B U L L E T I N O F T H E A M E R I C A N A S S O C I A T I O N O F P E T R O L E U M G E O L O G I S T S VOL. 42, NO. 5 (MAY, 19581, PP. 1047-1063, 7 FIGS.

TliME-STRATIGRAPHYi

HARRY E. WHEEr,ER2 Seattle, Washington

ABSTRACT

As in lithostratigraphy and biostratigraphy, time-stratigraphic units may be classed on the basis of their vertical, lateral, and vertico-lateral contiguity.

Those space-time units defined by their position in vertical succession are of two kinds: (1) units of variable time value without specific time limitations, including holostrome, hiatus, lacuna, and erosional vacuity; and (2) units of constant maximum time limits, including (a) units of constant time value such as era, period, epoch, and age, and (b) units of variable time value within specified maximum time limits such as system, series, and ^tage.

Holostrome and hiatus are analogous to formation in lithostratigraphy and zonule in biostratig­raphy. Neither era-period-epoch-age, nor system-series-stage, have strict analogies in lithostratig­raphy' or biostratigraphy.

Laterally contiguous time-stratigraphic facies based on comparative ratios of two or more of the afore-mentioned time-variable units exist in concept, and are amenable to cartographic portrayal if desired. Such facies in time-stratigraphy are analogous with lithofacies and biofacies.

Vertico-laterally contiguous (mutually intertongued) combined holostromes comprise deposi-tional holosomes, and combined hiatuses comprise hiatal holosomes. These are the time-stratigraphic analogies of lithosome and biosome. Their primary function is to serve as a basis for envisaging the complete regional or interregional, continuous, and differential migration of depositional and non-depositional environments in space and time.

INTRODUCTION

Concepts in time-stratigraphy have been troublesome largely because the temporal factor is a fourth stratigraphic dimension (Wheeler and Beasley, 1948, p. 80). The geologist must portray the complexly variable, three-dimensional rela­tionships of rock- and biostratigraphy; yet only two of these three spatial dimen­sions can be graphically illustrated simultaneously and without distortion on a two-dimensional medium. Moreover, if the geologist desires to illustrate com­parative duration, the vertical dimension of his cross section (or even a three-dimensional model) must be shared in common by both thickness and time. Be­cause these factors are mutually variable, the thickness of lithic or faunal units and the duration of their accumulation can not both be shown in the same illustra­tion without distortion of one or the other. This fact is doubly unfortunate in that the temporal values of such significant events as non-deposition and erosion are reduced to zero in a section whose vertical dimension is adjusted to thickness of the stratal record. Thus non-deposition, erosion, and other interpreted events of space-time value can be represented graphically only in a framework of both space and time. This, in turn, leads to the conclusion that all time-stratigraphic units are three-dimensional, and that there are no "pure" time units in geology.

' Manuscript received, January 10, 1958.

^ Professor of Geology, University of Washington. The writer expresses special gratitude to V. Standish Mallory, University of Washington, for his

many helpful suggestions, and to Professors R. C. Moore, University of Kansas, H. G. Schenck and Fred Binford, Stanford University, and L. L. Sloss, Northwestern University, tor their critical read­ing of the manuscript. Those who have contributed to this study through occasional oral discussion are too numerous to be named individually. The writer is, nevertheless, grateful to all of them.

1047

1048 HARRY E. WHEELER

If this is logically sound, there should not only be differences between time-stralig-raphy and the other two kinds, but also ways in which they correspond.

In three previous papers, V. S. Mallory and the writer have at tempted to demonstrate: (1) the necessity and utility of the arbitrary cut-ofif as a standard procedure in the designation of certain kinds of stratigraphic units (Wheeler and Mallory, 1953); (2) factors and consequent concepts embodied in the delinea­tion and classification of lithostratigraphic units (Wheeler and IMallory, 1956); and (3) the role of factors and concepts in biostratigraphy (Wheeler, 1958). The present paper gives essentially the same treatment to time-stratigraphy.

Soon after this series of studies began it became apparent that the same three modes of unit separation either have been or may be advantageously employed in each of the three major stratigraphic categories; i.e., most lithic and paleontologic data and lime interpretations may all be logically and usefully segregated vertically, vertico-laterally, and laterally (Wheeler and i la l lory , 1954). Trial employment of litho-, bio-, and time-stratigraphy as one parameter, and vertical, vertico-lateral, and lateral separation as the other, gives a classifica­tion with nine categories. This in turn has led to the observation that with a few exceptions, units occurring in vertical succession (regardless of whether they are litho-, bio-, or time-stratigraphic in nature) are similar in dimensional configura­tion; that all vertico-laterally segregated (intertongued) units are dimensionally alike; and that all laterally contiguous (facies) units are also "bodies" of similar form.

This approach to t ime-stratigraphy is based on the initial premises that all stratigraphic units are three-dimensional; tha t all rock and biostratigraphic units are defined as bodies in three-dimensional space; that all time-stratigraphic units are definable only as space-time entities in which relative geologic time is used in place of the vertical spatial dimension employed in lithostratigraphy and bio­strat igraphy; and that time-stratigraphic units of one kind or another should be designed to account for complete stratigraphic space-time, just as rock and bio­stratigraphic units of some kinds are designed to account for the complete pre­served record in stratigraphic space.

