B U L L E T I N O F T H E A M E R I C A N ASSOCIAT ION O F P E T R O L E U M G E O L O G I S T S VOL. 49, NO. 4 (APRIL. 1965). PP. 445-459, 12 FIGS.

G E N E R A L T H E O R Y OF E P E I R I C CLEAR W A T E R S E D I M E N T A T I O X i

M. L. IRWIN^ Denver, Colorado

ABSTRACT

Shallow marine sedimentation is described in contrast to oceanic, coastal deposition. Shallow marine sedimentation occurs in non-oceanic epeiric seas where great width of shelves, their low order of slopes (less than one foot per mile), and extreme shallowness of the waters on them are sufficient in themselves to restrict or eliminate circulation. Oceanic, coastal deposition occurs where steep bottom slopes are the rule and where waves and tides extend to the shore.

Three marine, hydraulic energy zones, which are in effect environments of deposition, are thought to occur in epeiric seas: (1) a hundreds-of-miles wide, low-energy zone prevailing in the open sea beneath wave depth where marine currents are the only form of hydraulic energy acting upon the bottom (Zone X); (2) an intermediate, high-energy belt, tens-of-miles wide, beginning where waves first impinge upon the sea floor and thus expend their kinetic energy upon the bottom, extending landward to the limit of tidal action (Zone Y); (3) an extremely shallow, low-energy zone, tens- to hundreds-of-miles wide, occurring landward of Zone Y in which there is little circulation of water, where tides are essentially wanting, and in which the only wave action is that produced by local storms (Zone Z).

Reasons are given to show that in epeiric seas, which have received no invasion of terrigenous elastics ("clear water"), sediments formed in Zone Y are basically of biogenic origin, are sand-size or coarser, and are those most likely to become well-winnowed primary reservoirs; that those sediments formed in Zone Z are primarily fine-grained and tight, and are generally of chemical origin; that Zone X contains sediments which are mainly fine-grained detritus from Zone Y.

During a marine transgression, as each sedimentation zone migrates toward shore, it overlaps older sediments which earlier were deposited in an environment similar to the one then prevailing in the zone adjoining landward. In this manner, an ascending sequence of sediments, deposited under conditions of progressively less restricted circulation, the same as the pattern extending at a single time from land to sea, is established. During regression, each sedimentation zone migrates seaward, so that the ascending sequence of sediments is deposited under progressively more restricted conditions, and is the same as the one reaching at a single time from sea to land. Thus, in a regressive sequence, a rock type found on top of another would also occur landward from it; during transgression, its position would be seaward. This process of continuous sedimentary overlap, as the sea advances and retreats, is the principle which explains cyclic sedimentation. It is noted that the age of geologic formations (lithofacies) becomes Drogressively younger in the direction of migration, emphasizing the necessity for time control.

Two examples are presented together with further discussion which points out how the regular sedimentary patterns may be altered by barriers (such as reefs), by climate, and by degree of bottom slope; and how an analysis of both the normal and altered patterns may be used for exploration and grading purposes.

INTRODUCTION Bahama Banks, which are not considered in this

I n this paper "clear water sedimentat ion" refers Presentation to be epeiric but possibly are "horst-

to those processes tha t ideally would produce ^'^^" ^.^^^'''^'' ^"'^'^^'^ '•°™ ^""^ surrounded by

sediments in epeiric, marine waters which have " received no influx of terrigenous elastics. ^ h e familiar s tatement , "The present is the key

From the viewpoint of the sedimentologist, it is, ^o the pas t , " may be a misleading one when con-indeed, unfortunate that we are living at a t ime in ^^^^""^8 '^P"" '^ sedimentation. We simply have no geologic history when continental landmasses existing models of epeiric seas to guide our investi-probably cover as much of the earth as they ever S^'ions, and although it is true tha t many simi-have. For this reason, our studies of recent ma- ' " ' ^ i e s do pertain between the pas t and the pres-

j - „ i t - t ••- ^ t. u ^ J • ent, it is equally t rue tha t many differences exist rine sedimentation, of necessity, must be made m ' ^ •' •' . .

, , • , 1, 4.1, as well. Two important factors, neither of which is oceanic, coastal areas or m places such as the ' '

duplicated in a present-day setting bu t which ' Manuscript received, November 27, 1964. easily may be overlooked, are "mobi l i ty" and ' Senior geologist. Shell Oil Company, Incorporated. " t ime . " Nature is never static. I n the geologic past ,

The writer wishes to thank S D. Theodosis F. P eatmc seas transgressed and regressed over the Van West, and especially A. B. Shaw who critically read . , . . , ^ , . the manuscript and have offered numerous, helpful sug- continents during numerous periods. These far-gestions. Permission to publish this paper kindly has reaching movements required time, been given by Shell Oil Company, Incorporated. Q present-day investigations must integrate

Ihe author died in an automobile accident, June, t- j o o 1964.—Editor's note. ancient models, many of which are fragmentary,

445

446 M. L. IRWIX

with observations of current conditions which we believe approximate, at least in part, those of the past. This pafier, then, has five purposes:

1. Outline a general theory to explain the spatial relations of lithologic types, source beds and reservoir beds in all epeiric "clear water" marine sediments;

2. Show how this theory may be applied as a tool for oil and gas exploration;

3. Demonstrate its use in the geologic grading of paleogeographic provinces;

4. Explain how the theory may provide a basis for time subdivision of rocks which is independent of paleontology; and

5. Discuss differences between epeiric and present-day oceanic coastal sedimentation.

Certain qualifications are inherent in the theory. We are concerned here only with shallow marine sedimentation: that which is believed to have occurred in non-oceanic, shoaling epeiric seas where the great width of shelves, their low order of slopes (generally reckoned to be less than one foot per mile), and the extreme shallowness of the waters on them were sufficient in themselves to restrict or eliminate circulation. Of intrinsic importance is the qualification that the interpretation of these sediments must be based on a consideration of time relations.

