Architectural Elements and Bounding Surfaces

One of the fundamental objectives of stratigraphy is to describe and interpret the three-dimensional architecture of the sediments filling sedimentary basins. On the largest scale, this involves the disciplines of lithostratigraphy, biostratigraphy, allostratigraphy and chronostratigraphy (Chap.7), the reconstruction of major depositional sequen- ces and the sequence-stratigraphic architecture, perhaps with the aid of regional seismic sections (Chaps. 5 and 6), an assessment of the roles of sea-level change, and the various mechanisms of basin subsidence (Miall 2010; Allen and Allen2013). At an intermediate to small scale, basin archi- tecture depends on the interplay among subsidence rates, sea-level, and the autocyclic processes governing the dis- tribution and accumulation of sediments within a particular

Fig. 3.61 “Log-shape analysis”. Certain petrophysical logs, particularly gamma ray and SP logs, when working with clastic successions, may be very useful indicators of cyclic processes in vertical profiles (Cant1992)

Fig. 3.62 Examples of typical petrophysical log profiles through coastal plain and shelf clastic sequences based on examples from the Gulf Coast. Left log gamma ray or S.P.; right log resistivity. Center bar shows scale subdivisions of 30 m (100 ft) (Fisher et al.1969)

range of depositional systems. Exploration for stratigraphic petroleum traps and for many types of stratabound ore bodies require close attention to basin architecture at the intermediate scale. For example, the search for and the interpretation of subsurface trends in potential reservoir rocks is a major preoccupation of petroleum geologists. Since the late 1950s many improvements have been made in techniques of basin analysis that facilitate this task.

For some time, we have been quite good at predicting the location and trend of such major depositional entities as alluvial fans, barrier reefs, various types of delta, and barrier island sand bodies (to take some examples at random). But one of the outcomes of modern sedimentological research on depositional processes has been a much improved under- standing of the composition and architecture of the various facies assemblages formed within each type of depositional system. The predictive value of sequence models has also

proved its usefulness. We now have the ability, therefore, to construct much better models of the heterogeneities that exist within clastic reservoirs. Conventional techniques of sedimentological analysis and facies modeling place primary emphasis on the use of Walther’s law and the interpretation of vertical profiles, particularly for the study of clastic sed- iments (Reading1996; James and Dalrymple2010). Mean- while, the study of sequence stratigraphy has reached a level of maturity that terminologies and techniques have largely stabilized (Catuneanu2006; Catuneanu et al.2011).

A better understanding of the types of depositional units or elements that comprise stratigraphic sequences, including data describing in three dimensions their scales, lithofacies compositions, length of time required for accumulation, preservation potential, and their spacing in time and space, facilitates quantitative modeling of clastic depositional sys- tems. A more immediate practical application of these data is Fig. 3.63 Some examples of

actual log profiles from the Beaufort-Mackenzie Basin (Young et al.1976)

to improve the information used by reservoir engineers and geologists in their models offluid flow in clastic reservoirs. It has been estimated that complex internal architectures are responsible for intrareservoir stratigraphic entrapment of an average of 30 % of the original oil in place, amounting to as much as 100 billion barrels of movable, nonresidual oil in the United States alone (Tyler et al.1984). A better under- standing of architectural complexities of petroleum reser- voirs would facilitate improved primary production and would increase the success rate of enhanced recovery projects.

There are two important interrelated ideas (Miall 1988a,

b,c):

1. Thefirst is the concept of architectural scale. Deposits consist of assemblages of lithofacies and structures over a wide range of physical scales, from the individual small-scale ripple mark to the assemblage produced by an entire depositional system. Recent work, particularly in eolian,fluvial, tidal, and turbidite environments, sug- gests that it is possible to formalize a hierarchy of scales. Depositional units at each size scale originate in response

to processes occurring over a particular time scale and are physically separable from each other by a hierarchy of internal bounding surfaces.

