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Chapter 7 Epicontinental Marine Environments, II
Chapter 7
EPICONTINENTAL MARINE ENVIRONMENTS, I1
CYCLES WITH SIGNIFICANT QUANTITIES OF SANDSTONE, IRONSTONE AND PHOSPHORITE: MINOR CYCLES WITH BITUMINOUS LAMINAE
In this chapter we shall deal with a wider variety of lithological types than in the previous chapter. Generally speaking they are of subordinate importance to Iimestoneshale cycles.
CLAY-SANDSTONE-LIMESTONE CYCLES
The best-known cycles composed of sequences of argillaceous, arenaceous and calcareous deposits, such as those in the late Palaeozoic of northern England (Yoredale facies) and the mid-continent region of the United States, are dealt with fully in Chapters 4 and 5 , since they have generally been regarded as indicative of oscillating marine and continental environments. The few other examples discussed here may be dealt with most appropriately under the heading of epicontinental marine environments. In his classic work on the Jurassic System in Great Britain ARKELL (1933) observed that as long ago as 1822 Conybeare and Phillips had remarked on the regular manner in which clay is followed by sandstone and the latter by limestone in the socalled Oolitic sequence. Arkell went on to elaborate this tripartite scheme, dividing almost the whole of the Jurassic in southern England into major cycles. His scheme is given below, with the clay, sandstone and limestone units signified by capital letters: Portland Stone (L) Portland Sand (S) (9) Kimmeridge Clay (C) Westbury Ironshot Oolite (L) Sandsfoot Grit ( S ) (8) Sandsfoot Clay (C) Osmington Oolite, etc. (L) Bencliff Grit, etc. (S) (7) Nothe Clay, etc. (C)
184
EPICONTINENTAL MARINE
ENVIRONMENTS, I1
Berkshire Oolite (L) Lower Calcareous Grit (S) (6) Oxford Clay (C) Kellaways Rock (L) Kellaways Sand (S) (5) Kellaways Clay ( C ) Cornbrash Main Forest Marble Limestone (L) Hinton Sand, etc. (S) (4) Bradford Clay, etc. (C) Great Oolite Series (L) Stonesfield Slate beds ( S ) (3) Fuller's Earth (C) Inferior Oolite Series (L) Upper Lias Sands ( S ) (2) Upper Lias Clay (C) Marlstone Rock-bed (L) Middle Lias Sands (S) ( I ) Middle Lias Clay (C) Lower Lias clays, etc. It is easy to criticise aspects of this scheme as an example of cyclic sedimentation. As Arkell himself freely admitted, there was no question of clay, sandstone and limestone being deposited over the whole area simultaneously. No lithological details are given, distinguishing, for instance, different types of limestone. Distortion is involved equating major formations like the Oxford and Kimmeridge Clay with minor and local units like the Sandsfoot Clay; the Marlstone consists of sandstone and ironstwe besides limestone, and so on. Nevertheless one cannot disagree with Arkell that there is a striking overall periodicity involved, phases of widespread deposition in very shallow water (oolites, coral limestones, sandstones) being interrupted by phases of deeper water (clays and subsidiary argillaceous limestone). Arkell followed the Germans in attributing these oscillations to periodic epeirogenic movements of the sea floor. We may note, however, that the beginnings of cycles 2 , d (best taken together with the much thinner and less conspicuous cycle 5) and 9, all suggesting widespread and pronounced deepening of the sea, correspond with important worldwide transgressions and hence signify eustatic control (HALLAM, 1963~).The same may be true of others of Arkell's cycles.
CLAY-SANDSTONE CYCLES
185
Another example where the cycles are major stratigraphic units, ranging up to several hundred metres in thickness, comes from the Upper Pliocene and Pleistocene deposits of Wairarapa, New Zealand (VELLA,1963). Vella gave the following as a typical marine cycle, together with his interpretation: (Disconformity) shallowing culmination. Sandstone with shell beds shallowing. Sandy mudstone with scattered fossils deepening culmination. Sandstone with shell beds deepening. Coquina limestone deepening. (Disconformity) shallowing culmination. Vella attempted an estimation of depth of deposition based on known distribution of modern relatives of the contained fossils, and concluded that the cycles were expressions of depth changes ranging from about 50 to 160 m. These were in phase at least over half of North Island and, allowing for local tectonic disturbance, glacioeustatic control was proposed. To conclude this section an instance of a minor cyclic sequence will be cited from the Rhaetian in southwest England (HAMILTON, 1962). In the Cotham Beds (for example) four cycles can be distinguished, about a metre in thickness, with the following characteristics: ( I ) a basal calcareous horizon, above which there is usually a gradual reduction in the amount of carbonate, (2) shell or sand beds, or ripple lenticles, most common near the base of a cycle but which decrease in frequency upwards, and (3) an increase in the proportion of clay upwards in each cycle. Hamilton considered that the basal bed of each cycle was deposited under conditions of greatest current and/or wave action, with a gradual reduction of energy input from the environment as the cycle progressed. Such conditions were thought to be related possiblyto relatively small oscillations of the strand line. It is interesting to note, as Hamilton pointed out, that these minor cycles occur within a Rhaetian sequence which itself forms a good example of a cycle of Kliipfelian type.
CLAY-SANDSTONE CYCLES
As in the previous case, it is difficult to find many cited instances in the literature of clay-sandstone cycles in stable shelf regimes which do not involve non-marine deposits. Infact, both of the examples which will be considered here pass laterally into nonmarine beds within short distances. They are nevertheless most appropriately dealt with here. Major marine and non-marine units of the Eocene of the Anglo-Franco-Belgian Basin (STAMP, 1921) interfinger diachronously as indicated diagrammatically in Fig.73. The marine part of the sequence tends, in its fullest development, to show the sequence: basal gravel-sand-clay-sand-gravel. The gravel may be only poorly developed or absent, but the plane of erosion between the marine cycle and the underlying beds is almost invariably well marked. To give a specific example, the London Clay
186
EPICONTINENTAL MARINE
-.
2
I,
%I
LOWER
HEADON
BEDS
ENVIRONMENTS, I1
OLIGOCENE
1BARTON I AN LEDlAN LUTETIAN
BAGSHOT
'SANDS
7-7 YPRESIAN
READING,
BEDS
1
Fig.73. Major cycles in t& Lower Tertiary of the Hampshire Basin. Stippled areas represent marine beds, unornamented areas hon-marine beds. (After STAMP,1921.)
