Absolute Water Depth Limits of Late Devonian Paleoecological Zones

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Absolute Water Depth Limits of Late Devonian Paleoeeologieal Zones

By ASHTON F. EMBRY, III, Calgary, and J. EDWARD KLOVAN, Calgary *)

With 10 figures

Zusammenfassung njkjEin verfeinertes Schema zur Klassifizierung von Riffkalksteinen, das die Bestandteile

fiber 2 mm (den konglomeratischen Anteil) und die Art der organischen Bindung bes- ser berficksichtigt, erm6glicht eine genauere Faziesbeschreibung von organischen Rif- fen. Die Klassifizierung wurde auf oberdevonische Rifle angewandt, die im nord- 6stlichen Tell der Banks Island, Canadian Arctic Archipelago, anstehen. Die Fazies- verteilung und -abfolge in einem der Rifle erm6glichte es, die absoluten Wassertiefen von drei wesentlichen oberdevonischen Zonen zu bestimmen. Korallen waren die vorherrschende Fauna in Wasser tiefer als 21 m; tabulare Stromatoporiden wuchsen zwischen 21m und 9m Wassertiefe; massive Stromatoporiden waren die vorherrschende Fauna zwischen 9 m und Meeresspiegel. Der wichtigste Faktor, der die Tiefen der Zonengrenzen bestimmte, war die Wellenenergie (normaler Wellentiefgang 9 m; Sturmwellentiefgang 21 m).

Abstract

A refined scheme o~ reefal limestone classification, which places more emphasis on the > 2 mm components (conglomeratic fraction) and on the mode of organic binding, allows for a more detailed facies description of organic buildups. The classification has been applied to Late Devonian organic buildups which outcrop on northeastern Banks Island, Canadian Arctic Archipelago. The distribution and sequences of facies in one organic buildup has led to the determination of absolute water depth limits of three major Late Devonian paleoecological zones. Corals were the dominant fauna below 70 feet (21 m.); tabular stromatoporoids flourished between 70 feet (21 m.) and 30 feet (9 m.) of water depth; massive stromatoporoids were the dominant fauna between 80 feet (9 m.) and sea level. The main controlling factor on the depth limits of the zones was wave energy (normal wave base, 80 feet [9 m.]; storm wave base, 70 feet [21 re.l).

R6sum~

Une classification sch6matique et d6taill6e des calcaires de r6cif sup~rieur insistant sur les constituents d'une grosseur ~ 2 mm. (fraction comglomeratique) et sur la mani~re avec laquelle les constituents ont 6t6 li6 spar des organismes permet une d6scription plus d6taill6e des facies d'6difices organiques. Elle a 6t6 appliqu6e ~ l'6tude de r6cifs qui affleurent dans la partie Nord-est des Banks Island dans l'archipel arctique canadien.

Lad distribution et la succession des facies dans un 6difice organique ont permit de pr6ciser les limites des profondeurs absolues d'eau de trois zones pal6o6cologiques principales du d6vonien sup6rieur. Des coraux formaient la faune pr6pond6rante au- dessous de 21m.; stromatop6roides tabulaires abondaient entre 2Ira. et 9m. de profondeur; des stromatoporoides massifs formaient la faune pr6pond6rante entre la surface de la mer et 9m. de profondeur. Le facteur principal fixant les limites de profondeur des zones pal6o6cologiques 6tait l'6nergie des vagues (base des vagues nor- males 9 m., base des vagues de temp~te 21 m.).

*) Authors' addresses: ASaTON F. EMBRY, III, Mobil Oil Canada Ltd., Calgary, Al- berta, Canada; J. EDWARD KLOVAN, The University of Calgary, Calgary, Alberta, Canada.

