way into the interpretations passed on to you.

As in the current research project, Bob Dalrymple will be the PI because of the advantageous arrangement regarding overheads that Queen’s University offers (i.e., money will flow through to Steel and Gingras without an overhead tax), thereby maximizing the research value of your dollars. ReferencesAinsworth, R.B., and Walker, R.G., 1994, Control of estuarine valley-fill deposition by

fluctuations of relative sea level, Cretaceous Bearpaw-Horseshoe Canyon transition, Drumheller, Alberta, Canada. In R.W. Dalrymple, R. Boyd, and B.A. Zaitlin (eds.), SEPM Special Publication 51, p. 159-174.

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Yang, B.C., Dalrymple, R.W., Chun, S.S., Johnson, M.F., and Lee, H.J., in review, Tidally modulated storm sedimentation on open-coast tidal flats, southwestern Korea. Submitted to SEPM Special Publication “Recent Advances in Coastal and Shelf Sedimentation”.

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Yoshida, S., Steel, R. J. and Dalrymple, R. W., in prep., Depositional process changes in a sequence stratigraphic context: an ingredient in a new generation of sequence stratigraphic models. Manuscript in preparation for submission to the Journal of Sedimentary Research.