in the Hampshire Basin begins with thin sandy pebble beds and passes up into silty clay; the top is sandy once more. This succession is thinner and sandier in the west, where it is interrupted by non-marine beds. The thickness of the London Clay cycle varies from nearly 100 m in Whitecliff Bay to 70 m in Alum Bay. Thin intercalations of gravel within the clay were not regarded by Stamp as marking minor cycles. Stamp was able to demonstrate the essential contemporaneity of major cycles within the whole area considered. They were related to periodic transgressions and regressions of the sea, with the clays signifying deeper water conditions than the sands. Cycles very similar to those just described also occur in the Tertiary deposits of the Gulf Coast Region of the United States (BORNHAUSER, 1947; LOWMAN, 1949; FISHER, 1964; and Table XXIX). Gulfwards the deposits pass from a fluvial and lagoonal facies into an argillaceous marine facies. The cycles occur where marine clays are intercalated between marginal marine and continental arenaceous beds. Fisher (1964) has given a detailed account of cyclic deposits in the Eocene. His transgressive phase of a given cycle consists of sands and marls overlain by so-called restricted or marginal marine clays. These are laminated and have an apparently dwarfed fauna of thin-shelled bivalves. This phase overlies a basal condensed unit, marking a transgressive marine disconformity, rich in glauconite, phosphorite pebbles, reworked fragments and shark teeth. The inundutive phase (Bornhauser's term) marks the maximum advance of the sea and is characterised by normal argillaceous beds. Fisher's regressive phase has a basal arenaceous unit (fluvial or marginal marine) and an upper argillaceouscarbonaceous unit (mostly lagoonal or flood plain) in the northern Gulf Coast Region. These strata pass into entirely marine beds southeastwards. Fisher recognised five cycles in the Claibornian of Alabama but only one in the
IRONSTONE-BEARING CYCLES
187
Jacksonian (cf. Table XXIX). The thickness of the marine phases in this region rarely exceeds 30 m while the non-marine phases may be much thicker. While there appears to be general agreement that these Tertiary cycles in both Europe and North America result from the advance and retreat of the sea, no-one seems to have considered that such oscillations might have been the local expressions of eustatic movements until HALLAM (1963a) pointed out that the major transgressions and regressions in the two regions appeared to coincide in time, at least to the degree of refinement indicated by such age designations as end-Palaeoceneand Middle Eocene. kn this connection it is noteworthy that the basal transgressive beds appear to be condensed, giving an overall asymmetry to the cycles which compares with the Mesozoic limestone cycles discussed in the previous chapter. Another point of comparison is that the laminated shales, most likely anaerobic deposits, occur in the lower part of the cycle. IRONSTONE-BEARING CYCLES
One of the major problems in sedimentary geology concerns the origin of the banded ironstones or itabirites found in the Precambrian of every continent (GEIJER,1957; JONES,1963). Though most abundant in the Late Precambrian, where they compose formations up to 450 m in thickness (SAKAMOTO, 1950) they also occur less commonly in the Archaean and Palaeozoic. We are not concerned here, however, with the general problem of iron enrichment but with the nature and origin of the cyclic alternations between chert on the one hand and one or more iron-bearing minerals (silicates, carbonates and oxides) on the other. Specific examples will be taken from the famous deposits of banded ironstone in the Huronian of the Lake Superior region in North America, considered to have been laid down in a restricted marine environment (JAMES,1954). One common type of banding consists of regular alternations of chert and siderite averaging 1 cm in thickness. Stylolites are common and slump structures not rare. A more spectacular rock is the jaspilite of the Marquette Range, composed of alternations of reddish jasper and grey haematite typically 0.25-1.25 cm thick. These layers may themselves be laminated, with laminae about 0.025 mm thick. The bedding is wavy rather than straight and individual layers show pinch-and-swell structure. Parts of the succession are oolitic, both in the chert and haematite, though oolitic structure is more obscured in the latter lithology. A notable feature of the various types of banding is the remarkable constancy of individual horizons. James was primarily concerned with the general environment of deposition and the conditions governing the formation of various iron minerals and did not address himself to accounting for the banding. Sakamoto, however, dwelt on this subject at length in a general review of Precambrian ores (SAKAMOTO, 1950). He developed an ingenious hypothesis treating the banding as annual. A monsoonal climate was proposed, with alternating wet and dry seasons. During the wet season, it was argued, acid conditions would have prevailed in the weathering regime, allowing iron to migrate in
188
EPICONTINENTAL MARINE ENVIRONMENTS, I1
TABLE XXIX MAJOR CYCLES IN THE TERTIARY OF THE GULF COAST OF THE U.S.A.
(After FISHBR, 1964) Lithology
Sand, shale, lignite -.
Group
Formation Mississippi
Louisiana
Catahoula
+ post-Catahoula
Cycle Phase
Pliocene
Regression
-.
?
?
Miocene
Inundation
Chickasawhay L. Byram Marl Marianna L. Forest Hill/ Red Bluff
Vicksburg
Vicksburg (Oligocene)
Transgression
Grey shale, marl
Yazoo
Jackson Shale
Jackson
Inundation
Marl, glauconitic sand
Moody’s Branch
Moody’s Branch (Eocene)
Transgression
Sand, grey shale, lignite
Cockfield
Cockfield
Regression
Dark shale
Limestone, marl, shale
V
VI
-
Dark shale
Cook Mountain -
Limestone, shale, sand
(Wantubee)
Inundation
Cook Mt. Shale Clayborne _ _ _ ~ (Eocene) Cook Mt. L.
_
Transgression
Regression
Sand, shale, lignite Sparta (Kosciuska) Sparta Dark shale
Zilpha
111
Inundation
Cane River Shale
I1
~~
Glauconite sand, marl, shale, glauconite sand
Sand, shale, lignite Wilcox Dark shale
Porter’s Creek
Marl, calc. shale
Clayton
UPPER CRETACEOUS
Transgression
Winona Tallahatta Cane River Marl
Wilcox
Wilcox (Eocene) Regression
Midway
Midway
Inundation Transgression
I
IRONSTONE-BEARING CYCLES
189
solution, whereas silica would remain behind as insoluble material. Conditions would (1950) considered that the banded have been reversed in the dry season. SAKAMOTO ores were deposited in lakes. Iron hydroxide would have been precipitated in alkaline waters in the dry season, with the pH varying from 9 to 5. JONES(1963) has also proposed that the primary control involved was pH (in contrast to the Eh control of different iron minerals, as James argued). Sakamoto thought that banded ironstones were formed only during a period in the Precambrian under conditions which have never recurred since. A major difficulty in Sakamoto’s interesting hypothesis is the assumption of striking changes in pH at frequent and regular intervals. They would be remarkable even in lakes, but it is highly questionable whether the Precambrian banded ironstones formed in lacustrine environments. JAMES (1954, p.243) has argued cogently for marine conditions in the case of the Lake Superior ores, and the wide lateral extent of such deposits in different parts of the world is more readily explicable on this assumption. It is well-known that sea water is a well-buffered chemical system, with pH varying only between narrow limits (about pH 7.6-8.1). The problem therefore remains. Perhaps the possibility of a diagenetic control of the banding should be explored, as in the case of some small-scale cycles in limestone, shale and chert (see Chapter 6). KREJCI-GRAF (1964, p.485) has argued, for instance, that the cause of the banding might have been processes of solution and precipitation within the sediment due to periodic or episodic changes of Eh in connection with the rhythm of sedimentation of organic matter. There seem to be one or two pointers towards a hypothesis of diagenetic unmixing in James’ description of the Marquette jaspilites. The bands are themselves laminated, and where oolitic structure occurs it evidently ignores the banding, occurring in both lithologies. It is true that James argued for the primary origin of the chert, but the evidence he put forward (remarkable lateral constancy, stylolites and chert veinlets cutting bands, slump structures involving chert) is not decisive and the socalled primary chert could be accounted for by diagenetic migration fairly soon after deposition. A totally different type of cycle involving ironstone has been described from the Lias of Yorkshire, England, by HEMINGWAY (1951). Three major cycles were distinguished. The best developed corresponds with the Toarcian and is about 100 m thick. Following silty grey shales at the base come finely laminated dark brown bituminous shales of the Jet Rock. These pass gradually upwards into fine grey shales and thence via silty shales into fine-grained sandstone (Fig.74). At the top of the cycle in Rosedale are a few metres of sideritic chamosite oolite. Another cycle corresponds almost exactly with the Domerian, shales passing up via sandstone into shales with nodules of siderite mudstone and thence into chamosite oolite (Cleveland Ironstone). The third cycle is taken to embrace almost the whole of the Lower Lias, but in this case a regular sequence is not apparent and the relationships of the different lithological units are more complex than Hemingway assumed. Hemingway interpreted the cycles as resulting from tectonically-induced vertical
EPICONTINENTAL MARINE ENVIRONMENTS, I1
STRATA Rosedale Ironstone Blea Wyke Beds Striahitus Shales
SUB STAGES
Peak Shales
'TOARCIAN
Alum. Shales
The Hard Shales The Bituminous Shale; Jet Rock The Grey Shales
1
Cleveland Ironstone Series
DOMERIAN
The Sandy Series
Shales with ironstone concretions
CAR I X I A N
Shales with hard sandstones 5 I NEMUR I AN
Shales with occasional shell limestones
J R Iu1
5 0 0 feet omitted
Grey qrccn shale Black shale
-
HETTANGIAN
RHAETIC
Fig.74. Major cycles involving ironstones in the Liassic succession of Yorkshire, with interpretation of relative depth of deposition of the different facies. (After HEMINGWAY, 1951.)