Analysis of t ime-stratigraphy on these bases has led to the observation that not all time-stratigraphic units are entities of constant temporal value. Some of those regarded as most useful for purposes of regional synthesis, and thus as a basis for historical interpretation, occur as space-time variables. Although at first glance these views may seem somewhat radical, they are based, neverthe­less, largely on long-established but neglected fundamentals presented in Eliot Blackwelder's (1909) classic s tudy of unconformities. In addition, and perhaps no less important , is the fact that this synthesis embodies and impels the use, in stratigraphic interpretation, of Joseph Barrell 's (1917) frequently neglected base-level concept.

NATURE OF TIME-STRATIGRAPHIC UNITS

The framework of t ime-stratigraphy differs primarily from lithostratigraphy and biostratigraphy in three respects (Fig. 1).

riME-STRA TIGRA FllY 1049

First, all tliree dimensions directly concerning rock and biostratigraphic units are spatial. Only the lateral dimensions are spatial in t ime-stratigraphy; here the "ver t ical" dimension is measured in terms of time.

Second, lithostratigraphic and biostratigraphic units, insofar as they are properly delineated on the basis of tangible distribution of rocks or fossils (as taxonomic entities rather than rock constituents) in three-dimensional space, are intrinsically objective. The temporal element in time-stratigraphy, however, is inferential and therefore subjective. Any estimate of geologic age, nevertheless, must be based on either biostratigraphy or l i thostratigraphy (or their physical manifestations such as radioactivity),—or preferably on the integration of both of these mutually variable factors.

Third, while rock and faunal units are material bodies delineated in three-dimensional space (within the tangible preserved stratigraphic record), time-stratigraphic units, because their vertical dimension is complete time rather than

/ / / / LITHOSTRATIGRAPHIC/' /

/ /BIOSTRATIGRAPHIC / /

/ 1 OBJECTIVE

j MATERIAL

I ONLY

^

. T A ^ t j

/ ^ /

//

/

/ ^ 1 SUBJECTIVE

M A T E R I A L A N D

N O N - M A T E R I A L

\

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FIG. 1.—Block diagrams emphasizing three-dimensional nature of both lime-stratigraphy and litho-bioBtratigraphy, together with their fundamental differences.

thickness, must incoporate not only the tangible preserved record, but also those non-material parts of the total space-time "volume" represented by non-deposi­tion and those presently negative parts from which the stratigraphic record has been erosionally removed. In other words, just as rock units of one kind or another are defined to fill all space occupied by the determinable present geologic record, time-stratigraphic units of one kind or another must be conceived or designated to account for all interpretable space-time. Otherwise the bases for interpreta­tion of geologic history are inadequate.

Despite the three-dimensional character of rock and faunal units, this should not be construed to imply that lithostratigraphic or biostratigraphic units may not be envisaged, illustrated, or discussed in a space-time framework. Formations, lithosomes, zones, zonules, and biosomes, for example, may have little historical significance until this step has been taken. Nevertheless, such substitution of time for thickness does not destroy the fundamental nature of these rock or bio­stratigraphic units, but merely distorts their form. Thus a formation is a rock unit regardless of whether its illustrated vertical dimension is a measurement of its thickness or an estimate of its temporal value. Conversely, some time-strati­graphic units (system, series, and stage) may be usefully distorted to fit a three-dimensional spatial framework, although their fundamental space-time nature is not destroyed.

1050 HARRY E. WHEELER

These views are not in complete accord with those of the American Commis­sion on Stratigraphic Nomenclature (1952, pp. 1828-29). For example, that Com­mission report initially staled that time-stratigraphic units are "material rock units"; but it further pointed out that "their boundaries, as extended geographi­cally from the type section ['or otherwise designated type'], are essentially iso­chronous surfaces, representing everywhere the same horizon in geologic time." These statements are incompatible. As demonstrated on following pages, only a portion of a boundary between two such material units can be isochronous—that portion which falls within depositional continuity from the one unit to the other. Elsewhere the isochronous surface departs from the material entity involved, and enters a non-material, space-time body or lacuna; there the material units under consideration are separated from the intervening lacuna by time-variable (non-isochronous) surfaces.

In brief summation, the discussion to this point argues that time-stratigraphic units must include all three-dimensional entities {material, non-material, or combi­nations of both) which are related to stratal relationships, and which are defined in a framework consisting of two lateral space dimensions and a vertical time dimension. In order that stratigraphic space-time may be completely accounted for, these sub­jective units must delineate all interpreted deposition {sedimentary or volcanic), non-deposition, post-deposition erosional removal, or whatever combinations of these may be useful in the interpretation and visualization of geologic history.

The following analysis is based on the essential validity of the preceding statement.

UNITS D E F I N E D BY POSITION IN VERTIC.4L SUCCESSION

SEQUENCE AS OBJECTIVE FRAMEWORK OF REFERENCE

As in the case of lithostratigraphy (Wheeler and Mallory, 1956) and bio-stratigraphy (Wheeler, 1958), some time-stratigraphic units clearly occur in ver­tical succession; while others may be defined either in accordance with then lateral contiguity or their vertico-lateral (intertonguing) relationship (W'heeler and Mallory, 1954). The vertically successive units are considered first.