At a specific time in an epeiric sea, lateral variations in organic activity, hydrauhc energy, dis

solved chemicals, temperature, depth, and clarity of water are capable of producing simultaneously several unlike sedimentary environments, in each of which discrete, indigenous lithologic types (hthofacies) arc able to form. In Figure 1, as an example, animal skeletal calcisiltite may be deposited in one environment at "A" at the same time that animal skeletal calcarenite is forming in another environment at "B" , pellets at "C", dolomite at "D", and evaporites at "E" . During a transgression of the sea (as in Fig. 1), each environment, attempting to preserve an established chemical, physical, and biological equilibrium, would migrate in a landward direction, maintaining meanwhile continuous sedimentation (the direction of migration would be seaward during a regression). In this manner, continuous rock bodies of similar lithologic types may be deposited over considerable distances. They are, however, by the very nature of their origin, diachronous.

This paper makes no attempt to discuss sedimentation such as prevails today on the Bahama Banks and on the Gulf and Pacific Coasts. None of these is epeiric in the sense here defined and on oceanic coasts the slopes of the bottoms receiving sediments are many times greater than those which prevailed during much of the geologic past when epeiric sediments were forming upon the continents.

It is difficult to conceive of bottoms having slopes of less than one foot per mile which extend

I ''S' '• ^ I ANIMAL SKELETAL CALCISILTITE

I tf !>J^ i ANIMAL SKELETAL CALCARENITE

NON SKELETAL CALCARENITE (PELLETS)

1 3 SYNGENETIC CHA LKY DOLOMITE

I A A A I EVAPORITES

FIG. 1.—Time relations of epeiric sediments. Not to scale.

EPEIRIC CLE.\R WATER SEDIMENTATION' 447

FLORIDA STRa TS

SLOfC 052 Fr/Wie

GfiEAT BAHAMA SANK ArjCROS ISLAND TONGUE OF OCEAN

n s SiOPe Ii3 FT/UtiE SLOPE 2.00 FT./MILE SLOPE 105L FF/UILE

TEXAS COAST

SLOPE 02 FT/UILE

UNCOMPAHGfiE UPLIFT

N WILES

;ol Stole 0

F'iG. 2.—Degree of bottom slopes beneath present day (coastal) and epeiric seas.

for hundreds and even thousands of miles, but this is the order of magnitude which must be considered. The depositional province of the Great Plains Devonian, as an example, once reached from north of the Arctic Circle in North America, across Canada to at least as far south as the Sioux upKft in South Dakota, and over this great distance its depositional slope was probably in the order of one-tenth foot per mile. Within it at least three major tectonic basins formed: the Mackenzie, the Alberta, and the Williston. Yet, each of these basins contains sediments that comprise no more than a part of the whole model, although in order to comprehend the magnitude and significance of epeiric sedimentation, the complete complex must be perceived.

The Paradox basin is part of another depositional province which prevailed during Pennsyl-vanian time in Arizona, New Mexico, Utah, and Colorado. The lowest profile in Figure 2 is at true scale and extends from the Uncompahgre uphft, across the Paradox basin to the San Juan River reefs in southeastern Utah. If it is assumed that the reefs were deposited in about 50 feet of water or less, because of the algal debris the)' contain, they force upon us the conclusion that the net slope over the region was on the average of 0.2 foot per mile. For comparison, the second profile has been drawn across the Texas Gulf Coast seaward to the 100-fathom line just west of the Mississippi River delta. The third extends from the Florida Coast across the Florida Straits, over the Bahama Banks to the eastern edge of the Tongue

of the Ocean. Other than the oceanic depths indicated on the Bahama Banks profile, none of the slopes shown in Figure 2 is discernible to the eye when drawn at true scale, and yet a comparison between the Gulf Coast and Uncompahgre profiles shows that the slopes on the Gulf shelf are 6-50 times those which must have obtained between the Uncompahgre uplift and the San Juan River reefs.

It may be noted here that the word "basin" has several connotations. It is applied exclusively by some when considering tectonism alone. Others make no distinction between a tectonic basin and a bathymetrically controlled, sedimentary complex, holding that the two are of similar size, form simultaneously at the same place, and produce progressively thicker sediments toward their deepest portions. Generally, the concept of sediments thickening basinward is true only in a tectonic sense or in "graben-produced" areas within a continent. In epeiric seas the thickest indigenous deposits tend to form high on the sedimentary shelf where organic and chemical activity is greatest, and where "rapid" sedimentation, together with concurrent subsidence beneath continuously shallow water, may be most pronounced. Considered in this sense, basins are mainly discrete, tectonic features which are only parts of a much larger sedimentary framework. In this paper, therefore, the expression "depositional province" supplants "depositional basin," and the term "basin" is restricted to tectonic down-warps.

448 M. L. IRWIN

CLEAR WATER CON'CEPT

This part of the paper is devoted to a discussion of the sediments that ideally would form in marine waters which have received no influx of terrigenous elastics. The expression "clear water concept" is applied to this situation. The discussion includes:

1. Consideration of those sediments which would be fashioned when nothing is added to the sea, and

2. Appraisal of the sedimentation that would prevail upon the addition of rain and river water.

Carbonates.—Because the processes forming carbonates are mainly chemical and biochemical, students of the subject have often ignored the physical forces at work in the sea and the physical properties of carbonate sediments which form as a result. This is at variance with the study of terrigenous elastics which, of necessity, is concerned mainly with physical properties. Actually, a consideration of physical grains rather than one of chemically formed crystal textures may be a more fruitful approach to the study of carbonates than many of those undertaken in the past.