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A. F. EMBRY, J. E, KLOVAN - - Absolute Water Depth Limits of Late Devonian

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Introduction

Devonian organic buildups 1) are present in many parts of the world and the Canadian Arctic Archipelago is no exception. Numerous Late Devonian organic buildups are magnificently exposed in an area of 1500 square miles (3900 square kilometres) en northeastern Banks Island (Fig. 1). Organic buildups, along with inter-organic buildup strata, constitute a 200 foot (61 metre) limestone unit within a thick (3600 feet [1100 metres]) sequence of Upper Devonian elastic rocks. The stratigraphic nomenclature assigned to the strata (KLOvAN & EMBnY, 1971) is shown in Fig. 2. The sequence represents the development of a clastic wedge which built southward during Late Devonian time. Upper Devonian strata of Banks Island record the gradual change from marine shelf strata (Weatherall Formation) to near-shore strata (Hecla Bay Formation) culminating in coastal plane strata (Griper Bay Formation). The limestone unit, of Late Frasnian age, has been termed the Mercy Bay Member of the Weatherall Forrr~ation (EMBer & KLOVAN, 1971). It represents the development of a reef tract during a transgressive episode. Fig. 3 depicts the interpreted paleogeography at the time of deposition of the Mercy Bay Member.

The Mercy Bay Member contains a multitude of organic buildups which dis- play a marked variation in character in an east-west direction. Organic buildups along the eastern (seaward) margin of the reef tract are narrow, linear bioherms trending north-south. They are encased in younger quartz sandstones and shales which filled in the inter-reef areas after cessation of reef growth. To the west, organic buildups are more numerous, and consist of lower bioherms and upper biostromes. The lower bioherms trend east-west, and the inter-bioherm strata are penecontemporaneous, argillaceous limestones. The organic buildups along the western outcrop edge of the Mercy Bay Member are large bioherms oriented in a north-south direction. These bioherms exhibit a marked asymmetry re- presenting lateral eastward growth. All of the organic buildups show a vertical faunal zonation with corals and tabular stromatoporoids in the lower portion and massive stromatoporoids e) in the upper portion. Fig. 4 is a schematic east-

~) The term organic buildup is applied to any carbonate rock body which is com- posed predominantly of mega-fossils, regardless of the shape or origin of the rock body. Organic buildups are classified according to shape (bioherm or biostrome) and mode of origin (reef or bank).

2) The dividing line between tabular stromatoporoids and massive stromatoporoids was arbitrarily drawn at one inch (2.5 cm.) in thickness.

673

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Fig. 1. A. Canadiarl Arctic Archipelago. B. Banks Island showing location of Upper Devonian outcrop and study locality.

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Fig. 2. Stratigraphic Nomenclature -- northeastern Banks Island, N.W.T.

674

A. F. EMB~Y, J. E. KLOVAN - - Absolute Water Depth Limits of Late Devonian

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Reefal limestone classification

Before proceeding with the description and interpretation of the Mercy Bay Member outcrop, we must digress slightly and briefly describe a new scheme of nomenclature for reefal limestones. The classification was devised because it became apparent that other limestone classifications were inadequate for describing the diverse lithologies which occur in organic buildups. The proposed classification, illustrated in Fig. 5, is an expanded version of the excellent classification of DUNttAM (1962). Essentially all that has been done is to place more emphasis on the carbonate conglomerates and to subdivide the so-called boundstones.

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Fig. 5. Classification of limestones according to depositional texture.

Division into organically bound and non-bound rocks is still the primary basis of subdivision of limestones. In the non-bound group we have added one level in the hierarchy by using the ~ 2ram. component, that is the conglomeratic fraction, as a basis of subdivision. This results in six rock types: mudstone, wackestone, packstone, grainstone, floatstone, and rudstone. The first four are used exactly as defined by DtrNHAM (1962).

The terms floatstcne and rudstone have been coined for limestones which contain greater than ten per cent ~ 2 turn. component, that is the carbonate conglomerates. The need to recognize these two rock types is obvious because

2 mm. particles are the most important constituents for describing and inter- preting rocks of organic buildups. The difference between the two rock types is that the ~ 2 mm. particles form the supporting framework in a rudstone whereas in a floatstone the ~ 2 mm particles "float" in a finer grained matrix. Thus floatstone is the conglomeratic analogue of wackestone while rudstone corresponds to packstone and grainstone.