movements altering the local depth of sea. Based on a comparison with recent deposits in the Black Sea, a gradual shallowing was deduced up the succession (Fig.74). It was presumed that the topographically subdued land masses which bordered the Liassic sea were subjected to deep chemical weathering in a warm humid climate. Ironstone was formed in small, shallow restricted arms of the sea only after the cessation of mechanical erosion at an advanced stage of peneplanation. Certain aspects of Hemingway's hypothesis may be criticised, for instance the dubious assumption that sand and clay were derived from different sources, and the claim that each of these marine cycles corresponds with a cycle of erosion on the land. Nevertheless the general interpretation in terms of changing depth of sea in a special chemical environment seems reasonable. It is interesting in this connection that a
PHOSPHORITE-BEARING CYCLES
191
cycle very similar to that in the Toarcian of Yorkshire is present in rocks of the same and MAUBEUGE, 1943), while the Domerian age in northeastern France (THEOBALD cycle and the major change in depth of sea between Domerian and Toarcian is recognisable over a much wider area than Yorkshire (HALLAM, 1961). Further examples of cyclic sedimentation involving ironstones were described by REID (1965) from the Precambrian of Yampi Sound, Western Australia. Two types of cyclic variation are distinguishable. In individual beds, often less than 3 cm thick, there is a textural gradation from a predominance of haematite at the base to a predominance of quartz at the top. Reid compared this with graded bedding, with the difference that the variation here is expressed by the specific gravities of two different minerals rather than grain size. In addition to this there is a regularly repeated cycle, from about 10 to 40 m thick, of haematite-rich, schistose and quartzose beds. Within a succession of 17 such cycles a few lithological phases are missing locally but no phase occurs out of position. The Yampi iron ores were considered to be clastic sediments but the origin of the cyclic sequence was not discussed.
PHOSPHORITE-BEARING CYCLES
Although many sedimentary cycles deposited in shelf seas contain layers of phosphatic nodules we are concerned here with deposits containing thick beds of economically exploitable phosphorite. One of the best-known sequences of such deposits is the Permian Phosphoria Formation of western Wyoming and neighbouring states. SHELDON (1963) has recognised two major cycles in these deposits, ranging from about 20 to 60 m in thickness. The idealised (composite?) sequence, not completely present at any one locality, is as follows: The underlying sequence in reverse order, i.e., 1 1 passing up to 1. (11) Dark carbonaceous mudstone. (10) Dark pelletal phosphorite. (9) Dark dolomite. (8) Light-coloured bioclastic phosphorite. (7) Bedded chert. (6) Nodular or tubular chert. (5) Interbedded light-coloured bioclastic limestone and calcareous sandstone. ( 4 ) Light-coloured dolomite and dolomitic sandstone. (3) Light-coloured mudstone. (2) Red beds. ( I ) Conglomerate overlying erosion surface. These various units were considered to be approximate time-rock horizons and represent transgressive and regressive phases of areally zoned depositional environ-
192
EPICONTINENTAL MARINE
ENVIRONMENTS, I1
ments. The erosion surface and conglomerate mark the phase of maximum regression, the dark mudstone maximum transgression. Lateral facies variations (Fig.75) support this interpretation, with land lying to the east and deep water to the west. Sheldon stated that the cycles tend to be skewed towards the base, with the upper half of the sequence being more fully developed. This suggested to Sheldon that the transgressions took place more rapidly than the regressions, but we may note that transgressive-regressive cycles are frequently asymmetrical in this way, with transgressive phases being marked by comparative condensation of the sedimentary sequence, and do not necessarily signify asymmetry intheunderlyingcontrol (see 178p.). Sheldon thought that the cycles, which extend over a very wide area, were the result of tectonically controlled fluctuations in depth of sea.
MINOR CYCLES WITH BITUMINOUS LAMINAE
A common feature of many marine bituminous shales, siltstones and limestones is a fine lamination consisting of layers of organic matter alternating regularly with mineral matter. BRADLEY (1931) seems to have been the first to suggest that the organicmineralic couplets corresponded to a year’s deposition and hence were true varves. Bradley studied several examples, from the Genesee Shale (Devonian) of New York, the Hannibal Shale (Mississippian) of Illinois, the Hartshorn Sandstone (Pennsylvanian) of Oklahoma and the Modelo Formation (Miocene) of California. The mean varve thickness ranged from 0.025 mm in the Modelo Formation to 0.40 mm in the Genesee Shale. Unlike glacial lake varves the mineralic layers showed no signs of graded bedding, hence rendering unlikely an origin by selective settlement from suspension in the sea water. The dark organic layers were thought by Bradley to represent autumn or winter settlement of dead plankton, such as diatoms and dinoflagellates, following intensive summer growth in the surface waters, a phenomenon that has been observed in certain large lakes. It was thought unlikely that successional phases of organic productivity in spring and summer would give more than one sharply defined layer each year. In fact it is now known that there may be several blooms of diatoms each year, with corresponding deposition more than once in the year, but the supernumerary layers 1953). can usually be recognised as such (DEEVEY, Direct support for Bradley’s principal contention, that the couplets are annual, comes from work on recent deposits. The deposition of sediment in the Clyde Estuary in Scotland was studied over a period of several years by H. B. MOORE(1931), who was able to show that each year correlated with the formation of a thin, peaty layer not more than 2 mm and a light band from 3 to 7 mm thick. Apparently the silica of the diatoms, composing the bulk of the organic matter, was quickly dissolved, so that all trace of organised structures disappeared at an early stage. This observation may heIp to account for the evident lack of structure in similar fossil deposits. Further confirmation comes from the work of SEIBOLD (1958) on euxinic sediments in an isolat-
193
MINOR CYCLES WITH BITUMINOUS LAMINAE Phosphatic Shale
Chert
Carbonate Rock
Light-
BJE . ..l co o r e d Mudstone
,::\\\\\
Red
Precycle
*
W Southeastern Idaho
Central Wyorni n 9
I
A PERIOD OF INITIAL SEDIMENTATION OF CYCLE AND OF MAXIMUM REGRESSION
B
C PERIOD OF MAXIMUM TRANSGRESSION
D PERIOD OF MAXIMUM REGRESSION
Fig.75. Interpretation of the formation of major cycles involving phosphates in the Phosphoria Formation of Wyoming. (After SHELDON,1963.)