Although all time-stratigraphy is subjective or abstract, it is imperative that its interpretation have a factual basis. Moreover, if it is agreed that time-stratig­raphy must involve the space-time distribution of non-record as well as that of accumulated and preserved record, it appears to be equally mandatory that the objective basis for time-stratigraphic concepts shall include not only the pre­served depositional record, but the stratigraphic evidence of physical erosion as well. As a tangible framework on which to hang pertinent faunal and lithic data, the sequence of Sloss, Krumbein, and Dapples (1949, pp. 110-11) generally ful­fills these requirements. Paraphrasing these authors' discussion, a sequence com­prises an assemblage of strata exhibiting similar responses to similar tectonic environments over wide areas, separated by objective horizons without specific time significance. Although the lack of precision here is doubtless intentional, it is note-

TIME-STRA riGRA PUY 1051

worthy that most of the "objective horizons" sei)araling the sequences desig­nated by Sloss et al. are either unconformities or their lateral manifestations in the form of abrupt changes in depositional character.

In any event, the migration of depositional environments in space and time can be interpreted adequately only by the recognition of assemblages of strata separated by unconformities. I t also seems evident that such stratal successions not only fit the broad definition of sequence, but answer to the character of most specifically delineated sequences as well. A sequence, as the term is employed in the present discussion, is thus defined as a preserved stratal assemblage which is unconformably separated from underlying and overlying rocks. Inclusion of the word preserved in this definition is necessary for two reasons: (1) the sequences employed by Sloss el al. comprise only the present stratal record; and (2) as indi­cated below, complete time-stratigraphic integration involves the distinction between the objective preserved record and the part ly subjective restored succes­sion.

Transposition from objective spatial s trat igraphy to subjective time-stratig­raphy may be illustrated by a case in which preserved records of several re­gional episodes of subsidence and accumulation are represented by sequences whose bounding unconformities manifest intervening episodes of deformation and erosion. Such a case is presented in the extremely generalized cross section (Fig. 2) showing the writer's interpretation of late Paleozoic and early Mesozoic Cordil-leran relationships as they would have appeared at the close of Triassic time. This section diagrammatically illustrates the spatial distribution of four regionally extensive unconformities and their intervening sequences occurring within the interval from upper Mississippian to the top of the Triassic. The base of the illustrated succession is t ime-stratigraphic and arbitrary. I t diagrammatically shows the position of the base of the Upper Mississippian or Chesterian Series which is believed to have a relationship of continuity with underlying rocks throughout the U.S. Cordilleran region. The top is arbitrarily limited by the Triassic-Jurassic boundary which is also regarded as a horizon of depositional continuity throughout the region. The sequences, as defined by their vertically bounding unconformities and laterally terminating arbitrary cut-offs, are given arbi trary letter designation. All post-Triassic structure has been "ironed ou t " in this section, and the structure developed during this Mississippian-Triassic in­terval is extremely generalized in order to simplify graphic illustration.

No formations, lithosomes, or other units which would illustrate spatial distribution of lithologic types are shown. In spite of their significance in ultimate historical interpretation, these are not included for two reasons: first, in order to emphasize the sequences and intervening unconformities; and second, because they do not affect the concepts involving the time-stratigraphic units under discussion. The same may be said of biostratigraphic units, although they too, are most essential to the correlations involved.

No a t tempt is made here to demonstrate the correctness of the regional inter-

1052 HARRY I'.. WHEELER

pretations upon which this discussion is based. Although these stratigraphic interpretations represent the writer's current views, their strict accuracy should not significantly affect their acceptability as examples illustrating time-strati-graphic relations or concepts involved in any such regional situation.

The first of the afore-mentioned unconformities separates Sequences Y and Z in the Rocky Mountain region on the east. I t marks the contact between such formations as the Casper, Tensleep, and Weber of Pennsylvanian-Permian age

WEST EAST

EROSION SURFACE (expressed as lime-stratigraphic truncation)

EROStONAL VACUITY /removal during hiatal interval/

LACUNA (nondeposition and erosion)

HOLOSTROME C (restored complete deposition)

FiG. 2.—Generalized cross section approximately along Fortieth Parallel from Pacific Coast to Rocky Mountains showing principal sequences and intervening unconformities from base of Upper Mississippian (Chesterian) to Triassic-Jurassic boundary.

FIG. 3.—Time-stratigraphic cross section through lateral extent of Sequence C in Figure 2, illustrating derivation of holostrome and hiatus as primary components of regional stratigraphic cycle.

FIG. 4.—Time-stratigraphic cross section showing holostromes and hiatuses derived from se­quences shown in Figure 2. Horizontal reference lines are boundaries of periods designated in Figure S.

77M1--STRA TJGRA PHY 1053

above, and Late Mississippian or older rocks below. This is the westerly counter­part of one of the most widespread unconformiiies in ihe historical record of North America. Eastward across the continent this break is variously known as pre-Cherokee, pre-Manchester, pre-I*ottsville, pre-Lee, etc. Westerly from its termination, distinction between Sequences Y and Z is not possible on the basis

FIG. 5.—Time-stratigraphic cross section showing geologic periods superimposed on deposition-non-deposition pattern derived from Figure 4.

FIG. 6.—Facies based on deposition-non-deposition ratios in space-time. FIG. 7.—Depositional holosome intertongued vith hiatuses (potential hiatal holosomes).

1054 HARRY E. WHEELER

of unconformity. Here in the easterly par t of the miogeosyncline, Sequence R appears to have accumulated from Devonian to early Jurassic time without significant interruption. This does not imply that Sequence R (Devonian lo lower Jurassic) in all cases should be treated as a unit for purposes of regional stratigraphic analysis. I t may be validly and usefully subdivided into lesser intervals on the basis of "objective horizons" other than unconformities, and in accordance with the recommendations of Sloss et al. (1949). I t may be noted that these sequences, as defined for the purpose of this discussion, must all terminate laterally against "ver t ical" surfaces (arbitrary cut-offs) interesecting the termini of their limiting unconformities, and extending vertically in each case from the preceding unconformity to the one following. Of the four cut-offs shown, only the one separating sequences B and C from K is complete. This, of course, is be­cause the unconformities bounding both top and bot tom of only B, C, and K lie within the limits of the illustrated succession.