2. The second is the concept of the architectural element. An architectural element is a lithosome characterized by its geometry, facies composition, and scale, and it is the depositional product of a particular process or suite of processes occurring within a depositional system.

3.5.11.1 Architectural Scale and Bounding Surface Hierarchies

As noted by Allen (1983, p. 249)

The idea that sandstone bodies are divisible internally into “packets” of genetically related strata by an hierarchically ordered set of bedding contacts has been exploited sedimento- logically for many years, although not always in an explicit manner. For example, McKee and Weir (1953) distinguished the hierarchy of the stratum, the set of strata, and the coset of sets of strata, bedding contacts being used implicitly to separate these entities.

Allen (1966) showed that flow fields in such environ- ments as rivers and deltas could be classified into a Fig. 3.64 Wireline log showing the typical“funnel-shaped” profile, with three examples: b, d tertiary delta-front cycles, Arctic Canada; c deltaic bay-fill succession, Carboniferous, Kentucky (log diagram from Pirson1977)

hierarchical order. His hierarchy was designed as an aid to the interpretation of variance in paleocurrent data collected over various areal scales, from the individual bed to large outcrops or outcrop groups. The hierarchy consists offive categories, small-scale ripples, large-scale ripples, dunes, channels, and the integrated system, meaning the sum of the variances over the four scales. Miall (1974) added the scale of the entire river system to this idea, and compiled some data illustrating the validity of the concept (Sect.6.7.4).

Brookfield (1977) discussed the concept of an eolian bedform hierarchy and tabulated the characteristics of four orders of eolian bedform elements: draas, dunes, aerody- namic ripples, and impact ripples. These four orders occur simultaneously, superimposed on each other. Brookfield showed that this superimposition resulted in the formation of three types of internal bounding surface. His first-order

surfaces are major, laterally extensive, flat-lying, or convex-up bedding planes between draas (macroforms, in the terminology of Jackson1975). Second-order surfaces are low- to moderately-dipping surfaces bounding sets of cross-strata formed by the passage of dunes across draas (mesoforms). Third-order surfaces are reactivation surfaces bounding bundles of laminae within crossbed sets and are caused by localized changes in wind direction or velocity (mesoforms to microforms).

Similar hierarchies of internal bounding surfaces have been recognized in some subaqueous bedforms. Shurr (1984) developed a fivefold hierarchy of morphological elements of shelf sandstone bodies, ranging from the litho- some (widespread stratigraphic units) to individual facies packages. Yang and Nio (1989) carried out a similar type of study on some ancient tidal sandstone bodies.

Fig. 3.65 a, c Wireline logs showing the typical“bell-shaped” profile of a fining-upward succession. b Point-bar, Carboniferous, Alabama; dfluvial channel-fill, Tertiary, Arctic Canada (log diagram from Pirson1977)

Brookfield’s (1977) development of the relationship among the time duration of a depositional event, the physical scale of the depositional product, and the geometry of the resulting lithosome was a major step forward that has been of considerable use in the analysis of eolian deposits. Brookfield (1977), Gradzinski et al. (1979), and Kocurek (1981) showed how these ideas could be applied to the interpretation of ancient eolian deposits. Kocurek (1988) has found thatfirst-order surfaces include two types of surfaces, the most laterally extensive of which he terms super-sur- faces. Characterization of eolian deposits therefore now requires a four-fold hierarchy of bounding surfaces.