676

A. F. EMBRu J. E. KLOVAN - - Absolute Water Depth Limits of Late Devonian

Limestones which were organically bound at the time of deposition have been subdivided on the basis of the nature of the organic binding. Three rock types have been recognized: framestone, bindstone and bafflestone. Framestone contains in situ massive fossils which built a rigid, three dimensional framework at the time of deposition. Bindstone contains in situ laminar fossils which enerusted and bound the sediment during deposition. In bindstone the in situ fossils do not form a three dimensional framework as they do in framestone. Bafflestone contains in situ, stalk-shaped fossils which acted as sediment baffles. To identify one of these rock types, the geologist must decide if the fossil organisms bound the. sediment during deposition and if so, in what manner. If, for some reason, this latter decision cannot be made, we recommend that the term boundstone be used.

Modifiers such as particle type, further grain size qualification (Wentworth Scale), impurities and color can easily be added to the temainolo.gy of the basic classification.

The proposed limestone classification can be used in two ways. The different classes can be used both as rock names as well as textural modifiers for describing the matrix of a rock type. An example of the use of the classification is: thamnoporid floatstone with a fine-grained, skeletal, wackestone matrix. In this case floatstone is used as the rock name whereas wackestone is a textural modi- fier. Another example is: tabular stromatoporoid bindstone with a thamnoporid floatstone matrix with a fine-grained, skeletal waekestone matrix. In this case, where a boundstone is being described, the matrix of the rock has to be described on two scales: the > 2mm. particle size scale and the < 2ram. particle scale. This results in "the matrix having a matrix".

The proposed classification has been specifically designed to adequately describe what are normally coarse textured rocks. When the size of the rock specimen or exposure is not sufficient to show this texture, then it will be diffi- cult, if not impossible to properly identify the rock. Bit cuttings, for example, would not show the necessary criteria to permit identification of most of the new classes proposed here. It should be noted, however, that within the boundstones, there is no stipulation as to the size of the binding, baffling or frame- building organisms.

The classification may seem complicated at first but it has been found that it conveys a much more complete picture of the rock type than do other limestone classifications.

Outcrop, description

One of the best exposures of the Mercy Bay Member occurs on the valley walls of an unnamed river which flows northward into M'Glure Strait 10 miles (16 km.) east of Mercy Bay (Fig. 6). The locality is marked by a star on Fig. 1. The organic buildup consists of a lower bioherm, 110 feet (88 m.) thick, capped by two layers of biostrome, 100 feet (80 m.) thick. The inter-organic buildup strata are poorly exposed and outcrops are sparse.

The Mercy Bay Member is underlain by a fine-grained, argillaceous, quartz sandstone which contains scattered corals, brachiopods and crinoids. The initial biohelmal buildup is 15 feet (4.5 m.) thick and consists predominantly of Alveo-

677

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lites bindstone and disphyllid coral bafflestone both with a wackestone or mudstone matrix (Fig. 7, A). Tabular stromatoporoids, thamnoporid corals, braehiopods, crinoids and gastropods are also present within this unit. The central core is massive whereas the flanks are cludely bedded bindstone. Only in the lateral extension of the unit, which forms an extensive 2 foot (6m.) bed, is coral floatstone the dominant lithology.

The next unit in the bioherm consists of 40 feet (12 m.) of tabular stromatoporoid bindstone with a tharnnoporid floatstone matrix with a skeletal wackestone matrix (Fig. 7, B). The core again is massive. The flanks, which also consist of bindstone, are bedded with depositional dips up to 20 ~ This unit differs from the preceding one in that tabular stromatoporoids have replaced corals as the predominant fauna and the unit is more extensive having overstepped the coral unit a considerable distance, to the SOUth.

The upper portion of the bioherm consists of two units separated by a distinct break. The first unit is 30 feet (9 m.) thick, and has a massive core and bedded flank beds. The core consists of massive stromatoporoid frarnestone intermixed with stromatoporoid rudstone, both with a medium-grained, skeletal grainstone or packstene matrix (Fig. 7, C). In situ stromatoporoids are often laterally and vertically continuous over large

~78

Fig. 6. O~ttcrop of the Mercy Bay Member. The organic buildup is well exposed, and consists of a lower bioherm overlain by two layers of biostrome. Inter-bioherm strata are exposed to the left of the organic buildup. The size of the organic buildup can be appreciated by noting the man standing at the base of it (the black dot to

which the arrow points). View looking east.