The second unconformity separates Sequences A and B in the coastal states and western Great Basin, and Sequences A and K in the central Great Basin. I t occurs between such late Mississippian or early Pennsylvanian formations as the Baird, Spotted Ridge, Battle, Tomera, Diamond Peak, MiUigen, and older rocks below, and overlying formations such as the Wood River, Strathearn, Havallah, Antler Peak, McCloud, and Coyote But te . The basal beds of this overlying se­quence may vary in age from mid-Pennsylvanian to early Permian.

The third unconformity marks the base of Sequence C of Guadalupian age, which contains the Nosoni, Dekkas, Trail, Robinson, Clover Creek, Seven Devils, Casto, Koipato, and Edna Mountain formations.

Finally, Sequences C, K, and N are followed unconformably by sequence D, the basal beds of which are regionally transgressive from early to late Triassic, and which includes such formations as Candelaria, Excelsior, Luning, Gabbs, Pit, Hosselkus, Brock, Dinwoody, Thaynes, Timothy, Higham (Shinarump), Wood (Chinle), Mart in Bridge, and Hurwal.

Thus as presently integra,ted, the major Cordilleran late Paleozoic and early Mesozoic preserved sequences and intervening unconformities are exemplified as to content and grossly illustrated in Figure 2. They diagrammatically express the tangible relationships in space which will serve as an objective framework of reference on which to hang other objective data, and from which one may derive various time-stratigraphic units. These in turn will directly portray the most fundamental events in geologic history—the space-time distribution of deposition and non-deposition—or, in other words, the alternating progression and regres­sion of base-level (Barrell, 1917). Such derivation of time-stratigraphic units, however, involves the conversion of the fragmentary rock record in space, in which neither non-deposition nor erosion are given any evaluation whatever, into a subjective but nevertheless complete time-stratigraphic synthesis in which, ideally, all space-time is accounted for by three-dimensional evaluation of all original deposition, non-deposition, and post-depositional erosional removal.

TIME-STRA TIGRA PHY 1055

L'NITS OF VARIABLE TIME VALUE WITHOUT SPECIFIED TOTAL LIMITS

The first step in the conversion of physical stratigraphy to time-stratigraphy is to plot the pertinent physical stratigraphic surfaces of record in order to de­velop a configuration with lateral spatial dimensions and a vertical dimension of time. This may be illustrated in another cross section (Fig. 3), using the Guada-lupian Sequence C as the example.

Ideally, through the integration of all bio- and lithostratigraphic evidence of geologic age, the base of this cross section is delineated by estimating the relative age of the base of Sequence C at a number of localities. Similarly, the regionally variable upper limit of Sequence C is determined by estimating the age of its youngest preserved strata at points of their depositional contact with rocks of any later age or their exposure at the present surface. In other words, this is generally an eroded surface, expressed as the space-time profile of truncation of Sequence C by the succeeding unconformity or any later erosion surfaces, including the pres­ent. Thus the rock unit Sequence C is projected in space-time.

On the basis of integration of all available physical and biostratigraphic data, the space-time profile of the termination of the episode of deposition represented by the more-or-less fragmentary Sequence C is also estimated (dashed line). The space-time volume between the erosion surface and this estimated cessation of accumulation is therefore that part of a regionally transgressive-regressive suc­cession which has been removed by subsequent erosion. This "restored" part of the succession is designated as erosional vacuity on the section. Thus Sequence C (preserved record) viewed in space-time, together with the erosional vacuity, comprise an estimate of the space-time distribution of a complete regional trans­gressive-regressive depositional succession (C) which is here designated as a holostrome (Ao/o = complete plus 5/row = layer). By the same procedure, Holo-stromes A', B', D' , K', N' , R', Y', and Z' may be delineated (Fig. 4).

The cycle represented by Sequence C is not complete, however, without sim­ilar time-stratigraphic evaluation of the base of the next following holostrome which, in this case, is established on the basis of interpretation of the relative age of the basal strata of Sequence D at as many localities as feasible. The total age range of Sequence D in western United States is believed to extend from early Triassic to early Jurassic, but its base is interpreted as transgressing from early Triassic to late Triassic. This is plotted on the cross section as the base of D (Fig. 3). The interpreted complete regional space-time value of nondeposi-ticn or hiatus is thus represented by the variable interval between the top of Holostrome C and the base of Holostrome D'. Thus the principal elements of the complete stratigraphic cycle comprise the entire space-time "volume" represented by both the surface accumulation (holostrome) and non-deposition (hiatus) interpreted for the interval separating two successive unconformities. Holo-stromes and hiatuses, so identified, are seen to be the time-stratigraphic analogies of group, formation, member and lithostrome in lithostratigraphy, and of zonule in biostratigraphy. In all three categories these units are segregated on the basis

1056 IIAKRV E. WHEELER

of position in vertical succession, but again only by virtue of the employment of an artificial mode of lateral termination - the arbitrary cut-off.