We begin by investigating the formation of carbonate sands and muds. The terrigenous clastic equivalents of these sediments are largely results of the physical processes of weathering and erosion, and are derived from the land. On the other hand, carbonate sands and muds are mainly indigenous to the sea and are products of physical, chemical, and biochemical forces. There are three classes of carbonates which may be considered in terms of their grain size:

1. Those composed mainly of animal and plant skeletal remains (bioclastics),

2. Those made up on non-skeletal, chemically and biochemically precipitated particles (pellets, muds, etc.), and

3. elastics which are derived from the broken fragments of pre-existing carbonates (litho-clastics).

Because of the ready availabihty of nutrients, the principal site of invertebrate animal growth is at the edges of marine banks or shelves where the shallow, turbulent sea water contains considerable dissolved oxygen and is of normal temperature and salinity. The skeletal remains of these animals

form sand and coarser particles many of which, when subjected to boring and burrowing organisms and to the pounding of waves and tides, break up into fine sand, silt, and mud-size fractions. Further wave action and winnowing distribute the broken particles over the sea floor simply as clastic fragments, so that they become progressively finer away from the region of high marine energy. These are sands, silts, and muds (bioclastics) which have formed as a result of both biochemical and physical activity, and have been distributed by physical forces. Actually, most of epeiric, deeper open sea carbonates are composed of silt-size detritus from the zone of marine animal life. Smaller fractions, although present in an open epeiric sea, are not abundant.

Reefs are a special type of biological sediment, and in this paper they are considered in a restricted sense. Reefs include only those structures whose parts are bound together to become wave-resistant and are composed of animal skeletons and symbiotic algae (colonial forms), which grow upward, either as a result of the bottom sinking or sea-level rising, to produce various mound-shape forms in which there is no appreciable bedding ("bioherms"). Animal colonies which grow laterally ("biostromes") are not considered in this presentation to be reefs; although "reef-like" complexes may include "true" reefs and biostromes, each forming at different times in response to varying transgressions, stillstands, or regressions of the sea.

Another type of carbonate sand and associated shale forms landward from the edges of banks and shelves. Cool sea water is swept into this zone to be warmed, evaporated to some extent, and to come into contact with algae and bacteria. These agencies all remove dissolved carbon dioxide from the water which increases the pH. This in turn may result in the precipitation of calcium carbonate.

Precipitation may occur in turbulent, high-energy waters which are only a short distance landward from the zone of marine animal life. In this high-energy environment, precipitated particles, shell detritus, or algal fragments may be rolled about on the sea bottom, forming a nucleus around which additional calcium carbonate can accumulate by a process of interstitial precipitation and accretionary growth to produce pellets having concentric and superficially concentric structures. In this zone, turbulence probably acts

EPEIRIC CLK.\R W.ATKR SEDIMENTATION 449

as an additional release for carbon dioxide which further aids in the precipitation of calcium carbonate. When pellets are not generated in this region, algal bioclastic carbonates may take their place.

When precipitation occurs still farther back on the shelf beyond the zone of waves and tidal action, where the waters are no longer turbulent, but where algae and bacteria flourish, algal carbonate mud (lithographic or chalky) is deposited. Here also is an environment in which fecal pellets (structureless) and pellets having a fully developed concentric structure with a small nucleus may occur. The latter type of pellet structure is concretionary in habit and can form in both quiet and turbulent water. It may be noted that the terms "ooHte," "pseudo-oolite," and "superficial-oolite" have been applied by some workers to one or more of the pellet types. Because this terminology may be ambiguous, it is omitted in this paper and only the term "pellet" is used.

The dual role of algae in carbonate sedimentation should be mentioned. These plants aid in the precipitation of calcium carbonate from sea water by extracting carbon dioxide from it for use in their fife processes, and, in addition, their skeletal remains may form part of the sediment. They are commonly constituents and binders of reefs and may be sediment trappers in shallow, non-turbulent water. Thus, many shallow-water hmestones are characterized by the presence of algae. This differentiates them from sediments deposited in the deeper open sea where algal growth is inhibited by insufficient light.

The third class of carbonate sands and muds is composed of the broken fragments of pre-existing carbonates. It is the one usually recalled to mind when fragmental or clastic limestones are considered. However, this type of sediment is comparatively rare in the geologic section. Fragments from carbonate strata exposed to weathering and erosion seldom are redeposited in the sea, for the soft and soluble nature of their composition inhibits their being transported over any significant distance. Breccia from submarine faulting of carbonate strata, talus from sea cliffs composed of limestone, and tidal flat sediments, intermittently exposed to weathering and erosion, may constitute the largest source of these rocks.

All primary marine carbonates may be referred to the three classes of sands and muds already discussed. Crystal textures have not been consid

ered. Their common presence in chemically and biochemically formed primary sediments allows us to disregard them. Our interest is in the larger particles, themselves composed of crystals. The study of crystal textures assumes greater importance, when diagenetic changes are being considered. It is important to note that effective primary porosity (that having appreciable permeability) is associated with the coarser fractions, and that, as sediments become finer, their permeability is reduced progressively. (Doubling the grain size increases permeability 30 times—G. R. Ritten-house, personal communication.) Well-winnowed sands, whether they are composed of carbonates or some other substance, constitute reservoirs which may contain and emit fluid. However, carbonate sands, being indigenous for the most part to the sea, form in response to known chemical, biochemical, and physical forces, so that the site of their occurrence in the geologic section is predictable.