Fig. 7. Major lithologies of the organic buildup. A. Alveolites bindstone and disphyllid coral bafflestone. One division on the pole is 1 foot (8 m.) B. Tabular stromatoporoid bindstone. C. Massive stromatoporoid framestone (pencil is 6 inches [15 cm.] long). D. Stromatoporoid rudstone. Bar is 1 era. E. Stromatoporoid float-

stone. Bar is 1 cm.

A. F. EMBRY, J , E. KLOVAN - - Absolute Water Depth Limits of Late Devonian

Fig. 7 679

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areas (50 square feet [4.5 square, metres]). Flank beds consist of stromatoporoid rudstone and floatstone both with a fine-grained skeletal packstone matrix (Fig. 7, D, E). The large stromatoporoid fragments become less abundant down dip, and the beds become argillaceous and contain thin shale partings. The flank beds eventually grade into argillaceous, skeletal wackestone and packstone of the inter-bioherm strata.

A one foot (.3 m.) thick recessive interval consisting of argillaceous, stromatoporoid and coral rudstone occurs at the top of the unit.

The next massive stromatoporoid unit, which is 25 feet (7.5 m.) thick, is litho- logically and faunally very similar to the underlying unit. However, this unit consists of two laterally separate buildups of massive stromatoporoids.

In summary, the lower, essentially continuous bioherm is 110 feet (33 m.) thick, 600 feet (183 m.) wide and of unknown length. It shows a marked, vertical faunal zonation beginning with corals, passing through tabular stromatoporoids and ending with massive stromatoporoids. The bioherm is asymmetrical; the north side is linear and slopes steeply upward to the south, the south side is irregular with the various units overstepping each other to the south.

Inter-bioherm strata, stratigraphically equivalent to the above described bio- herin, consist of horizontal beds of dark grey, argillaceous, very fine-grained, skeletal packstone and wackestone. Braehiopods and ostracods are the only mega- fossils in these strata. These strata are: for the most part contemporaneous with the bioherm but the uppermost strata are subsequent.

Above the bioherm is an areally extensive biostrome which lies on top of both the bioherm and inter-bioherm strata. The unit is 30 feet (9 m.) thick over the underlying bioherm compared with only 20 feet (6 m.) over the inter-bioherm strata. It is massive and consists of massive stromatoporoid framestone and stromatoporoid rudstone both with a skeletal grainstone or packstone matrix.

Overlying the biostrome is a 35 foot (10 m.) covered interval. The lithology of this interval is inferred to be similar to that of the inter-bioherm strata.

The capping unit of the organic buildup is a 40 foot (12 m.) thick biostrome. The unit is well bedded with massive stromatoporoid framestone and rudstone being the dominant lithologies. However, beds of coral and tabular stromatoporoid bindstone do occur in the middle portion of the unit. The Mercy Bay Member is overlain by a dark grey, marine shale.

Fig. 8 summarizes the distribution of the various lithologies which compose the Mercy Bay Member at this locality.

Depositional history

Interpretation of the depositional history of the Mercy Bay Member is based on the distribution and sequence of lithologies and faunas which have been described above. The interpretation has relied heavily on the well-established Late Devonian paleoecology model (LEcOMPTE, 1958; KLOVAN, 1964). This model is reviewed and discussed in the next section.

The following sequence of events summarizes the depositional history of the Mercy Bay Member at the described locality and is schematically illustrated in Fig. 9.

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Fig. 9. Depositional History of the Mercy Bay Member.

A. F. EMBRY, J. E. KLOVAN - - Absolute Water Depth Limits of Late Devonian

1. During a transgressive episode in the development of a clastic wedge, influx of terrigenous sediment onto a portion of the marine shelf became negligible. Coral growth became prolific and corals built a small biogenie bank in the relatively deep, quiet water (Fig. 9, A). The muddy nature of the matrix and bound aspect of the flank beds both suggest a relatively quiet water environment.