194
EPICONTINENTAL MARINE ENVIRONMENTS, I1
ed bay in the Adriatic. He recognised fine laminations in the deepest part of the bay, with laminae averaging about 0.25 mm. These could be proved by historical events and seasonal variations to be annual deposits. Seibold considered that the light layers were deposited in summer and the dark layers, rich in organic matter and iron sulphide, throughout the remainder of the year. Two further examples of laminated recent sediments attributed to an annual cycleare the black euxinic muds in the Black Sea (ARCHANGELSKY, 1927) and diatomrich clays in the Santa Barbara Basin off southern California, also deposited in conditions of oxygen deficiency (HULSEMANN and EMERY,1961). Taken together with the evidence from similar deposits in lacustrine environments (NIPKOW,1928) there seems to be good empirical support for Bradley’s ideas, even though there is no general agreement as yet on the exact time of deposition of the organic-rich layers. Evidently the possibility of more than one phytoplankton flowering in the year does not seriously distort this picture. Recently, in a study of diatom-rich varved sediments from the central Gulf of California, CALVERT(1966) has produced evidence to suggest a radically different mode of origin. While phytoplankton production is reasonably constant throughout the year, river discharge varies greatly in different seasons. The varves in this case are therefore attributable to an increased sedimentation rate as a result of summer floods. The term varve may be ascribed with confidence to the type of minor cycle under discussion. On the other hand, indiscriminate use of the term for laminated sediments of a variety of other types (e.g., KORN,1938) is to be discouraged. It seems highly probable that organic-mineralic varves can only form under anaerobic or near-anaerobic conditions. If oxygen is available in abundance in the bottom waters benth0ni.c organisms rapidly destroy the fine lamination and the organic matter is oxidised (cf. CALVERT, 1964). Though such varves may form at any depth they are most likely to be common where organic productivity is high, namely in shallow coastal waters and shelf seas or in areas of upwelling. A number of examples of lamination interpreted as fossil marine varve deposits has been described, in addition to those already mentioned. The oldest so far recorded occur in the Precambrian Nama Limestone in South West Africa (KORNand MARTIN,1951). Fine lamination in the limestone is composed of couplets of lightishcoloured calcite bands and organic-rich quartz bands. Calcite precipitation was evidently the principal control on variations in thickness. Bituminous shales occurring in the Blue Lias of southern England are seen in thin section to consist of alternations of layers of dark reddish-brown structureless organic matter ranging from 9 to 17p in thickness, and clay-calcite layers from 16 to 25p thick (HALLAM, 1960). Some of the intervening limestones are also laminated, with individual laminae being much thicker (average 0.23 mm) because of the greater abundance of calcite. North American examples include, besides those cited by Bradley, laminae from 0.1 to 0.2 mm thick in the Miocene Monterey Formation of California (BRAML E m , 1946) and laminae in the Upper Cretaceous Beds of the Black Hills region
195
MINOR CYCLES WITH BITUMINOUS LAMINAE
THE VARVED CLAStlC-ORGANIC-EVAPORITE ANNUAL CYCLES
LAMlNATlONS (diaqrommatic)
STRATIGRAPHIC SECTION
CYCLE
FORMATION APPROXIMATE CLIMATE TIME REQUIRED FOR DEPOSITION
I
COOL WARM OR
2- FOLD
CYCLE clartic and wsum
(erriron
ormation
OR
\
WET
DRY
I ndrtona
Fig.76. Minor cycles (varves?) in the Jurassic Todilto Formation of New Mexico. (After R. Y. ANand KIRKLAND, 1960.)
DERSON
(RUBEY,1930). Rubey actually distinguished three types of lamination, grading into each other and averaging about 0.2 mm in thickness, only one of which showed a variation in content of organic matter. The others consisted of alternations of calcite and quartz silt and of silt and clay. All three types were attributed to annual climatic cycles of unspecified type. R. Y. ANDERSON and KIRKLAND (1960) have described limestone varves from the Jurassic Todilto Formation of New Mexico which have three components. The thickest consist of limestone bands variable in thickness but averaging 0.13 mm (Fig.76). They were considered to have been deposited in summer, in conditions of higher temperature, evaporation and/or photosynthetic activity. The organic layers, described as sapropel, are more constant and average 8,u in thickness. They contain subordinate
196
EPICONTINENTAL MARINE ENVIRONMENTS, I1
10-
5-
Fig.77. Amplitude spectrum of Upper Devonian Ireton Shale varves. (Adapted from R. Y. ANDERKOOPMANS, 1963.)
SON and
fragments of vascular plants and were regarded as autumn-winter layers resulting from plankton mortality. The third component is detrital quartz sand, intermittent but persistent areally. This is most probably a winter deposit, both wind- and streamborne. Varved deposits possess a special interest for those who hope to detect the past
MINOR CYCLES WITH BITUMINOUS LAMINAE
197
existence of solar cycles of varying magnitude. The eleven-year sunspot cycle is by far the most well-known (KORN,1938; R. Y. ANDERSON, 1961) but a number of others of greater period has been claimed from a variety of data. Sunspot cycles are supposed to affect the weather and hence ultimately the thickness of tree rings and varves. Thus increased precipitation of rain results in the thickening of the clastic component of organic-silt varves in certain Russian lakes (SHOSTAKOVICH, 1936). Temperature variations may affect the thickness of chemical precipitates. The subject of climatic cycles and their influence on sedimentation will be dealt with separately in the last chapter. We are concerned here only with the detection of thickness periodicity within varve sequences in non-evaporitic marine deposits. A number of attempts has been made to detect such periodicity in varves of all types by personal estimations of abnormal thickness, but these have been insufficiently objective to be convincing. Statistical techniques such as power spectrum or time series analysis are available for more rigorous analysis and should be used wherever possible in this type of study in an attempt to separate “noise” of a stochastic nature from genuine non-random “signals” (see Chapter 1). SEIBOLD and WIECERT(1960) undertook a type of sequential Fourier analysis of the Adriatic varves described by the first-named author in 1958 and recognised weak periods close to 6,8, 11 and 14 years throughout the sequence. R. Y. ANDERSON and KOOPMANS (1963) made power and amplitude spectrum analyses of several extensive varve sequences. They found, for instance, that varves in the Upper Devonian Ireton Shale of Alberta (R. Y. ANDERSON, 1961) registered a significant peak at 22 years with both techniques, while the amplitude spectrum analysis revealed lesser peaks at 1 1 + , 12+ , 8 + and 6 + years (Fig.77). R. Y . Anderson and Koopmans also analysed a varve sequence in the Nama Limestone, using data from KORNand MARTIN (1951). A strong long-term trend was not obvious, though a weak peak at about 100 years was present, overshadowed by stronger peaks near 25 and 12 years. A cluster of high peaks occurs at about 6-8 years, with other peaks at 3.7 and 2 years.