Evaporites.—So far, we have examined only one chemical and biochemical product of the sea: calcium carbonate. However, there are other minerals that are equally important when considering "clear water" sedimentation in an arid climate, wherein there is no appreciable influx of rain or river water to the shoreward part of an epeiric sea. Heating and evaporation of essentially stagnant sea water increase its chemical saturation to the point where compounds are able to come out of solution. Theoretically, in epeiric seas, where low-order slopes, great width of shelves, and shallow depth of water are sufficient in themselves to restrict or eliminate circulation, salinities increase progressively from the landward edge of an open sea shoreward, so that an orderly pattern of precipitated sediments is established. The sediments comprising this pattern from the open sea landward are:

1. Calcium carbonate, CaCOj (calcite or arago-nite),

2. Calcium magnesium carbonate, CaMg (C03)2 (dolomite),

3. Calcium sulfate, CaS04 (anhydrite or gypsum), and

4. Sodium chloride, NaCl (halite) and other salts.

Thus, primary dolomite and finally evaporites are formed landward from calcium carbonate. I t is important to note that when sedimentation and

450 M. L. IRWIX

subsidence arc in equilibrium, no vertical succession of minerals results from precipitation during a standing seu-levcl stage. The compounds listed above probably arc deposited concurrently.

ENERGY ZONES

Sedimentation responds in often predictable ways to the effects of hydraulic energy acting upon marine bottoms. There are three energy belts in epeiric seas, each of which helps to control the type of sediment forming upon the sea floor:

1. A hundreds-of-miles wide, low-energy zone occurring in the open sea beneath wave depth where marine currents are the only form of hydraulic energy acting upon the bottom (Zone X),

2. An intermediate, high-energy belt, tens of miles wide, beginning where waves first impinge upon the sea floor and thus expend their kinetic energy on the bottom, extending landward to the hmit of tidal action (Zone Y), and

3. An extremely shallow, low-energy zone, which may be hundreds of miles wide, occurring landward of Zone Y in which there is little circulation of water, where tides are essentially wanting and in which the only wave action is that produced by local storms (Zone Z).

Figure 3 is an idealized section through these energy belts. In the foregoing discussion and on the section the zones are referred to as Zone X, Zone Y, and Zone Z. This practice is followed in the remainder of this paper.

Zone Y is characterized by the presence of sediments that are basically biogenic. Animal life, including reefs, flourishes on the seaward part of it; algal and chemically formed carbonate sands occur toward its landward side. It is the belt in

which winnowed sands form, and is the place, therefore, where we should expect to find well-developed, primary reservoirs. The pH in this environment is less than 0.7 and the Eh is high so that oxidizing conditions prevail and preservation of organic material consequently is inhibited. The zone is no more than tens of miles wide, for shoreward, friction quickly dissipates both wave and tidal actions and seaward, increasing depth places the sea floor beneath the reach of these forces.

Zone Z may be hundreds of miles wide, though within it water depths are extremely shallow, reaching to no more than a few feet at any place, being limited seaward by the depth of Zone Y, which must be in the photic zone and landward by mean sea-level. The nearly flat slope of its bottom permits wide expanse of very shallow water. It follows that water circulation is inhibited greatly here, and that this zone, as a consequence, becomes the natural site for the deposition of sediments which are basically of chemical origin. On this type of shelf, salinities increase shoreward, as there is progressively less introduction of fresh sea water, so that a regular pattern of facies from dolomite, through anhydrite or gypsum, to salt and other evaporites is established. If the bottoms here are tectonically unstable and there is continued downwarp as sedimentation proceeds, tremendous thicknesses of evaporites may accumulate beneath continuously shallow water. This is the suggested origin of "evaporite basins." Actually, in this writer's opinion, evaporites are extremely shallow-water deposits, which require inhibited circulation and an almost complete lack of fresh water for their precipitation.

Very little sediment probably forms in Zone X, although it is the site for an accumulation of finegrained detritus derived principally from Zone Y. Temperature decreases in the bathymetrically deeper water seaward from Zone Y, allowing it to

- HUNDREDS OF MILES -

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LOW ENERGY

uaJ(xxJou(X)9^^xou(xxxxxxxxx)^j6axxxx^

"Y"

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- UP TO HUNDREDS OF MILES

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i!:^^

7 • y M O X x ; -LITTLE CIRCULATION; TIDES RESTRICTED SEAWARD ABSEM SHOREWAROi LOCAL STORM WAVE ACTION ONLY. /

SEA BOTTOM ELEVATION KINETIC ENERGY FROM WAVES ACTING UPON BOTTOM; IN EFFECT TRANSPOSES ZONE " Y " SEAWARD.

POINT WHERE WAVES FIRST STRIKE BOTTOM,

POINT AT WHICH WAVE AND TlOAL ACTIONS ARE LARGELY DISSIPATED BY FRICTION.

EDGE OF LAND

KINETIC ENERGY FROM WAVES ACTING UPON BOTTOM; STRONG TIDAL ACTION.

FIG. 3.—Section showing energy zones in epeiric seas. Not to scale.

J-:PI;IRIC CLEAR WATER SEDIMENTATION 451

\ ZONf FROM WHICH F L U I O CfiN

SEA BOTTOM

ELEVATION--..