2. Rapid organic growth raised the coral bank into shallower, more agitated water where tabular stromatoporoids became the predominant fauna. The lack of detrital flank beds suggests, however, that turbulence was not high. Tabular stromatoporoids continued to build the biogenic bank both vertically and laterally (Fig. 9, B).

3. The biogenic bank continued its upward growth into shallower water until wave base was reached. At this point massive stromatoporoids colonized the upper surface of the biogenic bank and began building a wave-resistant reef. The reef grew upward until sea level was reached and a reef flat developed on the top of the reef. (The thin, recessive interval at the top of the first massive stromatoporoid unit is interpreted to be a reef fiat deposit.) Continual erosion and regeneration of the reef contributed skeletal detritus to the inter-reef area (Fig. 9, C).

4. A relative rise in sea level occurred, and massive stromatoporoids colonized the reef flat. Two separate centres of stromatoporoid growth were established and the two reefs grew up to sea level. Reef flats formed when the reefs reached the surface (Fig. 9, D).

5. Inter-reef areas continued to fill with skeletal detritus. Eventually the inter- reef areas became so shallow that reef growth ceased. The inter-reef areas continued to receive detritus until they were completely infilled resulting in an extensive, level surface mar, fled by skeletal debris. This conclusion is supported by the fact the next massive stromatoporoid unit extends laterally over the bio- herin and the inter-reef areas, thus illustrating the lack of topographic relief at the end of this depositional event (Fig. 9, E).

6. A rise in sea level created a high energy environment over the area. An areally extensive massive stromatoporoid reef formed and grew up to sea level (first biostrome) (Fig. 9, F).

7. Sea level rose again but massive stromatoporoids did not colonize the area. Instead the area received inter-reef sediment (covered interval). Eventually massive stromatoporoids colonized the area and built an extensive reef up to sea level (second biostrome) (Fig. 9, G).

8. An influx of terrigenous sediment, due to the seaward migration of the northern shoreline, occurred, and reef growth ceased despite any relative rise in sea level (Fig. 9, H).

The interpretation of the lower 85 feet of the Mercy Bay Member is based on the assumption that reef growth commenced in fairly deep water mad developed upward to the water surface. This implies a static sea level situation - - neither subsidence nor up-lift of the sea floor being invoked. The position of the Mercy Bay Member in the over-all stratigraphic sequence and other stratigraphie evidence indicate that this is a plausible assumption (KLovAN 8~: EMBRY, 1971).

683

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Absolute depth limits of Late Devonian paleoecologieal zones

Paleoecological studies of Late Devonian organic buildups (LEcOMPTE, 1958; KLOVAN, 1964) have established a fatmal zonation model. This model forms the basis of the interpreted depositional history presented above.

Three major paleoecological zones are recognized in the model: 1. An underturbulent or quiet water zone which receives a minimum amount

of wave agitation and is located well below wave base. This zone is characterized by corals (Alveolites, disphyllids and thanmoporids).

2. A sub-turbulent or semi-rough zone, below average wave base but still within the reach of storm waves. This zone is characterized by tabular stromatoporoid s.

8. A turbulent or rough water zone which receives the maximum amount of wave agitation, is above wave base and is characterized by massive stromatoporoids.

Previous authors (LEcoMPTE, 1958; KLOVAN, 1964) established the relative water .depths of the paleoecological zones, but they did not relate the zones to absolute water depths. The interpreted depositional history of the 85 foot thick, biohermal portion of the Mercy Bay Member (Events 1--8; Fig. 9, A--C) leads to an interpretation of the absolute water depths of the three paleoecological zones. If, as icostulated, this biohermal part of the organic buildup was built upward from the sea floor to the water surface during a time of static sea level, it is possible to calculate water depth limits for the three paleoecological zones from the thick- nesses of the lithological and faur.al units. The coral baffiestone and bindstone unit is 15 feet (4.5 m.) thick, the tabular stromatoporo~d bindstone unit is 40 feet (12 m.) thick, and the first massive stromatoporoid unit is 80 feet (9 m.) thick, for an aggregate thickness of 85 feet (26 m.)