EPICONTINENTAL MARINE ENVIRONMENTS, I1
CYCLES WITH SIGNIFICANT QUANTITIES OF SANDSTONE, IRONSTONE AND PHOSPHORITE: MINOR CYCLES WITH BITUMINOUS LAMINAE
In this chapter we shall deal with a wider variety of lithological types than in the previous chapter. Generally speaking they are of subordinate importance to Iimestoneshale cycles.
CLAY-SANDSTONE-LIMESTONE CYCLES
The best-known cycles composed of sequences of argillaceous, arenaceous and calcareous deposits, such as those in the late Palaeozoic of northern England (Yoredale facies) and the mid-continent region of the United States, are dealt with fully in Chapters 4 and 5 , since they have generally been regarded as indicative of oscillating marine and continental environments. The few other examples discussed here may be dealt with most appropriately under the heading of epicontinental marine environments. In his classic work on the Jurassic System in Great Britain ARKELL (1933) observed that as long ago as 1822 Conybeare and Phillips had remarked on the regular manner in which clay is followed by sandstone and the latter by limestone in the socalled Oolitic sequence. Arkell went on to elaborate this tripartite scheme, dividing almost the whole of the Jurassic in southern England into major cycles. His scheme is given below, with the clay, sandstone and limestone units signified by capital letters: Portland Stone (L) Portland Sand (S) (9) Kimmeridge Clay (C) Westbury Ironshot Oolite (L) Sandsfoot Grit ( S ) (8) Sandsfoot Clay (C) Osmington Oolite, etc. (L) Bencliff Grit, etc. (S) (7) Nothe Clay, etc. (C)
184
EPICONTINENTAL MARINE
ENVIRONMENTS, I1
Berkshire Oolite (L) Lower Calcareous Grit (S) (6) Oxford Clay (C) Kellaways Rock (L) Kellaways Sand (S) (5) Kellaways Clay ( C ) Cornbrash Main Forest Marble Limestone (L) Hinton Sand, etc. (S) (4) Bradford Clay, etc. (C) Great Oolite Series (L) Stonesfield Slate beds ( S ) (3) Fuller's Earth (C) Inferior Oolite Series (L) Upper Lias Sands ( S ) (2) Upper Lias Clay (C) Marlstone Rock-bed (L) Middle Lias Sands (S) ( I ) Middle Lias Clay (C) Lower Lias clays, etc. It is easy to criticise aspects of this scheme as an example of cyclic sedimentation. As Arkell himself freely admitted, there was no question of clay, sandstone and limestone being deposited over the whole area simultaneously. No lithological details are given, distinguishing, for instance, different types of limestone. Distortion is involved equating major formations like the Oxford and Kimmeridge Clay with minor and local units like the Sandsfoot Clay; the Marlstone consists of sandstone and ironstwe besides limestone, and so on. Nevertheless one cannot disagree with Arkell that there is a striking overall periodicity involved, phases of widespread deposition in very shallow water (oolites, coral limestones, sandstones) being interrupted by phases of deeper water (clays and subsidiary argillaceous limestone). Arkell followed the Germans in attributing these oscillations to periodic epeirogenic movements of the sea floor. We may note, however, that the beginnings of cycles 2 , d (best taken together with the much thinner and less conspicuous cycle 5) and 9, all suggesting widespread and pronounced deepening of the sea, correspond with important worldwide transgressions and hence signify eustatic control (HALLAM, 1963~).The same may be true of others of Arkell's cycles.
CLAY-SANDSTONE CYCLES
185
Another example where the cycles are major stratigraphic units, ranging up to several hundred metres in thickness, comes from the Upper Pliocene and Pleistocene deposits of Wairarapa, New Zealand (VELLA,1963). Vella gave the following as a typical marine cycle, together with his interpretation: (Disconformity) shallowing culmination. Sandstone with shell beds shallowing. Sandy mudstone with scattered fossils deepening culmination. Sandstone with shell beds deepening. Coquina limestone deepening. (Disconformity) shallowing culmination. Vella attempted an estimation of depth of deposition based on known distribution of modern relatives of the contained fossils, and concluded that the cycles were expressions of depth changes ranging from about 50 to 160 m. These were in phase at least over half of North Island and, allowing for local tectonic disturbance, glacioeustatic control was proposed. To conclude this section an instance of a minor cyclic sequence will be cited from the Rhaetian in southwest England (HAMILTON, 1962). In the Cotham Beds (for example) four cycles can be distinguished, about a metre in thickness, with the following characteristics: ( I ) a basal calcareous horizon, above which there is usually a gradual reduction in the amount of carbonate, (2) shell or sand beds, or ripple lenticles, most common near the base of a cycle but which decrease in frequency upwards, and (3) an increase in the proportion of clay upwards in each cycle. Hamilton considered that the basal bed of each cycle was deposited under conditions of greatest current and/or wave action, with a gradual reduction of energy input from the environment as the cycle progressed. Such conditions were thought to be related possiblyto relatively small oscillations of the strand line. It is interesting to note, as Hamilton pointed out, that these minor cycles occur within a Rhaetian sequence which itself forms a good example of a cycle of Kliipfelian type.
CLAY-SANDSTONE CYCLES
As in the previous case, it is difficult to find many cited instances in the literature of clay-sandstone cycles in stable shelf regimes which do not involve non-marine deposits. Infact, both of the examples which will be considered here pass laterally into nonmarine beds within short distances. They are nevertheless most appropriately dealt with here. Major marine and non-marine units of the Eocene of the Anglo-Franco-Belgian Basin (STAMP, 1921) interfinger diachronously as indicated diagrammatically in Fig.73. The marine part of the sequence tends, in its fullest development, to show the sequence: basal gravel-sand-clay-sand-gravel. The gravel may be only poorly developed or absent, but the plane of erosion between the marine cycle and the underlying beds is almost invariably well marked. To give a specific example, the London Clay
186
EPICONTINENTAL MARINE
-.
2
I,
%I
LOWER
HEADON
BEDS
ENVIRONMENTS, I1
OLIGOCENE
1BARTON I AN LEDlAN LUTETIAN
BAGSHOT
'SANDS
7-7 YPRESIAN
READING,
BEDS
1
Fig.73. Major cycles in t& Lower Tertiary of the Hampshire Basin. Stippled areas represent marine beds, unornamented areas hon-marine beds. (After STAMP,1921.)