\

MIGRATE TO RESERVOIR ••

"X" SEfl BOTTOM ELEVATION

- t — t — ) — i — t — t — r — t — t — t — I — i — t — t — f - ^ ' i — i — \ — I — h

FIG. 4.—Plan view showing relations of sources to reservoirs. Conditions at 1 and 2 are preferable to those at 3. Not to scale.

remain undersaturated so that precipitates are inhibited from forming inorganically. Then too, much of the bottom here is near the lower limit or beyond the zone of photosj'nthesis, which discourages plant (algal) growth. In addition, because the sea floor is below wave base, little turbulence prevails to agitate and supply oxygen to it, and reducing conditions, consequently, tend to develop. This is not an environment in which benthonic animal life flourishes. An abundance of organic matter, however, is supplied to the bottoms here, being swept in virtually as an "organic rain" from Zone Y, and accumulating from the fallout of indigenous plankton and nekton in Zone X itself. Part of this material is animal derived; part is chlorophyll from phyto-plankton. It is relatively undisturbed after deposition, for marine currents are the only form of energy at work on the sea floor in this area. The sum total of these effects is to produce an environment, which seems to the writer to be suited to the accumulation, preservation, and subsequent chemical and biochemical reduction of organic matter. Zone X, then, appears to be a principal source of hydrocarbons to the reservoirs forming in Zone Y. Further, these conditions produce dark-colored sediments that, in the gross view, tend to thin toward the open sea and thicken toward the bathymetri-cally shallow, sedimentary shelf.

Although the waters in Zone X are deeper than elsewhere in epeiric seas, they are still relatively shallow, so that any tectonic or topographic eleva

tion of the sea floor is apt to bring the bottom above the base of wave action and, in effect, transpose Zone Y seaward (Fig. 3). Such a juxtaposition of Zones X and Y produces a natural strati-graphic trap, sealed and surrounded by its own source beds, for it follows that a reservoir which is surrounded by source beds is able to draw upon more of the contained organic material than one whose contact with it is linear (Fig. 4).

SEDIMENTATION ZONES

In the preceding parts of this presentation, broad zones of "clear water" sediments have been discussed, and an attempt has been made to show-that these sediments respond in predictable ways to the physical, chemical, and biochemical forces which have acted upon them. In addition, the effect of hydraulic energy at work upon the sea floor has been investigated, resulting in the recognition of three discrete marine energy belts, which are, in effect, environments of deposition. In this section of the paper, energy zones and sedimentation zones are considered together, emphasizing the interrelationship of the two. The relation of energy and sedimentation zones in an epeiric sea, which is free from the influx of both rain and river water and terrigenous elastics, is summarized on line J in Figure 5. The sedimentation zones are easily referred to by number, and this practice will be followed in the remainder of this paper.

Only one sedimentation zone is present in Zone X; Zones Y and Z each includes two sedimenta-

4,S2 M. L. IRWIN

tion zones. Salinity increases from Zone II landward due to increasing restriction of circulation in that direction, and from the inward side of Zone III landward, the sediments become finer-grained and therefore less suitable for primary reservoirs. The chemical nature of Zones IV and V, together with their dense matrix, makes them equally undesirable for either source or reservoir. It becomes apparent that the farther exploration is carried on shelfward, the less likelihood there should be for finding accumulations of oil and gas in primary "clear water" sediments.

When rain and river water are added to epeiric seas, the pattern is altered to the situation shown on line K of Figure 5. This water freshens the highly saline sea on its shoreward side and produces an environment in which algae are able to flourish. The presence of these plants results in the formation of algal carbonate muds (Zone VI) which lie shoreward from evaporites. Indeed, the finding of carbonates on two or more sides of thick evaporites has led some people to believe that evaporites form in bathymetrically deep, highly saline, restricted basins which freshen toward their edges. If, however, the climate is sufficiently humid and enough fresh water is added to the sea, Zones IV and V may not develop at all, so that an expanded Zone III may grade shoreward into Zone VI. The presence of a physical barrier, such as a reef in Zone II, also may alter the pattern by permitting only slight turbulence in the area

shoreward from the barrier, which would prevent the formation of carbonate sands in Zone III . In this situation Zone II would grade shoreward into Zones IV, V, and VI, or if the chmate were sufficiently humid, through a fine-grained, attenuated Zone III into Zone VI (Fig. 5).

Most of this relationship (line K of Fig. 5) was first noted by the writer in 1955 when he was engaged in a study of the Mississippian System in parts of North and South Dakota (Fig. 6). Much of the present theory resulted from deductions which were made during the course of the study, while attempting to explain the sedimentary patterns observed in this area (Figs. 7, 8). Since that time, similar models have been described in the literature, notably one presented in an excellent paper by R. W. Edie (1958). However, to the writer's knowledge, the present paper is the first attempt to qualify the subject in its broader aspects and to record differences that exist between epeiric and present-day, oceanic coastal sedimentation.

To these two factors, climate and barriers, which may govern the width of sedimentation zones, must be added a third: slope of the sea floor. In effect, the steeper that the slope of the bottom is, the narrower Zones Y and Z become. In oceanic, coastal areas, where steep slopes are the rule, Zone Z may be extremely narrow or missing altogether, so that wave and tidal actions extend to the shore, producing a beach; whereas, in

-HUNDREDS OF MILES-

X

LOW ENERGY

(MARINE CURRENTS!

BASICALLY DETRITUS FROM ZONE E

TENS Of MILES--

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(WAVES AND TIOESI SEDIMENTS BASICALLY BIOGENIC

• j - —TENS TO HUNDREDS OF MILES -

3 LAND

LOW ENERGY (LITTLE CIRCULATION; TIDES RESTRICTED SEAWARD ABSENT SHOREWARD, LOCAL STORM WAVE ACTION ONLY) SEDIMENTS BASICALLY CHEMICAL

•• SEAWARD

ENERGY

ZONES

n 12 SEDIMENTATION ZONES

ANIMAL SKELETAL

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CASE

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'6 CONCENTRIC STRUCTURE WITH APPARENT SMALL NUCLEUS

C. SUPERFICIAL CONCENTRIC STRUCTURE.

D STRUCTURELESS (FAECAL)

HEAVY OUTLINE SHOWS ZONES MOST FAVORASLE FOR PRIMARY RESERVOIR POROSITY.

FIG. 5.—Clear water energy and sedimentation zones.