Holostromes and hiatuses are interpretive or subjective; they are conceived in both space and time; their time dimension varies from place to place; and they have no jire-determined or otherwise specified relation to standard isochronous surfaces such as period boundaries. They are, therefore, lime-stratigraphic units of variable time value without specified total limits. Only by their derivation can all stratigraphic space and time be accounted for, and they alone appear to provide the basic units of a subjective framework (Fig. 4) required to envisage the flow of events through the geologic past. In fact, they are manifestations of one of the most important geologic phenomena involved—the succession of major base-level changes.

Terminology.—As in most matters involving fundamentals in geology, little if any of the foregoing is new. Numerous authors have dealt with one or another of the concepts involved, although only Blackwelder appears to have incorporated all of them. In his s tudy of unconformities, Blackwelder (1909) either described or implied most of the relationships discussed here, and at tempted to develop appropriate nomenclature. However, he was primarily concerned with the un­conformity itself, rather than the role of both unconformities and preserved stratal records as the objective factors in complete time-stratigraphic integration. Following are brief definitions of these units of variable time value, together with discussion of their previous t reatment .

Holostrome.—Although he did not propose a term for it, Blackwelder's (1909, p. 299) concluding paragraph prophetically implied the existence and utility of such a stratigraphic ent i ty: "The entire geologic record then is not to be conceived as a pile of strata, but as a dovetailed column of wedges, the unconformities [hiatuses] and rock systems [holostromes] being combined in varying proportions." Many others have understood the need for recognizing and using stratal units separated by "breaks ." For example, the importance of the rock-stratigraphic unit se­quence of Sloss el al. (1949), although it comprises only the present fragmentary record, is increasingly appreciated. Gignoux (1955, p. 19) had this same general kind of unit in mind as the basis for one of his several conflicting concepts of "s tage ." That this particular concept of stage is synonymous with sequence of Sloss ei al. is shown by Gignoux's indication that such units are bounded by their intervening lacunas. Thus the significance and utility of stratigraphic units manifesting distinct regional episodes of subsidence and deposition have been appreciated by some for many years. Nevertheless, it appears that a specific kind of unit, which, together with hiatus, will account for complete stratigraphic space and time, has not been defined previously. Holostrome is therefore defined as a time-stratigraphic unit embodying the space-time value of a complete (re­stored) transgressive-regressive depositional succession.

The terms transgressive and regressive, as used here and on following pages, do not necessarily imply marine advance and retreat. I t refers to regional pro-

TIMESTRA TIGRA PHY 1057

gressive space-time development of environments of surface accumulation and their regressive termination in consequence of base-level shift, regardless of whether the accumulation consists of marine, non-marine, volcanic rocks, or their combinations.

Hiatus.—This term commonly has been used in two senses—either as the time value of an episode of non-deposition or the time value of non-deposition and ero­sion together. Grabau (1906a, p. 90) stated that a hiatus "marks an important physical break in the series, and a time interval unrepresented in this region by strata." The phrase "unrepresented in this region" may imply that only non-deposition is embodied. If this was Grabau's intent, this statement could have been based on the generalization that if a given interval had been represented regionally, some remnants would be preserved. Although this definition is some­what ambiguous, a clear indication of his intent to restrict this term may be seen in Grabau's (1906b, p. 616) illustration of regressive-transgressive relationships in time and space, in which the term hiatus is applied only to the "volume" representing non-deposition. Blackwelder (1909, p. 296, Fig. 3, and discussion) designated the time and space involved in non-deposition as the unconformity "time value"; but because this term appears to be in no way preferable, the ol­der term hiatus is accepted on the basis of priority. The time-stratigraphic unit hiatus is here defined (retaining what appears to be (irabau's intent) as the space-time value of nondeposilion during a regressive-transgressive epi­sode.

Erosional vacuity.—If holostromes and hiatuses are valid in concept, and if, as contended here, their envisagement is essential for any attempt at complete historical interpretation, the factor of differential erosional removal can not be escaped. This factor is complicated, however, not only by the fact that erosional removal is differential within a given sequence, but also because the erosional effects of a number of different hiatal episodes may combine in such fashion that the missing interval preceding any sequence may vary locally from values ap­proximating that of its own hiatus to values comprising major parts of the known geological column. In Figure 2, for example. Sequence D (Triassic) rests on Se­quence C (Guadalupian) in many areas; while elsewhere the Triassic may rest on Sequence A (Mississippian), as shown, or perhaps on even earlier Paleozoic strata. The total missing interval involved may differ vastly from its part which is interpreted to represent removal within the limits of a sequence. This total missing interval—that which may involve, say, Quaternary resting variously on pre-Cambrian, Jurassic, and Miocene—was defined as "stratigraphic hiatus" by Blackwelder (1909, p. 296, Fig. 3). Although this total missing interval must be recognized and dealt with in stratigraphy, its existence often tends to obscure the total history. Such total history can only be interpreted by reconstruction of holosomes and hiatuses; and these can not be reconstructed without distin­guishing between the hiatus (non-deposition) and that part of the "underlying" holostrome which is interpreted to comprise erosionally removed former deposi-

1058 IIARKY E. WHEELER

tion. This non-material part of a holoslrome which is interpreted as having been formerly represented by deposits is here termed erosional vacuity; and it may be defined as the space-time value of the erosionally removed part of a transgressive-regressive depositional succession.