in the Hampshire Basin begins with thin sandy pebble beds and passes up into silty clay; the top is sandy once more. This succession is thinner and sandier in the west, where it is interrupted by non-marine beds. The thickness of the London Clay cycle varies from nearly 100 m in Whitecliff Bay to 70 m in Alum Bay. Thin intercalations of gravel within the clay were not regarded by Stamp as marking minor cycles. Stamp was able to demonstrate the essential contemporaneity of major cycles within the whole area considered. They were related to periodic transgressions and regressions of the sea, with the clays signifying deeper water conditions than the sands. Cycles very similar to those just described also occur in the Tertiary deposits of the Gulf Coast Region of the United States (BORNHAUSER, 1947; LOWMAN, 1949; FISHER, 1964; and Table XXIX). Gulfwards the deposits pass from a fluvial and lagoonal facies into an argillaceous marine facies. The cycles occur where marine clays are intercalated between marginal marine and continental arenaceous beds. Fisher (1964) has given a detailed account of cyclic deposits in the Eocene. His transgressive phase of a given cycle consists of sands and marls overlain by so-called restricted or marginal marine clays. These are laminated and have an apparently dwarfed fauna of thin-shelled bivalves. This phase overlies a basal condensed unit, marking a transgressive marine disconformity, rich in glauconite, phosphorite pebbles, reworked fragments and shark teeth. The inundutive phase (Bornhauser's term) marks the maximum advance of the sea and is characterised by normal argillaceous beds. Fisher's regressive phase has a basal arenaceous unit (fluvial or marginal marine) and an upper argillaceouscarbonaceous unit (mostly lagoonal or flood plain) in the northern Gulf Coast Region. These strata pass into entirely marine beds southeastwards. Fisher recognised five cycles in the Claibornian of Alabama but only one in the
IRONSTONE-BEARING CYCLES
187
Jacksonian (cf. Table XXIX). The thickness of the marine phases in this region rarely exceeds 30 m while the non-marine phases may be much thicker. While there appears to be general agreement that these Tertiary cycles in both Europe and North America result from the advance and retreat of the sea, no-one seems to have considered that such oscillations might have been the local expressions of eustatic movements until HALLAM (1963a) pointed out that the major transgressions and regressions in the two regions appeared to coincide in time, at least to the degree of refinement indicated by such age designations as end-Palaeoceneand Middle Eocene. kn this connection it is noteworthy that the basal transgressive beds appear to be condensed, giving an overall asymmetry to the cycles which compares with the Mesozoic limestone cycles discussed in the previous chapter. Another point of comparison is that the laminated shales, most likely anaerobic deposits, occur in the lower part of the cycle. IRONSTONE-BEARING CYCLES
One of the major problems in sedimentary geology concerns the origin of the banded ironstones or itabirites found in the Precambrian of every continent (GEIJER,1957; JONES,1963). Though most abundant in the Late Precambrian, where they compose formations up to 450 m in thickness (SAKAMOTO, 1950) they also occur less commonly in the Archaean and Palaeozoic. We are not concerned here, however, with the general problem of iron enrichment but with the nature and origin of the cyclic alternations between chert on the one hand and one or more iron-bearing minerals (silicates, carbonates and oxides) on the other. Specific examples will be taken from the famous deposits of banded ironstone in the Huronian of the Lake Superior region in North America, considered to have been laid down in a restricted marine environment (JAMES,1954). One common type of banding consists of regular alternations of chert and siderite averaging 1 cm in thickness. Stylolites are common and slump structures not rare. A more spectacular rock is the jaspilite of the Marquette Range, composed of alternations of reddish jasper and grey haematite typically 0.25-1.25 cm thick. These layers may themselves be laminated, with laminae about 0.025 mm thick. The bedding is wavy rather than straight and individual layers show pinch-and-swell structure. Parts of the succession are oolitic, both in the chert and haematite, though oolitic structure is more obscured in the latter lithology. A notable feature of the various types of banding is the remarkable constancy of individual horizons. James was primarily concerned with the general environment of deposition and the conditions governing the formation of various iron minerals and did not address himself to accounting for the banding. Sakamoto, however, dwelt on this subject at length in a general review of Precambrian ores (SAKAMOTO, 1950). He developed an ingenious hypothesis treating the banding as annual. A monsoonal climate was proposed, with alternating wet and dry seasons. During the wet season, it was argued, acid conditions would have prevailed in the weathering regime, allowing iron to migrate in
188
EPICONTINENTAL MARINE ENVIRONMENTS, I1
TABLE XXIX MAJOR CYCLES IN THE TERTIARY OF THE GULF COAST OF THE U.S.A.
(After FISHBR, 1964) Lithology
Sand, shale, lignite -.
Group
Formation Mississippi
Louisiana
Catahoula
+ post-Catahoula
Cycle Phase
Pliocene
Regression
-.
?
?
Miocene
Inundation
Chickasawhay L. Byram Marl Marianna L. Forest Hill/ Red Bluff
Vicksburg
Vicksburg (Oligocene)
Transgression
Grey shale, marl
Yazoo
Jackson Shale
Jackson
Inundation
Marl, glauconitic sand
Moody’s Branch
Moody’s Branch (Eocene)
Transgression
Sand, grey shale, lignite
Cockfield
Cockfield
Regression
Dark shale
Limestone, marl, shale
V
VI
-
Dark shale
Cook Mountain -
Limestone, shale, sand
(Wantubee)
Inundation
Cook Mt. Shale Clayborne _ _ _ ~ (Eocene) Cook Mt. L.
_
Transgression
Regression
Sand, shale, lignite Sparta (Kosciuska) Sparta Dark shale
Zilpha
111
Inundation
Cane River Shale
I1
~~
Glauconite sand, marl, shale, glauconite sand
Sand, shale, lignite Wilcox Dark shale
Porter’s Creek
Marl, calc. shale
Clayton
UPPER CRETACEOUS
Transgression
Winona Tallahatta Cane River Marl
Wilcox
Wilcox (Eocene) Regression
Midway
Midway
Inundation Transgression
I
IRONSTONE-BEARING CYCLES
189
solution, whereas silica would remain behind as insoluble material. Conditions would (1950) considered that the banded have been reversed in the dry season. SAKAMOTO ores were deposited in lakes. Iron hydroxide would have been precipitated in alkaline waters in the dry season, with the pH varying from 9 to 5. JONES(1963) has also proposed that the primary control involved was pH (in contrast to the Eh control of different iron minerals, as James argued). Sakamoto thought that banded ironstones were formed only during a period in the Precambrian under conditions which have never recurred since. A major difficulty in Sakamoto’s interesting hypothesis is the assumption of striking changes in pH at frequent and regular intervals. They would be remarkable even in lakes, but it is highly questionable whether the Precambrian banded ironstones formed in lacustrine environments. JAMES (1954, p.243) has argued cogently for marine conditions in the case of the Lake Superior ores, and the wide lateral extent of such deposits in different parts of the world is more readily explicable on this assumption. It is well-known that sea water is a well-buffered chemical system, with pH varying only between narrow limits (about pH 7.6-8.1). The problem therefore remains. Perhaps the possibility of a diagenetic control of the banding should be explored, as in the case of some small-scale cycles in limestone, shale and chert (see Chapter 6). KREJCI-GRAF (1964, p.485) has argued, for instance, that the cause of the banding might have been processes of solution and precipitation within the sediment due to periodic or episodic changes of Eh in connection with the rhythm of sedimentation of organic matter. There seem to be one or two pointers towards a hypothesis of diagenetic unmixing in James’ description of the Marquette jaspilites. The bands are themselves laminated, and where oolitic structure occurs it evidently ignores the banding, occurring in both lithologies. It is true that James argued for the primary origin of the chert, but the evidence he put forward (remarkable lateral constancy, stylolites and chert veinlets cutting bands, slump structures involving chert) is not decisive and the socalled primary chert could be accounted for by diagenetic migration fairly soon after deposition. A totally different type of cycle involving ironstone has been described from the Lias of Yorkshire, England, by HEMINGWAY (1951). Three major cycles were distinguished. The best developed corresponds with the Toarcian and is about 100 m thick. Following silty grey shales at the base come finely laminated dark brown bituminous shales of the Jet Rock. These pass gradually upwards into fine grey shales and thence via silty shales into fine-grained sandstone (Fig.74). At the top of the cycle in Rosedale are a few metres of sideritic chamosite oolite. Another cycle corresponds almost exactly with the Domerian, shales passing up via sandstone into shales with nodules of siderite mudstone and thence into chamosite oolite (Cleveland Ironstone). The third cycle is taken to embrace almost the whole of the Lower Lias, but in this case a regular sequence is not apparent and the relationships of the different lithological units are more complex than Hemingway assumed. Hemingway interpreted the cycles as resulting from tectonically-induced vertical
EPICONTINENTAL MARINE ENVIRONMENTS, I1
STRATA Rosedale Ironstone Blea Wyke Beds Striahitus Shales
SUB STAGES
Peak Shales
'TOARCIAN
Alum. Shales
The Hard Shales The Bituminous Shale; Jet Rock The Grey Shales
1
Cleveland Ironstone Series
DOMERIAN
The Sandy Series
Shales with ironstone concretions
CAR I X I A N
Shales with hard sandstones 5 I NEMUR I AN
Shales with occasional shell limestones
J R Iu1
5 0 0 feet omitted
Grey qrccn shale Black shale
-
HETTANGIAN
RHAETIC
Fig.74. Major cycles involving ironstones in the Liassic succession of Yorkshire, with interpretation of relative depth of deposition of the different facies. (After HEMINGWAY, 1951.)