EPEIRIC CLE.\R WATER SEDIMENTATION 453

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6J WILSS

FIG. 6.—Stratigraphic section index map.

epeiric seas the zone of waves and tides may be hundreds of miles from shore, preventing the formation of beach deposits. This difference between the location of the high-energy belt (Zone Y) in oceanic coastal positions and epeiric areas is extremely significant. Measurement of the degree of the bottom slopes provides a tool to predict the distance which separates the primary reservoirs

formed in Zone Y from other types of sediments. However, this measurement can be made only from a datum which is a time equivalent.

River water flowing into the sea from the land may bring significant amounts of chlorophyll with it. The addition of this material to the already organically rich and chemically reducing environment prevailing on the floor of Zone VI, enhances

WEST

SHELL YOUNGBLOOD HOUME ANDERSON

SHELL WINTER

YOUNGBLOOD DRASKOVITCH

YOUNGBLOOD KELSTROM

PE«K OHLHMSER

TRflNSGRESSlVE

TflANSGRESSiVE

Ml TEflHIGENOUS CLASTICS

DATLM IS FIRST APPEARANCE OP SECIMENT-TREE TERRIGENOUS ELASTICS

FIG. 7.—Section showing clear water relationships.

454 M. L, IRVVIX

it as a region in wliirh petroleum source beds may form. Hydrocarbons in this environment would be constructed mainly from plant remains, whereas th<ise formed on the bottom of the open sea would be fashioned both from animals and from fallen, chlorophyll-rich phyto-plankton. It has been noted by Rainwater (1962), as well as by the writer, that hydrocarbon gases seemingly tend to be generated from plants, whereas the liquid fraction may be animal-derived. Although this is conjectural, a source of organic material principally of plant origin (Zone VI), could account for places, such as the Hugoton area in the Texas-Oklahoma Panhandle where the reservoirs are mainly gas-bearing.

THREE-DIMENSIONAL ASPECT

The discussion to here has been concerned, for the most part, with a consideration of the lateral relations of sedimentation and energy zones: those which form when sea-level is essentially constant. To transgressive and regressive epeiric seas occurring through time, a vertical dimension is added. Figure 9 shows the lateral and vertical patterns which form in transgressive and regressive epeiric seas. During transgression as each

sedimentation zone migrates toward shore, it overlaps older sediments which earlier were deposited in an environment similar to the one then prevaiHng in the zone adjoining landward. In this manner, an ascending sequence of sediments is deposited under conditions of progressively less restricted circulation, the same sequence as that established at a single time from land to sea. During regression, each sedimentation zone migrates seaward, so that the ascending succession of sediments is deposited under progressively more restricted conditions, and is the same as the one reaching from sea to land. If the sea completely withdraws from the continent. Zone Z would be exposed progressively in a seaward direction, permitting the formation of soils and other land forms. This surface, of necessity, would be dia-chronic, unless one were to conclude that an epeiric sea could regress "instantaneously" from the middle of a continent back to the ocean from which it came. The writer can conceive of no possible manner in which this could occur. Geologic time would be involved—probably in the order of millions of years. Thus, in a regressive sequence, a rock type found on top of another would also occur landward from it; during transgression its

HUNT-BROOKS

FEDERAL

TRANSGRESSIVE

TRANSGRESSIVE

[SSgn.M

E>>] FINE rERRIGENOUS CL4STICS

04TUM IS FIRST APPEi f t / lNCE OF SEDIMENT - FREE TERRIGENOUS CL iST ICS

FIG. 8.—Section showing clear water relationships.

ICPKIRIC CLEAR W.ATER SEDIM KNTA TION

b

4,S5

FIG. 9.—Horizontal and vertical relations of sedimentation and energy zones.

position would be seaward. This process of continuous sedimentary overlap, as the sea advances and retreats, is the principle which explains cyclic sedimentation. It should be noted that the age of geologic formations (Zone III sediments, for example—Fig. 9) becomes progressively younger in the direction of migration during both transgression and regression.

A perception of the spatial relations of the sedimentation zones becomes useful for grading and exploring provinces. A well at "a" on Figure 9, for example, would encounter only fine-grained, open marine sediments, which would indicate that exploration for primary reservoirs should be made landward. The section at "c" would be composed only of mud-size, chemically formed sediments, meaning that the search for primary reservoirs should be made seaward. More favorable conditions should prevail at " b " where the entire cycle would be present. Thus, an analysis of a single section would determine whether to proceed landward or seaward to find primary reservoirs; a knowledge of the width of the sedimentation zones would indicate how far.

The regressive part of a cycle is often prolonged, indicating a gradual withdrawal of the sea and the consequent formation of a regular succession of sediment types. In contrast, transgression is often "rapid," and results in an abrupt change from more saline to less saline-water deposits. This abrupt change is useful in recognizing correlative points to establish time equivalents. The major changes or "kick-backs" from more restricted to less restricted type sediments commonly can be traced over considerable distances. Minor fluctuations between transgressive and regressive sequences may be used locally. These minor fluctuations are brought about by local changes in the rate of subsidence or emergence in relation to the rate of deposition; for when the rate of subsidence is greater than the rate of deposition, transgressive sequences result, and when the rate of subsidence is less than the rate of deposition, regressive successions form. Care must be taken to differentiate local variations, which may be the result of this process and regional "kick-backs" brought about by regional transgressions and regressions. On Figure 10 a plot has been made of the vertical

45() M. L. IRWIN

relations of the sedimentation zones on three hypothetical sections: "A", "B" , and "C". The "kick-backs" from regressive to transgressive successions in an upward direction on the graphs indicate time markers wdiich may be correlated over considerable distances. The sections at " B " and "C", in addition, contain minor "kickbacks," which are useful for more local time correlations. Lithologic correlations diverge downward from time equivalents, and the facies thicken landward. Triangulation among three such points, not along depositional strike, would show the amount of this divergence in an area and so indicate the degree of diachronism of the formations. A "kick-back" from transgressive to regressive sequences in an upward direction may be used similarly to establish time correlations, although these are not as common. Diachronism in the order of 2 feet per mile, for example, has been demonstrated by A. B. Shaw (personal communication) between lithologic markers and paleonto-logic time equivalents in parts of the Mississip-pian section in Montana. This divergence is of considerable significance, amounting to 100 feet over a distance of 50 miles.