Several workers have attempted to develop a breakdown of the range of physical scales present influvial deposits. Allen’s (1983) study of the Devonian Brownstones of the Welsh Borders represents the first explicit attempt to for- malize the concept of a hierarchy of bounding surfaces in fluvial deposits and makes reference to Brookfield’s work in eolian strata as a point of comparison. Allen described three types of bounding surfaces. He reversed the order of

numbering from that used by Brookfield (1977), such that the surfaces with the highest number are the most laterally extensive. No reason was offered for this reversal, but the result is an open-ended numbering scheme that can readily accommodate developments in our understanding of larger scale depositional units, as discussed below. First-order contacts, in Allen’s scheme, are set boundaries, in the sense of McKee and Weir (1953). Second-order contacts“bound clusters of sedimentation units of the kinds delineated by first-order contacts.” They are comparable to the coset boundaries of McKee and Weir (1953), except that more than one type of lithofacies may comprise a cluster. Allen (1983) stated that“these groupings, here termed complexes, comprise sedimentation units that are genetically related by facies and/or paleocurrent direction.” Many of the com- plexes in the Brownstones are macroforms, in the sense defined by Jackson (1975). Third-order surfaces are com- parable to the major surfaces of Bridge and Diemer (1983). No direct relationship is implied between Allen’s three orders of surfaces and those of Brookfield because of the Fig. 3.66 a Blocky log response characteristic of some channel-fills, with an example (b) of a channel in the Cutler Group (Permian), New Mexico. c Serrated log (from Pirson1977) and an example (d), turbidite sandstones, France

different hydraulic behavior and depositional patterns of eolian and aqueous currents.

Miall (1988a,b,1996) found it useful to expand Allen’s classification to a seven-fold hierarchy to facilitate the def- inition offluvial macroform architecture and to include the largest, basin-scale heterogeneities in the classification (Fig.3.67). This hierarchy is summarized in Table3.3and is compared with other examples of architectural hierarchies erected for eolian deposits (Brookfield1977; Kocurek1988), coastal-estuarine sand waves (Allen 1980), hummocky cross-stratification cycles (Dott and Bourgeois 1982), shelf deposits (Shurr 1984), and turbidite depositional systems (Mutti and Normark 1987). The various rankings are cor- related mainly on the basis of the time duration represented by the deposits or the length of time stored in the bounding surfaces between them (column 2 in Table3.3). The num- bers in the first column refer to a hierarchy of

Sedimentation Rate Scalesthat is discussed in Chap.8. The formal bounding-surface classification shown in Fig.3.67is provided in column 3 of Table3.3. An example of an out- crop lateral profile showing the application of the bounding-surface classification is shown in Fig. 2.3. Note that the depositional units listed in Table 3.3 represent a range of 12 orders of magnitude in time scale. They also represent at least 14 orders of magnitude in size (area). Note also that although this hierarchy has, at the time of writing, only been applied to fluvial deposits (several examples are given in Miall and Tyler 1991), there is considerable potential for similar subdivision of all clastic deposits, including an adaptation of the other hierarchical systems illustrated in the table. Miall (1988c) noted the similarities of fluvial deposits to some coarse-grained submarine-fan deposits, particularly the channel fills, and suggested ways of applying the architectural concepts described here to these

Fig. 3.67 A six-fold hierarchy of depositional elements and bounding surfaces forfluvial deposits. Diagrams a–e represent successive enlargements of part of afluvial unit in which ranks of bounding surfaces are indicated by circled numbers. A similar hierarchy could probably be developed for channelized deposits in other environments (Miall1988a)

rocks. These ideas were picked up by Clark and Pickering (1996), who provided numerous illustrations of modern and ancient deep-marine channel systems.

First- and second-order surfaces in the sixfold fluvial hierarchy record boundaries within microform and meso- form deposits (Fig.3.67). The definition of first-order sur- faces is unchanged from Allen (1983). Second-order surfaces are simple coset bounding surfaces, in the sense of McKee and Weir (1953). Third- and fourth-order surfaces are defined when architectural reconstruction indicates the presence of macroforms, such as point bars or sand flats (Fig. 3.67). Third-order surfaces represent growth incre- ments of macroforms, such as the reactivation surfaces of