movements altering the local depth of sea. Based on a comparison with recent deposits in the Black Sea, a gradual shallowing was deduced up the succession (Fig.74). It was presumed that the topographically subdued land masses which bordered the Liassic sea were subjected to deep chemical weathering in a warm humid climate. Ironstone was formed in small, shallow restricted arms of the sea only after the cessation of mechanical erosion at an advanced stage of peneplanation. Certain aspects of Hemingway's hypothesis may be criticised, for instance the dubious assumption that sand and clay were derived from different sources, and the claim that each of these marine cycles corresponds with a cycle of erosion on the land. Nevertheless the general interpretation in terms of changing depth of sea in a special chemical environment seems reasonable. It is interesting in this connection that a
PHOSPHORITE-BEARING CYCLES
191
cycle very similar to that in the Toarcian of Yorkshire is present in rocks of the same and MAUBEUGE, 1943), while the Domerian age in northeastern France (THEOBALD cycle and the major change in depth of sea between Domerian and Toarcian is recognisable over a much wider area than Yorkshire (HALLAM, 1961). Further examples of cyclic sedimentation involving ironstones were described by REID (1965) from the Precambrian of Yampi Sound, Western Australia. Two types of cyclic variation are distinguishable. In individual beds, often less than 3 cm thick, there is a textural gradation from a predominance of haematite at the base to a predominance of quartz at the top. Reid compared this with graded bedding, with the difference that the variation here is expressed by the specific gravities of two different minerals rather than grain size. In addition to this there is a regularly repeated cycle, from about 10 to 40 m thick, of haematite-rich, schistose and quartzose beds. Within a succession of 17 such cycles a few lithological phases are missing locally but no phase occurs out of position. The Yampi iron ores were considered to be clastic sediments but the origin of the cyclic sequence was not discussed.
PHOSPHORITE-BEARING CYCLES
Although many sedimentary cycles deposited in shelf seas contain layers of phosphatic nodules we are concerned here with deposits containing thick beds of economically exploitable phosphorite. One of the best-known sequences of such deposits is the Permian Phosphoria Formation of western Wyoming and neighbouring states. SHELDON (1963) has recognised two major cycles in these deposits, ranging from about 20 to 60 m in thickness. The idealised (composite?) sequence, not completely present at any one locality, is as follows: The underlying sequence in reverse order, i.e., 1 1 passing up to 1. (11) Dark carbonaceous mudstone. (10) Dark pelletal phosphorite. (9) Dark dolomite. (8) Light-coloured bioclastic phosphorite. (7) Bedded chert. (6) Nodular or tubular chert. (5) Interbedded light-coloured bioclastic limestone and calcareous sandstone. ( 4 ) Light-coloured dolomite and dolomitic sandstone. (3) Light-coloured mudstone. (2) Red beds. ( I ) Conglomerate overlying erosion surface. These various units were considered to be approximate time-rock horizons and represent transgressive and regressive phases of areally zoned depositional environ-
192
EPICONTINENTAL MARINE
ENVIRONMENTS, I1
ments. The erosion surface and conglomerate mark the phase of maximum regression, the dark mudstone maximum transgression. Lateral facies variations (Fig.75) support this interpretation, with land lying to the east and deep water to the west. Sheldon stated that the cycles tend to be skewed towards the base, with the upper half of the sequence being more fully developed. This suggested to Sheldon that the transgressions took place more rapidly than the regressions, but we may note that transgressive-regressive cycles are frequently asymmetrical in this way, with transgressive phases being marked by comparative condensation of the sedimentary sequence, and do not necessarily signify asymmetry intheunderlyingcontrol (see 178p.). Sheldon thought that the cycles, which extend over a very wide area, were the result of tectonically controlled fluctuations in depth of sea.
MINOR CYCLES WITH BITUMINOUS LAMINAE
A common feature of many marine bituminous shales, siltstones and limestones is a fine lamination consisting of layers of organic matter alternating regularly with mineral matter. BRADLEY (1931) seems to have been the first to suggest that the organicmineralic couplets corresponded to a year’s deposition and hence were true varves. Bradley studied several examples, from the Genesee Shale (Devonian) of New York, the Hannibal Shale (Mississippian) of Illinois, the Hartshorn Sandstone (Pennsylvanian) of Oklahoma and the Modelo Formation (Miocene) of California. The mean varve thickness ranged from 0.025 mm in the Modelo Formation to 0.40 mm in the Genesee Shale. Unlike glacial lake varves the mineralic layers showed no signs of graded bedding, hence rendering unlikely an origin by selective settlement from suspension in the sea water. The dark organic layers were thought by Bradley to represent autumn or winter settlement of dead plankton, such as diatoms and dinoflagellates, following intensive summer growth in the surface waters, a phenomenon that has been observed in certain large lakes. It was thought unlikely that successional phases of organic productivity in spring and summer would give more than one sharply defined layer each year. In fact it is now known that there may be several blooms of diatoms each year, with corresponding deposition more than once in the year, but the supernumerary layers 1953). can usually be recognised as such (DEEVEY, Direct support for Bradley’s principal contention, that the couplets are annual, comes from work on recent deposits. The deposition of sediment in the Clyde Estuary in Scotland was studied over a period of several years by H. B. MOORE(1931), who was able to show that each year correlated with the formation of a thin, peaty layer not more than 2 mm and a light band from 3 to 7 mm thick. Apparently the silica of the diatoms, composing the bulk of the organic matter, was quickly dissolved, so that all trace of organised structures disappeared at an early stage. This observation may heIp to account for the evident lack of structure in similar fossil deposits. Further confirmation comes from the work of SEIBOLD (1958) on euxinic sediments in an isolat-
193
MINOR CYCLES WITH BITUMINOUS LAMINAE Phosphatic Shale
Chert
Carbonate Rock
Light-
BJE . ..l co o r e d Mudstone
,::\\\\\
Red
Precycle
*
W Southeastern Idaho
Central Wyorni n 9
I
A PERIOD OF INITIAL SEDIMENTATION OF CYCLE AND OF MAXIMUM REGRESSION
B
C PERIOD OF MAXIMUM TRANSGRESSION
D PERIOD OF MAXIMUM REGRESSION
Fig.75. Interpretation of the formation of major cycles involving phosphates in the Phosphoria Formation of Wyoming. (After SHELDON,1963.)