From these thickness it is postulated that coral biogenic bank began to grow :in a water depth of 85 feet (26 m.) with the tabular stromatoporoids becoming the predominant fauna at a water depth of 70 feet (21 m.). Massive stromatoporoids replaced the tabular stromatoporoids at a depth of 80 feet (9 m.) and were predominant up to sea level.

It is interesting to compare these depth limits postulated for the Late Devonian fatmal communities with the depth limits established for the modem Caribbean reef building communities. LOaAN (1969) has recognized the following ecological zones for the Recent organic buildups on the Yucatan shelf.

1. A quiet-low energy zone occurring at a depth of water below 70 feet (21 m.) and characterized by an Agaricia-Montastrea community,

2. An agitated to quiet - - intermediate energy zone occurring between 70 (21 m.) and 80 feet (9m.) of water depth and characterized by a Diploria-Mon- tastrea-Porites community.

8. A wave agitated - - high energy zone occurring between 80 feet (9 m.) and sea level and characterized by an Acropora palmata community.

Logan demonstrated that the depth limits of the zones correlate with two thresholds of wave action; one at 80 feet (9 m.) (normal wave base) and the other at 70 feet (21 m.) (storm wave base).

684

A. F. EMBI tY , J. E. KLOVAN - - Absolute Water Depth Limits of Late Devonian

O ECOLOGICAL ZONES

E RECENT

P ORGANIC BUILDUPS

l H YUCATAN SHELF

~L ILOGAN 19691

ACROPORA PALMATA

COMMUNITY

'50' (~z) DIPLORIA -

MONTASTREA-

PORITES

COMMUNITY

AGARIC IA -

MONTASTREA-

COMMUNITY

ENVIRONMENT

WAVE AGITATED

HIGH ENERGY

NORMAL WAVE BASE

AGITATED TO QUIET

INTERMEDIATE

ENERGY

STORM WAVE BASE

QUIET

LOW ENERGY

STRUCTURE

BUILDING

POTENTIAL

WAVE RES ISTANT

REEF

B IOGENIC

BANK

B IOGENIC

BANK

PALEOECOLOGICALZONES

LATE DEVONIAN

ORGANIC BUILDUPS

BANKS ISLAND

MASSIVE

STROMATOPOROID

COMMUNITY

TABULAR

STROMATOPOROID

COMMUNITY

CORAL

COMMUNITY

O

E

P

T

H SL

30' 19.2 M.

70" 21.4 M.

Fig. 10. Comparison of Recent and Late Devonian Ecological Zones.

Fig. 10 compares the Recent and Late Devonian faunal communities. The similarities between the two are remarkable giving added strength to the proposed depth limits for the Late Devonian faunal communities.

Conclusions

A refined scheme of reefal limestone classification, which places more emphasis on the > 2 mm components (conglomeratic fraction) and on the mode of organic binding, has been designed. Rudstone and floatstone are limestones which contain more than 10yo > 2 mm component (limestone conglomerates). Organically bound limestones have been subdivided on the basis of the nature of the organic binding. Three rock types are recognized: framestone, bindstone and bafflestone.

The classification has been applied to a Late Devonian organic buildup out- cropping on northeastern Banks Island, Canadian Arctic Archipelago. The organic buildup consists of a lower bioherm, 110 feet (88 m.) thick, overlain by two layers of biostrome, 100 feet (80 m.) thick. The lower 85 feet (26 m.) of the bioherm record the vertical change from coral bindstone and bafflestone (15 feet [5 m.]), through tabular stromatoporoid bindstone (40 feet [2 m.]), to massive stromatoporoid framestone mad rudstone (80 feet [10 m.]). This facies sequence is interpreted to represent the upward growth of an organic buildup from the sea floor to sea level during a time of static sea level. This interpretation leads to the determination of absolute water depth limits of the three major Late Devonian paleoecological zones. Corals were the dominant fauna below 70 feet (21m.); tabular stromatoporoids flourished between 70 feet (21 m.) and 80 feet (9m.); massive stromatoporoids were the dominant fauna between 80 feet (9 m.) and sea level. The main controlling factor on the depth limits probably was wave

44 Geologische Rundschau, Bd. 61 61}5

Aufs~itze

energy (normal wave base - - 80 feet [gin.l; storm wave base - - 70 feet [21 re.l). A comparison of these postulated depth limits with the established depth limits of modem Caribbean reef building communities supports the above conclusions.

