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Revisiting the Grand Canyon – Through the Eyes of Seismic Sequence Stratigraphy
By WARD ABBOTT*
Search and Discovery Article # 40018 (2001)
Adapted for online presentation from articles by same author, entitled “A New View of Something Grand” and “Canyon Offers Grand Seismic View” in Geophysical Corner, AAPG Explorer, July, 1998, and August, 1998, respectively. Appreciation is expressed to the author and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.
*Consulting geologist, Washington, Utah ( [email protected] ). Formerly Occidental International Exploration and Production, after Shell Oil Company.
The eastern portion of the Grand Canyon in northern Arizona is a geological paradise where previous group and formation designations can be redefined in terms of complete and incomplete unconformity-bounded depositional sequences. Many of these surfaces and their distinguishing characteristics can be recognized seismically.
This portion of the Grand Canyon is one of the most scenically spectacular and geologically instructive areas in North America. Perhaps at no other single locality are so many events, over such a long interval of Earth’s history (1.7 billion years), displayed in one view. From Zuni Point on the South Rim, the entire first-order sequence of the Paleozoic Era and the first-order sequence of the Proterozoic Eon can be observed.
The Grand Canyon offers a unique opportunity to use the Earth as a textbook (Figure 1) – and sequence stratigraphy not only offers a “quick look” approach to analyzing the hydrocarbon seals, reservoirs and source rocks, it also allows one to visualize and interpret the various trap types. One can use the Grand Canyon as a model and view angular unconformities, nonconformities, disconformities and local unconformities. First- through fifth-order depositional sequences are spectacularly displayed. Stratigraphic traps for hydrocarbon plays and prospects are highlighted by facies changes, onlap and truncation stratal patterns. Seals, source rock intervals, and maximum flooding surfaces can be clearly defined.
In addition, one of the most exciting facets of studying at the Grand Canyon is that it is one of the world’s best laboratories for comparing outcrop data to seismic data. This helps the explorationist avoid some of the pitfalls inherent in correlation.
This article is intended to explain the stratigraphy of the eastern Grand Canyon from a sequence-analysis viewpoint, in an effort to better describe the geologic history. It also relates the large-scale geologic phenomenon to seismic scale and shows how they can be recognized on 2-D seismic sections. The stratigraphy has traditionally been defined from a descriptive point of view and is presently assigned Group, Formation and Member designations. This has led to a lengthy and complicated nomenclature.
Figure 1: Hierarchy of depositional sequences —
Rocky Mountain area, USA.
Figure 2: Pima Point, looking
east. Here, the second-order Supai Group can be broken down into two third-order
sequences, and the lower third-order into two fourth-order sequences. The
second-order Mississippian Redwall sequence is seen in the massive cliff, and
karsting is evident at the top of the limestone unit. An incised valley fill is
present at the top of the Esplanade/base of the Hermit Shale.
Figure 3: This seismic line
shows two examples of collapsed karsting features on a highstand systems tract
carbonate shelf. They resemble downcutting, incised valleys on this single
seismic line – however, regional mapping indicates that they align parallel to
the shelf margin and are sporadic in occurrence. This, along with the collapsed
drape, indicates that they are collapsed karsting features.
Figure 4: Pima Point,
where downcutting erosional truncation and onlap stratal patterns define the
lowstand-systems-tract incised valley of the Hermit Shale, indicating a surface
of erosion and nondeposition. Basinward, LST deposits can be predicted.
Figure 7: This seismic line
illustrates the downcutting erosional truncation and onlap stratal patterns that
define a lowstand-systems-tract incised valley.
Figure 8: This seismic line
illustrates the downcutting and erosional truncation of an incised valley.
Figure 9: Looking west from Zuni
Point, the uplifted and truncated, basal Proterozoic second-order sequence; note
the two sandstone units in the Tapeats.
Figure 10: A regional, northwest view from Yavapai Point showing the truncated
Proterozoic.
Figure 11: Partial seismic line
illustrates the onlap pattern of a basal transgressive-systems-tract sand as it
onlaps a basement high.
Figure 12: This photo, taken at
Grandview Point, shows the first second-order sequence of the Proterozoic, and
the first second-order Paleozoic sequence; the "Great Unconformity"
separates these two sequences.
Figure 13: Partial seismic line illustrates the
truncation, facies pinch-out and the potential hydrocarbon traps illustrated in
figures 9-10 and 12. This comparison of outcrop and seismic data demonstrates
the need for the outcrop model to interpret the seismic geometry and predict
facies correctly.
Figure 14: Seismic line
illustrates three regional unconformity surfaces and portrays the basic geometry
exposed in the Grand Canyon. Compare figures 9-10 and 12 for similar geometries
to infer facies on this seismic section.
Figure 15: These two photos from
the Grand Canyon show three regional unconformity surfaces that are comparable
to the seismic line shown in Figure 14.The Sequence Model and Grand Canyon Sequence Stratigraphy
The Sequence Model and Grand Canyon Sequence Stratigraphy
As sea, ocean, and lake levels rise and fall in response to tectonic, eustatic, and climatic events, in both active and passive tectonic settings, a predictable pattern of sedimentary fill for clastic and carbonate rocks can be established.