Jones and McCabe (1980). Individual depositional units (s- toreys or architectural elements) are bounded by surfaces of fourth-order or higher rank. Fifth-order surfaces are surfaces bounding major sand sheets, such as channel-fill complexes (Fig. 3.67). They are generally flat to slightly concave upward, but may be marked by local cut-and-fill relief and by basal lag gravels (these were termed third-order surfaces by Allen 1983, and are the major surfaces of Bridge and Diemer 1983). Sixth-order surfaces are surfaces defining groups of channels, or paleovalleys. Mappable stratigraphic units, such as members or submembers, and sequence boundaries, are bounded by surfaces of sixth-order and higher (Fig.3.67; not defined by Allen1983). Miall (1997, Table 3.3 Hierarchy of depositional units

SRS Time scale (years) Inst. sed. rate (m/ka) Bound surf.

Sedimentary process Eolian architecture (Brookfield) Coastal-estuarine architecture (Allen) Shelf architecture (Dott and Bourgeois, Shurr) Submarine fan (Mutti and Normark) 1 10−6[30 s] 106 0 Burst-sweep cycle 2 10−5to 10−4[5–50 min]

105 0 Ripple migration Ripple [E3

surface]

3-surface in HCS 3 10−3[8 h] 105 1 Dune migration, foreset

bundles

Tidal bundle [E2 surface] 2 surface in HCS 4 10−2to 10−1 [3–36 days] 104 2 Diurnal variability to normal meteorological floods (dynamic events)

Neap-Spring bundle [E2] storm layer HCS sequence [1 surface] 5 100to 101 102to 103

3 Seasonal to 10-yearflood: macroform growth increment Reactivation [3rd order surface] 6 102to 103 102to 103 3, 4 Long-term (100-year) flood: macroform, point bar, splay

Dune [2nd order surface]

Sand-wavefield, wash over fan

Facies package (V) Macroform [5] 7 103to 104 100to 101 5 Long-term geomorphic process: channel, delta lobe, coal seam

Draa [1st order surface]

Sand ridge, tidal channel, barrier island Elongate lens (IV) Minor lobe, channel-levee [4]

8 104to 105 10−1 6 Channel belt, delta, orbital cycle [5th order]

Erg [supersurface] Regional lentil (III) Major lobe [turbidite stage 3] 9 105_{to 10}6 ₁₀−2 to 10−1 7 Depositional system, alluvial fan, major delta complex, orbital cycle [4th order] Erg [supersurface] Sandstone sheet (II) Depositional system 10 106 10−1 to 100 7 Rapid subsidence of convergent-margin basins, syntectonic clastic progradation, growth strata Lithosome (I) 11 106to 107 10−2 to 10−1 Basin-fill complex, tectonic cyclothem (e.g.,“clastic wedge”)

Coastal-plain complex

Lithosome (I) Fan complex

12 106to 107 10−3 to 10−2

Very low-accommodation cratons

SRS Sedimentary rate scale; HCS Hummocky cross-stratification

Table 15.3) suggested that sixth-order surfaces, as defined here, are equivalent to the “F” surfaces of Nio and Yang (1991) and to the bounding unconformities of allomembers or submembers, in the sense used by some workers in the area of sequence stratigraphy (Chap.6). Seventh-order sur- faces define individual depositional systems. Eighth-order surfaces bound major basin-fill complexes. Stratigraphic sequences are bounded by surfaces of sixth- and higher-order rank.

Surfaces of fifth and higher order are potentially the easiest to map in the subsurface because of their wide lateral extent and essentially simple,flat, or gently curved, chan- nelized geometry. Many examples of the mapping of chan- nel and paleovalley surfaces have, in fact, been reported in the literature (e.g., Busch 1974). Doyle and Sweet (1995) reported on an attempt to apply Miall’s hierarchical system of bounding surfaces to a subsurface study of a fluvial sandstone succession. Considerable potential now exists for mapping these surfaces with three-dimensional seismic data,

as described by Brown (2011; see Chap.6). All higher order surfaces may appear very similar in cores. They are best differentiated by careful stratigraphic correlation between closely spaced cores, an objective that is best achieved in the most intensively developedfields, where well spacing may be a few hundred meters, or less. Third- and fourth-order surfaces may be recognized by gentle depositional dips (usually <10°).