A time-stratigraphic analysis is of particular value when mapping paleogeography. From such an analysis, the position of ancient shorelines, the

configuration of buried submarine topography, and, of even greater importance, the history of paleostructural growth may be established. On the other hand, tlie intervals between formations may be thicker as the result of slow transgressions or regressions, or they may be thinner because_of more rapid invasions or withdrawals of the sea. Such intervals may indicate sedimentation rates, modified by compaction. These factors may have little or no bearing on paleostructure or submarine topography. Indeed, small variations in structure or topography in such shallow seas as we are considering can be of utmost importance, for, as pointed out in Figures 3 and 4, a small elevation of the sea floor in Zone X may place that bottom within the range of wave action and thus produce a high-energy environment suitable for the establishment of reefs or other well-winnowed carbonate sands.

A reef, actually, is a specialized sediment which may develop in Zone II . If reefs are to form at all, ideal conditions of depth, temperature, and clarity of water, food supply, and turbulence must prevail. Reefs of significant height form only'during marine transgressions; for it is only then that increasing water depth allows them to grow upward. However, once established, their very presence forms physical barriers to water circula-

R = regressive

FIG. lO.-Correlation of "kick-backs" (transgressions) to establish time equivalents,

EPEIRIC CLEAR W.\TER SEDIMEMATION 457

FIG. 11.—Reef pattern in center of transgressive sequence.

tion which influence the sedimentation patterns about them. In Figure 11 a reef growing at point "A" would affect the lateral position of the sedimentation zones in several ways: Zone I would be held seaward by this barrier during transgression, Zone III probablj' would not form at all, and Zones IV, V, and VI (Zone Z) would expand in width and be forced seaward to merge with the growing reef at its inner edge. The higher the reef grew, the greater would be both the lateral displacement and expansion of Zone Z and the lateral expulsion from its proper position of Zone X. Once reef growth ceased at point "A" and the barrier to water circulation no longer prevailed, the sedimentation zones rather quickly would revert landward to their normal stations in a transgressive sequence, and the next younger reef growth would begin shoreward at point "B . " Knowledge of this process, which could be repeated several times during a prolonged transgression, is useful for reef exploration. Thus, when reefing had been discovered in an area, a line of older reefs might be expected seaward, and a line of younger reefs might be found landward. Isolated reefs, in addition, could occur in the fore-reef areas in front of such barriers, due to elevations of the sea floor as we have considered previously.

The vertical succession of sediments in an area may indicate the presence of a barrier formed seaward during the initial stages of a transgression. Figure 12 is a graphic representation of the sedimentation patterns which might be found in a normal transgressive succession, in a reef section

and in a back-reef area. When vertical growth of the barrier-reef ceases, the open sea (Zone X) is able to transgress quickly over Zone Z deposits in the back-reef area, as it reverts to the position it would have occupied during a normal transgression. This rapid flooding excludes Zone Y sediments from their usual position in the ascending sequence, producing a "kick-back" on the graph in Figure 12 from "Z" to "X". This type of vertical succession also is apparent between points "A" and " B " and between points " B " and "C" in Figure 11. Thus, careful study of vertical sequences often enables us to predict lateral relationships which may occur in an area, and the kind of "clear water" reservoirs which may be expected. Such information can be unusually helpful when exploring regions in which there is meager stratigraphic control.

DiAGENESIS

In any discussion concerned with the geologic grading of paleogeographic provinces, the subject of secondary porosity development in carbonate rocks is likely to be raised. Although this paper mainly is directed toward a better understanding of the spatial occurrences of primary, carbonate reservoirs and to a minor extent their related source beds, it is perhaps apropos to include here a short discussion of secondary carbonate reservoirs and the influence it is believed they have upon an over-all grading program.

To review briefly, there are probably three fundamental types of primary carbonate poros-

458 M. L. I R W I \

ity: (1) that formed by intergranular winnowing of carbonate sands (pellets, bioclastics, and litho-clastics), (2) that produced by the decay of organic constituents in sediments which have lithi-fied early, and (3) that caused by borings of bottom-dwelling organisms, again in sediments which have solidified soon after deposition. Winnowed porosity is by far the most important of these, constituting the reservoirs in many of the world's major carbonate petroleum accumulations, such as the Redwater and associated limestone reef fields in Canada, the Scurry "reefs" in Texas, the McClosky fields in the Eastern Interior basin, the Williston basin Madison fields, and for most of the 112 billion barrels of proved reserves in carbonate fields in the Middle East.