The grouping together of unconformity-bounded genetic sequences of rocks establishes a complete depositional sequence, and its systems-tract deposits can be predicted for continental settings, coastal plains, shelf areas, platforms and basins. Therefore, when studying and/or correlating outcrop, well and seismic data the observer has the complete, predicted section to compare to his data to define areas where erosion or non-deposition have left only a partial or incomplete sequence.
Unconformity-bounded depositional sequence terminology and diagrams have appeared in geologic literature for the last decade. The basic nomenclature and sequence terminology as they pertain to the stratigraphy of the eastern Grand Canyon are shown on Figure 1.
From the Grand Canyon display, earth scientists can see that the sequence model has been repeated again and again as depositional base level rose and fell from the Precambrian all the way to the Pleistocene. This same pattern is repeated throughout the earth in different marine and continental settings. This method of analyzing different genetic sedimentary rocks allows the interpreter to visualize the varied phases of geologic history easily. It offers a “quick look” analysis of the occurrence of hydrocarbon reservoirs, seals, and source rocks.
The sediments deposited during the Paleozoic Era are assigned to a first-order sequence. Sediments assigned to Group designations are usually classed as second-order sequences and generally formation designations represent partial, or in some cases, complete sequences of third and fourth order (Figure 2). Each second-, third-, and fourth-order sequence typically has a sea-level rise, stillstand and fall phase designated as transgressive systems tracts (TST), highstand systems tracts (HST) and lowstand systems tracts (LST). Fifth-order sequences, however, are defined by a base-level rise and stillstand with no fall phase and are considered to be eustatic only. Second- through fourth-order sequence designations are primarily based on time, but thickness and areal extent also play a part.
Because of their great lateral extent and thickness, incised valleys and basin-floor turbidites of second- through third-order sequences are the only scale to be considered from an exploration point of view, while the fourth-and fifth-order parasequence categories are mainly used at production scale. Classic karsting and incised valleys (Figure 2) document the sequence boundaries of the parallel strata of the Grand Canyon.
Parallel stratal patterns are the most common in the stratigraphic record and the hardest to use to define sequence boundaries. Because of this, much of the geologic history contained in them is overlooked. Generally, there are no clues as to where to define the disconformity surface or sequence boundary. Therefore, knowledge of the sequence model can be of great assistance. If one knows the critical criteria to look for in defining the TST, HST and LST, proper placement can be achieved.
Facies criteria, paleo information, sedimentary structures and environmental data all can help in the correct placement of unconformity and sequence boundaries. The parallel stratal pattern is expressed in disconformity.
The seismic and outcrop expression of these phenomenon are detailed on Figures 3, 4, 5, 6, 7, and 8. With the “new eyes” provided by seismic-sequence stratigraphy, the accuracy of the “sequence model,” involving approximately 15,000 feet of sedimentary rocks exposed at seismic scale continuously for a distance of over 40 miles, is compelling. By studying the unconformities in the Grand Canyon, one can extrapolate and predict the missing sections for the incomplete sequences and forecast the lowstand systems tracts (shelf-margin prograding wedge, slope and basin-floor fans) for the deep-marine basin setting of eastern California, Nevada, and western Utah.
By understanding the different stratal patterns and unconformities of this unique geologic setting, an explorationist can use this earth model as a textbook to compare outcrop data to seismic data (even though Ordovician, Silurian, and Lower Devonian rocks are missing). The 15,000 feet of sedimentary rocks exposed continuously for a distance of over 40 miles are equivalent to 1.5-2.0 seconds. These extensive exposures allow the correlation of outcrop geometries and facies to seismic geometry; they allow inference of facies and environment and underscore the importance of having an outcrop model, at seismic scale, to interpret seismic data correctly and define potential hydrocarbon traps. These trap geometries are illustrated by photos, diagrams, seismic lines and a description of different stratal patterns in Figures 1, 9, 10, 11, 12, 13, 14, and 15.
Truncation and onlap stratal patterns that define upper and lower boundaries of depositional sequences can be observed in the eastern Grand Canyon area in the Proterozoic and Paleozoic stratigraphic sections. These stratal patterns are critical to the proper placement of unconformities.
Truncation patterns define the top sequence boundary in second, third and fourth-order sequences. There are two types:
· Those caused by uplift and erosion (regional scale).
· Those due to downcutting erosion (local scale).
In both cases the geologic history of the destroyed or missing section needs to be restored. The time of erosion and the time of fill need to be analyzed to establish proper correlations and geologic history. The truncation pattern is expressed in angular unconformity. The outcrop expression of these patterns is shown on Figures 9, 10, 12, and 15, and they are displayed seismically on Figure 11, 13, and 14.
Onlap patterns define the base-sequence boundary in second-, third-, and fourth-order sequences. This pattern indicates surfaces of nondeposition. Onlap patterns can be of local extent, defining the side of incised valleys, or on a more regional scale, defining marine, coastal and nonmarine coastal onlaps in transgressive systems tract settings. This pattern is expressed in nonconformity, disconformity and angular unconformity. The seismic expression of these patterns is shown on Figures 11, 13, and 14, and outcrop comparison is shown on Figures 9, 10, 12, and 15.
The data presented here are based primarily on the work of D.P. Elston, S.S. Beus, E.D. McKee and studies by the author carried out for Shell Oil and Occidental E&P.