Bibliography

DUNItAM, R. J.: Classification of Carbonate Rocks according to Depositional Texture. - - In: Ham, W.E., ed., Classification of Carbonate Rocks - - a symposium: Am. Assoc. Petroleum Geologists Mem. 1, 108--121, Tulsa 1962.

EMBrtY, A.F., & KLOVAN, J.E.: A Late Devonian Reef Tract on Northeastern Banks Island, N.W.T. - - Bull. Can. Petroleum Geology, 19, 4, Calgary 1971.

KLOVAN, J.E.: Facies analysis of Redwater reef complex, Alberta, Canada. - - Bull. Can. Petroleum Geology, 12, 1, 1--1O0, Calgary 1964.

KLOVAN, J.E., & EMBRY, A.F.: Upper Devonian Stratigraphy No~heastern Banks Is- land, N.W.T. - - Bull. Can. Petroleum Geology, 19, 4, Calgary 1971.

LECOMPTE, M.: Los recifs Paleozoiques en Belgique. - - Geol. Rdsch., 47, 1, 884--401, Stuttgart 1958.

LOGAN, B.W.: Carbonate Sediments and Reefs, Yucatan Shelf, Mexico. - - In: Logan, B.W., and MeBirney, A., eds. Yucatan- Bonaeca: Am. Assoc. Petroleum Geolo- gists Mere. 11, 129--198, Tulsa 1969.

Burial of Reefs by Shallow-water Carbonates, Silurian Gower Formation, Iowa, U.S.A.

By M. E. PmLcox, Liverpool *)

With 9 figures, 9. tables

Zusammenfassung

Die Sedimente der silurischen Gower-Formation im Staate Iowa wurden in zwei Phasen abgelagert. W~ihrend der ersten entwickelte sich in Krinoiden-Coelenteraten Riffkomplexe aus vielen nab benachbarten Kuppen ein Relief in urspriinglich 80 m Wassertiefe. Die Riffkomplexe sind asymmetrisch: die gut definierte Randzone in Windrichtung hat ein ausgepr~igtes Detailrelief; hinter einer zentralen Erhebung erstrecken sich die weniger steilen und ausgedehnteren Hicher der Lee-Zone. Soweit korreherbar, bestehen die riflemen Sedimente aus relativ feinem skeletalem Dolomit.

Die Ablagerungen der zweiten Phase repdisentieren eine Senkung des Meeresspie- gels; die topographisch tieferen Teile des vorhandenen Reliefs werden mit Sedimenten aus immer geringerer Wassertiefe gefiillt, haupts~ichlich mit laminierten Karbonat- schlammen und Feinsanden (Brady/Anamosa-Fazies-Gruppe). In der Randzone wurden sie zun~ichst als steile keilf6rmige Schichten auf Kuppenh~ingen abgelagert (Brady/Fa- zies); diese enthalten eine charakteristische Fauna und einzelne Stromatolithe. Die Kup- pen verbreiterten sich entsprechen zu Plateaus. Fossilarme, flacher liegende Schichten (laminierte Anamosa-Fazies) ffillten dann die Restsenken zwischen den Kuppen. Die entsprechenden Ablagerungen der Lee-Zone gehSren fast ausschlieBlich zur letzteren Fazies, und rifferne Sedimente sind wahrscheinlich im allgemeinen ~ihnlich.

*) Author's address: M.E. PHILCOX, Geology Department, University of Liverpool, Great Britain.

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Absolute Water Depth Limits of Late Devonian Paleoecological Zones