Four basic types of secondary carbonate porosity generally are recognized: (1) that produced by solution or leaching, (2) that formed by fracturing or jointing, (3) that caused by weathering, and (4) that resulting from recrystallization (do-lomitization of original limestone fabrics). Actually, the first two of these usually are related, and often

can be ascribed to tectonism, which produces the folding, faulting, fracturing, and unconformable relationships, which in turn, permit the secondary entrance of solvents and later hydrocarbons into what otherwise may be dense, inhospitable rocks. Even so, it is difficult to envisage a process by which fluids can enter structurally controlled fracture and solution reservoirs from off-structure impermeable sediments, unless some sort of conduit for such fluids is provided. The prolific LaPaz Cretaceous reservoir in western Venezuela, as an example (personal observation), is made up of nearly 2,000 feet of fractured limestone on a highly-faulted, sharply folded anticline. The middle CogoUo member of this sequence, however, is composed of approximately 50 feet of oil-impregnated, bioclastic, carbonate sand, in which primary porosity values as high as 21 per cent are encountered, but which is only slightly permeable (less than 1 md.). It is suggested that this bioclastic bed, which occurs over a wide area, served as a primary host rock for hydrocarbons, that subsequent tectonic activity produced the LaPaz

Y

n n Z

Normal tronsgressive Sequence

X I

L

in IE

-1

1 1 1 1

Z m

Reef growth ceases

Reet

Sequence

•Reef growlh begins

X Y

n m Z

Beck-reef Sequence

Kick-bock from " Z " to "x "

Elapsed times are equol at each section.

FIG. 12.—Normal, reef, and back-reef transgressive sequences.

i : i>EIRIC CLEAR \ V A ' | ] ; R S E U I M E X T A T I O X 459

structure and associated fracture reservoir, and that hydrocarbons then migrated sk)wly into this reservoir from and through the porous, bioclastic member. According to Dunningham (1958, p . 1229), a somewhat similar situation occurs in the .Ain Zalah and Butmah fields in Iraq where initial accumulations in neritic, carbonate sands later escaped upward into anticlinal fracture reservoirs.

I t has not been demonstrated satisfactorily tha t recrystalHzation (dolomitization) of original limestone fabrics is a process which may develop porosity in originally dense rocks. Actually, evidence exists which suggests tha t porosity may not be developed by this process; as for example, the frequent occurrences in the geologic section of dense, even lithographic, secondary dolomites. I t is suggested here tha t porosity in secondary dolomites, for the most part , is inherited directly from the original Kmestones, either from porous, carbonate sands in Zone Y, which subsequently are altered diagenetically, from recrystallized, weathered leached, and fractured rocks at unconformities or from dolomitized chalks in Zone VI which inherently (before diagenesis) possessed high porosities but had low permeabilities. The LeDuc and associated dolomite reef fields in Canada, the Silurian reefs in the Eastern Interior basin and the Turner A^alley and Jumping Pound fields in the Canadian Rockies Folded Belt are all examples of secondary dolomite reservoirs which originally were deposited as winnowed, carbonate sands (bio-clastics).

So it seems that most of the major, hydrocarbon accumulations found in carbonates throughout the world occur in three principal ways: (1) in primary, carbonate sand reservoirs, (2) in secondary reservoirs which have altered diagenetically from carbonate sands, and (3) in secondary reservoirs which are dependent on carbonate sands either as the original host rock or as hydrocarbon conduits. Based on these observations, it seems reasonably safe to assume tha t most significant, secondary carbonate reservoirs develop in the same general areas tha t we would expect to explore for the primary reservoirs, and tha t the grading principles set forth in this paper, therefore, are valid for an over-all evaluation of all marine carbonate sediments.

CoNCHDi.vG R E M A R K S

In the preceding discussion, an a t tempt has been made to show that "clear water" sediments form in response to known physical, chemical, and biologic forces, so that the horizontal and vertical positions tha t these rocks occupy are not random but can be predicted logically. In addition, an effort has been made to emphasize the differences tha t pertain between epeiric and oceanic coastal sedimentation, including:

(1) areal extent of depositional provinces, (2) degree of bot tom slopes,

(3) spatial relations of lithofacies, (4) importance of time and mobility, and

(5) positions of environments, suited to hydrocarbon reservoirs and source beds.

I t is the writer's hope tha t this paper will provoke ear th scientists to consider "clear water" sedimentation from this somewhat different point of view and to realize tha t there still remains much tha t can be learned from looking a t rocks.

SELECTED R E F E R E N C E S

Dunningham, H, V., 1958, Generation, migration, accumulation and dissipation of oil in northern Iraq, in Habitat of oil—a symposium: p. 1194-1250, Am. Assoc. Petroleum Geologists.

Edie, R. VV., 1958, Mississippian sedimentation and oil fields in southeastern Saskatchewan: Am. .\ssoc. Petroleum Geologists Bull., v. 42, no. 1, p. 94-125.

Ginsburg, R. N., 1957, Regional aspects of carbonate deposition: Soc. Econ. Paleontologists and Mineralogists Special Pub. 5, p. 80-100.

Hume, G. S., 1944, Petroleum geology of Canada: Canadian Geol. Survey Bull. 98, Geol. Ser., no. 54, p. 51-58.

Illing, L. v., 1954, Bahaman calcareous sands: Am. .•\ssoc. Petroleum Geologists Bull., v. 38, no. 1, p. 1-92.

McGehee, J. R., 1949, Pre-Waterways Paleozoic stratigraphy of Alberta Plains: Am. Assoc. Petroleum Geologists Bull., V. 33, no. 4, p. 606-613.

Plumley, W. J., Risley, G. A., Graves, R. W., Jr., and Kaley, M. E., 1962, Energy index for limestone interpretation and classification, in Classification of carbonate rocks—a sj'mposium: Am. Assoc. Petroleum Geologists Mem. 1, p. 85-107.

Rainwater, E. H., 1962, Stratigraphy and its role in petroleum exploration with special emphasis on the Gulf Coast: Am. Assoc. Petroleum Geologists Distinguished lecture.

Sugden, W., 1963, The hydrology of the Persian Gulf and its significance in respect to evaporite deposition: Am. Jour. Science, v. 261, no. 8, p. 741-754.

World Oil Editorial Staff, 1962, Canada's northernmost wildcat is projected to 14,000 feet: World Oil, v. 154, no. 1, p. 100-104.