194
EPICONTINENTAL MARINE ENVIRONMENTS, I1
ed bay in the Adriatic. He recognised fine laminations in the deepest part of the bay, with laminae averaging about 0.25 mm. These could be proved by historical events and seasonal variations to be annual deposits. Seibold considered that the light layers were deposited in summer and the dark layers, rich in organic matter and iron sulphide, throughout the remainder of the year. Two further examples of laminated recent sediments attributed to an annual cycleare the black euxinic muds in the Black Sea (ARCHANGELSKY, 1927) and diatomrich clays in the Santa Barbara Basin off southern California, also deposited in conditions of oxygen deficiency (HULSEMANN and EMERY,1961). Taken together with the evidence from similar deposits in lacustrine environments (NIPKOW,1928) there seems to be good empirical support for Bradley’s ideas, even though there is no general agreement as yet on the exact time of deposition of the organic-rich layers. Evidently the possibility of more than one phytoplankton flowering in the year does not seriously distort this picture. Recently, in a study of diatom-rich varved sediments from the central Gulf of California, CALVERT(1966) has produced evidence to suggest a radically different mode of origin. While phytoplankton production is reasonably constant throughout the year, river discharge varies greatly in different seasons. The varves in this case are therefore attributable to an increased sedimentation rate as a result of summer floods. The term varve may be ascribed with confidence to the type of minor cycle under discussion. On the other hand, indiscriminate use of the term for laminated sediments of a variety of other types (e.g., KORN,1938) is to be discouraged. It seems highly probable that organic-mineralic varves can only form under anaerobic or near-anaerobic conditions. If oxygen is available in abundance in the bottom waters benth0ni.c organisms rapidly destroy the fine lamination and the organic matter is oxidised (cf. CALVERT, 1964). Though such varves may form at any depth they are most likely to be common where organic productivity is high, namely in shallow coastal waters and shelf seas or in areas of upwelling. A number of examples of lamination interpreted as fossil marine varve deposits has been described, in addition to those already mentioned. The oldest so far recorded occur in the Precambrian Nama Limestone in South West Africa (KORNand MARTIN,1951). Fine lamination in the limestone is composed of couplets of lightishcoloured calcite bands and organic-rich quartz bands. Calcite precipitation was evidently the principal control on variations in thickness. Bituminous shales occurring in the Blue Lias of southern England are seen in thin section to consist of alternations of layers of dark reddish-brown structureless organic matter ranging from 9 to 17p in thickness, and clay-calcite layers from 16 to 25p thick (HALLAM, 1960). Some of the intervening limestones are also laminated, with individual laminae being much thicker (average 0.23 mm) because of the greater abundance of calcite. North American examples include, besides those cited by Bradley, laminae from 0.1 to 0.2 mm thick in the Miocene Monterey Formation of California (BRAML E m , 1946) and laminae in the Upper Cretaceous Beds of the Black Hills region
195
MINOR CYCLES WITH BITUMINOUS LAMINAE
THE VARVED CLAStlC-ORGANIC-EVAPORITE ANNUAL CYCLES
LAMlNATlONS (diaqrommatic)
STRATIGRAPHIC SECTION
CYCLE
FORMATION APPROXIMATE CLIMATE TIME REQUIRED FOR DEPOSITION
I
COOL WARM OR
2- FOLD
CYCLE clartic and wsum
(erriron
ormation
OR
\
WET
DRY
I ndrtona
Fig.76. Minor cycles (varves?) in the Jurassic Todilto Formation of New Mexico. (After R. Y. ANand KIRKLAND, 1960.)
DERSON
(RUBEY,1930). Rubey actually distinguished three types of lamination, grading into each other and averaging about 0.2 mm in thickness, only one of which showed a variation in content of organic matter. The others consisted of alternations of calcite and quartz silt and of silt and clay. All three types were attributed to annual climatic cycles of unspecified type. R. Y. ANDERSON and KIRKLAND (1960) have described limestone varves from the Jurassic Todilto Formation of New Mexico which have three components. The thickest consist of limestone bands variable in thickness but averaging 0.13 mm (Fig.76). They were considered to have been deposited in summer, in conditions of higher temperature, evaporation and/or photosynthetic activity. The organic layers, described as sapropel, are more constant and average 8,u in thickness. They contain subordinate
196
EPICONTINENTAL MARINE ENVIRONMENTS, I1
10-
5-
Fig.77. Amplitude spectrum of Upper Devonian Ireton Shale varves. (Adapted from R. Y. ANDERKOOPMANS, 1963.)
SON and
fragments of vascular plants and were regarded as autumn-winter layers resulting from plankton mortality. The third component is detrital quartz sand, intermittent but persistent areally. This is most probably a winter deposit, both wind- and streamborne. Varved deposits possess a special interest for those who hope to detect the past
MINOR CYCLES WITH BITUMINOUS LAMINAE
197
existence of solar cycles of varying magnitude. The eleven-year sunspot cycle is by far the most well-known (KORN,1938; R. Y. ANDERSON, 1961) but a number of others of greater period has been claimed from a variety of data. Sunspot cycles are supposed to affect the weather and hence ultimately the thickness of tree rings and varves. Thus increased precipitation of rain results in the thickening of the clastic component of organic-silt varves in certain Russian lakes (SHOSTAKOVICH, 1936). Temperature variations may affect the thickness of chemical precipitates. The subject of climatic cycles and their influence on sedimentation will be dealt with separately in the last chapter. We are concerned here only with the detection of thickness periodicity within varve sequences in non-evaporitic marine deposits. A number of attempts has been made to detect such periodicity in varves of all types by personal estimations of abnormal thickness, but these have been insufficiently objective to be convincing. Statistical techniques such as power spectrum or time series analysis are available for more rigorous analysis and should be used wherever possible in this type of study in an attempt to separate “noise” of a stochastic nature from genuine non-random “signals” (see Chapter 1). SEIBOLD and WIECERT(1960) undertook a type of sequential Fourier analysis of the Adriatic varves described by the first-named author in 1958 and recognised weak periods close to 6,8, 11 and 14 years throughout the sequence. R. Y. ANDERSON and KOOPMANS (1963) made power and amplitude spectrum analyses of several extensive varve sequences. They found, for instance, that varves in the Upper Devonian Ireton Shale of Alberta (R. Y. ANDERSON, 1961) registered a significant peak at 22 years with both techniques, while the amplitude spectrum analysis revealed lesser peaks at 1 1 + , 12+ , 8 + and 6 + years (Fig.77). R. Y . Anderson and Koopmans also analysed a varve sequence in the Nama Limestone, using data from KORNand MARTIN (1951). A strong long-term trend was not obvious, though a weak peak at about 100 years was present, overshadowed by stronger peaks near 25 and 12 years. A cluster of high peaks occurs at about 6-8 years, with other peaks at 3.7 and 2 years.