Lacuna.—Erosional vacuity comprises only a part of the non-material por­tion of the space-time value of a stratigraphic cycle. The remainder consists of the hiatus. This void representing combined hiatus and erosional vacuity nor­mally can be interpreted with greater facility than either of its parts, for it is commonly bounded by material entities (preceding and succeeding sequences). Because this void is more easily delineated, and because it must be envisaged in the process of distinguishing hiatus and erosional vacuity, it appears to deserve recognition—especially since it has been previously recognized with good purpose. Gignoux (1955, pp. 15-16) referred to this stratigraphic void as lacuna. He stated (p. 15) that "if, between . . . two formations [stratal successions], a period oc­curred during which sedimentation was either nil or, more likely, was replaced by erosion, it is said then that there has been a discontinuity, and this period corresponds to what is called a stratigraphic lacuna or, more briefly, a lacuna." Although this wording does not definitely distinguish between lacuna and either Grabau's hiatus or Blackwelder's "stratigraphic hiatus" (total missing interval), Gignoux's accompanying figure (p. 16) clearly indicates that the lacuna embodies hiatus together with the space-time value of that part of the preceding sequence which has been removed. In terms of the present discussion, a lacuna may be defined as a time-stratigraphic unit representing the interpreted space-time value of both non-deposition (hiatus and the erosionally removed part of the subjacent transgressive-regressive succession.

It should be noted that an erosional vacuity, like the total missing interval, may be the net result of erosion occurring entirely during the first hiatal episode following deposition or, in part, during any one or more subsequent hiatal epi­sodes. Note is also made of the ambiguity of Blackwelder's term "stratigraphic hiatus." From the viewpoint of this discussion, all hiatuses in stratigraphy are stratigraphic. For this total missing interval a term such as total vacuity would seem to be more appropriate. Such a total vacuity is clearly more than erosional; it is also in part non-depositional. In fact, it may incorporate any number of suc­cessive hiatuses and erosional vacuities.

UNITS WITH SPECIFIED MAXIMUM TIME LIMITS

Units of variable space-time value within specified constant time limits {system, series, and stage.—These time-stratigraphic units are here exemplified by system, which is commonly regarded as "a standard, world-wide division [which] con­tains the rocks formed during the fundamental chronologic unit, a period" (Ashley et al., 1933, p. 429). The phrase "contains the rocks formed during . . . a period" may imply that the completely restored succession (all combined holo-stromes or depositional holosomes), within period limits, comprises the system.

TlMIi-STRA riGRA FH V 1059

In other words, this definition conceivably incorporates rocks that subsequently have been removed or otherwise obscured.

On the other hand, the American Commission on Stratigraphic Nomenclature (1952, p. 1629) stated that systems are "material uni t s , " thus implying that only the actual record is embodied. In accordance with this restriction, the system would comprise all preserved s t rata (combined sequences or their parts), viewed in space-time, which fall within period limits.

Although either of these definitions of system would embody valid concepts, the latter is regaded as the more realistic, because it is based on the tangible, record. Thus restricted to the preserved record, system, series, and stage are time-stratigraphic units, based on the material stratal record; and they are units of variable (and even vertically intermittent) time value within the constant time-stratigraphic limits of the corresponding period, epoch, or age.

Units of constant time value (eon, era, period, epoch, and age.—Most stratig-raphers appear to agree to the conceptual validity and need for units of constant time value, and which encompass the earth 's entire stratigraphic sphere. At least such so-called " t ime" units as eon, era, period, epoch and age fall into this cate­gory. These, like other stratigraphic units have lateral dimensions in space and a vertical dimension expressed in time. They are laterally continuous and vertically contiguous, and they exist only in relation to the physical stratigraphic record. In other words, the only kind of time involved is that which relates to stratal relationships, fundamentally on the basis of the law of superposition. Therefore, the subjective units which the stratigrapher conceives are three-dimensional and time-stratigraphic, and only the one-dimensional columnar section view shows the time dimension alone.

Such units are diagramatically illustrated in Figure 5 which represents the same space-time cross section as Figure 4, but on which are shown the late Mis-sissippian, Pennsylvanian, Permian, and Triassic Periods. Positions of the period boundaries are established by estimation of their relation to the bio- and litho-stratigraphy of the depositional successions, and are projected through the hiatuses. I t therefore seems logical and proper to regard the geologic period as a subjective, conceptually world-wide time-stratigraphic unit with a constant vertical dimension or time value which is equivalent to the maximum vertical limits of the corresponding system.

The American Commission (1952, p. 1629) stated that geologic time is non-material; and from this they concluded that geologic time units are not strati­graphic units. The preceding present discussion, however, demonstrates that geologic time comprises only the vertical dimension in three-dimensional time-stratigraphy, and is thus analogous with columnar section in lithostratigraphy and local taxonomic range in biostratigraphy. As such, it is agreed that geologic time segments (in this one-dimensional sense) are not stratigraphic units in the three-dimensional sense employed here. Eon, era, period, epoch, and age, on the other hand, are generally conceived and used as three-dimensional, space-time

1060 HARRY /•:. WHEELER

units. The mere fact that they are subjective in nature should not exclude them from the category of time-stratigraphic units. In fact, as already shown, all lime-stratigraphic units are subjective or interpretive. Periods, epochs, etc., both logi­cally and in sound practice, must embrace both material (preserved stratal) and non-material (lacunal), three-dimensional, time-stratigraphic (space-time) en­tities. Thus, unless yet another set of new terms is to be introduced, the notion that such units are non-stratigraphic at present seems untenable.