Identification and correlation of these various bounding surfaces can clearly make a major contribution to the unraveling of the complexities of a channelized depositional system. They are likely to be particularly useful in the recognition and documentation of macroforms, about which much remains to be learned. Further discussion and illus- tration of thefluvial bounding surface hierarchy as applied to outcrop examples of fluvial deposits is given by Miall (1996).

The basis for a universal scheme with which to subdivide complex three-dimensional rock bodies is beginning to

Fig. 3.68 Principal sandstone and/or conglomerate architectural elements influvial deposits (modified from Miall1985)

emerge, as suggested by Table3.3, and the development of a common terminology may become necessary. Thus, the eolian super surfaces of Kocurek (1988) are equivalent to Miall’s (1988a) sixth-order surfaces, whereas Brookfield’s (1977) first-order eolian surfaces could perhaps be equated withfifth-order surfaces. Similarly, in the shelf environment, tidal sand waves and sand banks also contain a hierarchy of facies units and internal bounding surfaces (Allen 1980; Shurr1984; Berné et al. 1988; Harris 1988). The basal E1 surfaces of tidal sand waves (Allen1980) are equivalent to Miall’s third-order surfaces in terms of their time signifi- cance, and E2 surfaces are of first or second order, depending on their origin as diurnal or spring-neap erosion surfaces. The basal erosion surfaces of sand banks (Harris

1988) are offifth order, whereas the internal master bedding planes are equivalent to fourth-order surfaces. Note that these rankings are based on time significance, not on geometry. E2 surfaces of sand waves and master bedding surfaces of sand banks both show depositional dips and appear superficially similar to Miall’s third-order surfaces (e.g., compare Fig. 3.67 with Fig. 3.27), but represent

different processes acting over different time periods. These differences make the erection of a common terminology difficult.

3.5.11.2 Architectural Elements

Application of the bounding surface concept permits the subdivision of a clastic succession into a hierarchy of three-dimensional rock units. This facilitates description and also makes it easier to visualize the appropriate physical extent and time duration of the processes that controlled sedimentation at each level of the hierarchy. Rock descrip- tion involves the definition of lithofacies and the recognition of facies assemblages.

Most deposits may be subdivided into several or many types of three-dimensional bodies characterized by distinc- tive lithofacies assemblages, external geometries, and ori- entations (many of which are macroforms). Allen (1983) coined the term architectural element for these depositional units, and Miall (1985, 1996) attempted a summary and classification of the current state of knowledge of these elements as they occur in fluvial deposits, suggesting that

Fig. 3.69 Principal architectural elements in the overbank orflood- plain of river systems, based on an architectural diagram of afloodplain succession in the Lower Freshwater Molasse of Switzerland (Platt and Keller 1992), showing the range of elements to be expected in a

floodplain setting (Miall1996, Fig. 7.3). Element codes: CH channel, CR crevasse channel, CS crevasse splay, FFfloodplain fines, LA lateral accretion element, LV levee. P pedogenic (paleosoil) unit

there about eight basic architectural elements comprising the coarse units in fluvial depositional systems (Fig. 3.68). A comparable number offine-grained deposits make up the overbank orfloodplain suite (Fig.3.69).

Two interpretive processes are involved simultaneously in the analysis of outcrops that contain a range of scales of depositional units and bounding surfaces: (1) the definition of the various types and scales of bounding surfaces and (2) the subdivision of the succession into its constituent lithofacies assemblages, with the recognition and definition of macroforms and any other large features that may be present. Figure 3.70 illustrates a hypothetical set of cross-sections through an assemblage of architectural ele- ments, showing how a system of labeling and nomenclature may be used to clarify the sedimentology of the deposit.

In general, the most distinctive characteristic of a macroform is that it consists of genetically related lithofa-