CONCEPT OF TIMF,-STR,4TIGRAPHIC FACIES (LATERALLY CONTIGUOUS UNITS)

If Stratigraphic facies are to be defined essentially as they were by IMoore (1949, p. 32), Sloss, Krumbein, and Dapples (1949, pp. 92-93 et seq.), Wheeler and Mallory (1956, p. 2718) and others, they may be regarded as laterally segre­gated, statistical variants of a stratigraphic interval. Wheeler and Mallory (1956, p. 2717) demonstrated that facies in this sense are not only laterally con­tiguous, but also are separated by vertical surfaces (arbitrary cut-offs) extending from the bottom to the top of the stratigraphic interval under consideration.

In the case of the Cordilleran example employed throughout this discussion, it has been shown in Figures 3 and 4 that the time-stratigraphic values of holo-stromal and hiatal units are variable. Moreover, it may be noted that these interpreted variations in three-dimensional space-time are quite like those in­volving lithic relationships in three-dimensional space (Wheeler and Mallory, 1956, Figs. 1-6), and it may also be envisaged that they are like those involving biostratigraphic units (Wheeler, 1958).

The space-time distribution of deposition and non-deposition is again used as the base for Figure 6, which graphically illustrates that lateral variation in the time-dimension ratio of these time stratigraphic factors may serve as a basis for the derivation and delineation of facies in precisely the same manner as lithofacies or biofacies are derived. In fact, the ultimate potentialities here, as in lithofacies and biofacies, are limited only by the number of differing factors to be interpreted. For example, if the mapping of the resultant variation were deemed of value to the geologist, appropriate combinations of such factors as the time dimension of hiatus, erosional vacuity, lacuna, holostrome, and se­quence projected in space-time could be used. Moreover, if desired, "isochronal" maps of these variable time-stratigraphic units could be prepared which would be analogous to isopachus maps in lithostratigraphy.

The purpose here is neither to name nor to promote the use of time-strati­graphic facies units, but rather to demonstrate the concept of their existence, and to show that each of the fundamentally differing kinds of stratigraphic data or interpretation is logically subject to the same modes of treatment. Neverthe­less, the writer contends that such facies are not only conceptually valid, but may be of practical value as well, regardless of whether they are actually deline­ated or mapped. For example, an appreciation of lateral variation in relative amounts of geologic time represented by deposition and non-deposition, in gen-

TIME-STRATIGRAPHY 1061

eral is believed lo be no less important than lithic ratios {e.g., volcanic versus non-volcanic) as one of the several bases for the fades distinction (and there is no other) between the Cordilleran miogeosynclinal and eugeosynclinal belts. The accompanying time-stratigraphic cross section illustrate at a glance that the number and value of hiatuses increases westward from the miogeosyncline

INTERTONGUED TIME-STRATIGRAPHIC UNITS (V.ERTICO-LATERAL CONTIGUITY)

Wheeler and Mallory (1956, p. 2719 and Fig. 6) demonstrated, graphically illustrated, and defined as lithosomes, the bodies of uniform (or uniformly het­erogeneous) lithology which mutually intertongue with similar bodies of differing lithologic character. Also, precisely analogous intertongued biostratigraphic units have been defined as biosomes (Wheeler, 1958). In a like manner, Figure 7 implies that space-time units of irregular configuration, embodying interpreted total accumulation are mutually intertongued with hiatal units. These mutually intertongued time-stratigraphic "bodies" are termed holosomes (/?(?/o = complete plus soma = 'boAy). They are of two varieties—those comprising all combined contiguous holostromes, and those consisting of combined contiguous hiatuses. Comparison of Figures 4 and 7 shows that holostromes may occur as tongues of depositional holosomes, and hiatuses may occur as tongues of hiatal holo­somes, just as lithostromes (Wheeler and Mallory, 1956, pp. 2720-21) may repre­sent lithosomal tongues, and as zonules may occur as biosomal tongues (Wheeler, 1958); Though not demonstrated here, this implies, of course, that holostromes B' and/or C may "pinch out" westward beyond the limits of the section in order that at least two of the hiatuses may join.

Time-stratigraphic units of this type are variable within their total limits in both their vertical and lateral dimensions. They provide, when accompanied by a sufficient number of time-stratigraphic datum surfaces, a complete basis for visualization of the space-time migration of gradational processes. In fact, insofar as they are accurately delineated, the general continuous shift of the points of grade is marked by the boundary surfaces between depositional and hiatal holosomes. In other words, as a manifestation of Barrell's (1917) base-level con­cept, any point on such a holostrome base represents the estimated monetary space-time position of the local initiation of regional deposition. Any point at the top surface of a holostrome is the estimated space-time position of the local termination of the depositional episode.

Although the major regional holosomes and their component holostromes and hiatuses may be delineated in most cases with reasonable accuracy, the fact should be mentioned that the record of more local and ephemeral deposition within the space-time distribution of orogenies is generally removed by erosion before the onset of the ensuing regional episode of subsidence and consequent accumulation. For example, many of the intra-Cascadian ("Plio-Quaternary") deposits of the Cordilleran region are presently being removed; and, on the basis of past history, it is presumed that most of this scattered record will be

1062 HARRY R. WHEELER

lost before the conclusion of the orogenic-erosional episode. In other words, there is seldom any basis for the interpretation of gradational shifts within orogenies. Consequently, except for the interpretations based on inadequate structural cri­teria, most of the details of erogenic history are not decipherable.

Again, it is emphasized that these various relationships were also long ago in­dicated, illustrated or implied by Blackwelder (1909) and by numerous investi­gators since. Yet the subsequent lack of appropriate terminology, and neglect of the matter of their derivation, appears to be primarily responsible for the common failure to envisage the nature and interrelations of these and other time-stratigraphic units, and the analogies between them and their litho- and bio-stratigraphic relatives.

As a final analogy common to lithostratigraphy, biostratigraphy, and time-stratigraphy, the "tope" concept should be mentioned. If time-stratigraphic (paleogeographic) surfaces are subdivided on the basis of interpreted deposition and non-deposition, such areas in time-stratigraphy would indeed be analogous with lithotope and biotope (see Wheeler and Mallory, 1956, p. 2720; Wheeler, 1958).

CONCLUSION

The concluding recommendations of the report on time-stratigraphy by the American Commission on Stratigraphic Nomenclature (1952, p. 1636) state that:

The Commission recognizes that the subject is highly complex and that in the hght of growing knowledge, new appraisals are continually needed. Therefore, . . . it is intended that [these] views . . . should undergo a further period of weathering under exposure to the expression of opinions bj' geologists generally, before its tenets are incorporated formally in the Stratigraphic Code.

The present paper is primarily intended as such an expression of opinion and it attempts to incorporate the afore-mentioned complexities of the subject, with particular emphasis on the basic concepts involved and their reconciliation with sound stratigraphic practice.

An initially unanticipated outgrowth, however, has been the realization that two primary parameters appear to pervade the entire field of stratigraphy: (1) the kinds of data or interpretation (litho-, bio-, or time-stratigraphic), and (2) the modes of unit contiguity which geologists, over the years, have found con­venient and useful (vertical, vertico-lateral, and lateral). The writer presently believes that all valid stratigraphic units proposed to date will fall into one or another of the resulting nine categories (Wheeler and Mallory, 1954). If this is correct, as the numerous afore-mentioned analogies tend to indicate, not only may many of the long-sought-for solutions to our problems in time-stratigraphic classification and nomenclature be at hand, but some of those in litho- and bio­stratigraphy as well.

The ultimate test of any classification, of course, is not only its conceptual validity but also its practical utility. This discussion of the nature of time-strati­graphic units, however, does not emphasize their use in geologic procedure. Rather, the present purpose is to attempt fulfillment of the initial requisite—

TIME-STRA TIGRA PHY 1063

that all valid scientific data, nomenclature, and interpretation must be based on sound precept. Insofar as these notions are valid, and are considered in the light of valid concepts in litho- and biostratigraphy, the implications and possible attributes are manifold. Foremost among these attributes is that some units tend to force recognition of the essential fragmentary components of time-stratig­raphy; while others are designed to comprise the most meaningful elements of a complete space-time "volume" which, when properly assembled from adequate paleontologic and physical data, serves as a basis for interpretation of geologic history as the continuous flowage of events and phenomena that it is.

As a final specific illustration of this general attribute, and strongest among the arguments in support of the foregoing, it not only incorporates the often neglected fundamentals presented a half-century ago by Eliot Blackwelder (1909), but that it also appears to lend them renewed vigor. Moreover, and per­haps of equal importance, this analysis of time-stratigraphy gives expression to and demands employment of Barrell's (1917) indispensable concept of base-level, to which we frequently give lip-service, but commonly disregard.

REFERENCES

AMERICAN COMMISSION ON STRATIGRAPHIC NOMENCLATURE, 1952, "Nature, Usage, and Nomenclature of Time-Stratigraphic and Geologic Time Units," Bull. Amer. Assoc. Petrol. Geol., Vol. 36, pp. 1627-38.

ASHLEY, G. H . , ET AL., 1933, "Classification and Nomenclature of Rock Units," Ball. Geol. Soc. America, Vol. 44, pp. 423-59.

BARRELL, J., 1917, "Rhythms and the Measurement of Geologic Time," ibid.. Vol. 28, pp. 745-904. BLACKWELDER, E. , 1909, "The Valuation of Unconformities," Joar. Geol., Vol. 17, pp. 289-99. GiGNOTJX, M., 1955, Stratigraphic Geology. 682 pp. Freeman and Company, San Francisco. GRABATJ, A . W . , 1906a, "Guide to the Geology and Paleontology of the Schoharie Valley in Eastern

New York," New York State Mus. Btdl. 92, pp. 77-386. , 1906b, "Tj'pes of Sedimentary Overlap," Bull. Geol. Soc. America, Vol. 17, pp. 567-636.

MOORE, R . C , 1949, "Meaning of Facias, Geol. Soc. America Mem. 39, pp. 1-34. SLOSS, L . L., KRUMBEIN, W . C , AND DAPPLES, E . C , 1949, "Integrated Facies Analysis," ibid., pp.

91-124. WHEELER, H . E. , 1958, "Primary Factors in Biostratigraphy," Bull. Amer. Assoc. Petrol. Geol., Vol.

42, No. 3 (March), pp. 640-55. , AND BEASLEY, E . M . , 1948, "Critique of the Time-Stratigraphic Concept," Bull. Geol. Soc.

A merica, Vol. 59, pp. 75-86. , AND MALLORY, V. S., 1953, "Designation of Stratigraphic Units," Bull. Amer. Assoc. Petrol.

Geol., Vol. 37, pp. 2407-21. , AND , 1954, "Analysis and Classification of Stratigraphic Units" (abstr.). Bull. Geol.

Soc. America, Vol. 65, p. 1324. -, AND , 1956, "Factors in Lithostratigraphy," Bull. Amer. Assoc. Petrol. Geol., Vol. 40,

pp. 2711-23.