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Chapter 40. The Iperk sequence (Plio-Pleistocene) and its ostracod assemblages in the eastern Beaufort Sea

Chapter 40. The Iperk sequence (Plio-Pleistocene) and its ostracod assemblages in the eastern Beaufort Sea

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Nektoralik K-59









+N. lssungnak L-86


Tarsiut A-250

lssungnak 0-61


Kiggavik 14-43






1-Location map of the five wells and others in the Eastern Beaufort Sea. Inset map shows location of

this area.

periods of glaciation, part of the shelf was exposed, ice-scoured and depopulated. During warmer

periods the benthonic climatewascharacterised by annual swings in temperatureand salinity, and the

bottom was subject to ice gouging and rafting; such conditions become more extreme as the water

becomes shallower towards land. Bernard (1979) summarizes the probable early environment from

the biological point of view in a publication on bivalve molluscs in the Western Beaufort Sea. He

suggests that the fresh water outflow of the MacKenzie River acts as a barrier to migration of

benthonic species from east or west. It is not clear yet if this is true for ostracods and foraminifers;

records so far suggest that it is not.

The ostracod fauna from these wells was identified; it comprised some 36 taxa, 26 of them forming a typical marine shelf assemblage dominated by Heterocyprideis sorbyana, Paracyprideispseudopunctillata and Rabilimis septentrionalis. These are all tolerant of low salinity and range in depth

from a few to at least 200 metres.

This agrees with what McNeil has reported for the foraminifera1 assemblages of the Iperk

Sequence. However, in addition to the marine species, there were about ten ostracod taxa from

fresh water.

Once identified, the‘genera and species were grouped according to their known habitat preferences; this could be done with confidence because most are still living in the area. In addition,

Ostracod Assemblages in Eastern Beaufort Sea 535







K-59. (LEGEND:F = fresh water; R = reduced salinity and



depth; T = tolerant; M = marine; D = deep marine assemblages).

their ecology was discussed with W.M.Briggs, E.M.Brouwers, U.M.Grigg and R.M. Forester,

who are familiar with recent marine and fresh water benthonic ostracods from this region. The

material was easily divisible into four assemblages; three of these were marine, but the fourth,

presumably derived, was from fresh water.

In order to simplify the data, the commonest species from each assemblage were selected as

indicators of the presence of that assemblage. Any sample in which an indicator occurred was then

plotted on a graph showing the locality and depth (Table 1).

In such an environment one might expect to find that the commonest species are those which

are tolerant of the harsh and variable conditions inshore while also occurring in marine conditions

on the continental shelf. From there replacements would be available to repopulate inshore areas

where the fauna had been wiped out. In fact, the tolerant marine assemblage does contain the

largest number of both species and specimens. The species selected as indicators are still abundant

in modern benthonic samples (Briggs, Brouwers, Grigg, pers. comm.).

Ostracod Assemblages in Eastern Beaufort Sea 537

The indicator species are:

Heterocyprideis sorbyana (Jones, 1865)

Paracyprideis pseudopunctillata Swain, 1963

Rabilimis septentrionalis (Brady, 1866)

Cythererta teshekpukensis Swain, 1963

Sarsicytheridea bradii (Norman, 1865)

Of the other two marine assemblages, the inshore one contains few species, associated with

shallow depths and reduced salinity.

The indicator species are:

Cytheromorpha macchesneyi (Brady and Crosskey, 1871)

Pteroloxa cumuloidea Swain, 1963

The offshore marine assemblage is defined as that in which the species are associated with marine

salinities and sublittoral depths.

The indicator species are:

Cytheropteron spp.

Krithe glacialis (Brady, Crosskey and Robertson, 1874)

Rabilimis mirabilis (Brady, 1868)

Rabilimis paramirabilis (Swain, 1963)

Fresh water ostracods found in these wells were presumably derived from the adjacent land

drainage, especially the MacKenzie River system. All the genera represented were used as indicators

of the fresh water assemblage; these are:

Candona spp.

Cytherissa sp. (C. lacustris (Sars, 1863))

Zlyocypris sp. ( I . bradyi Sars, 1890)

Limnocythere spp.

The occurrences of these four assemblages, fresh water (F), inshore with reduced salinity (R),

tolerant (T), and marine or offshore (M), in the five wells are shown in Table 1. The distances

between the wells, from Adgo F-28 on the left, the most southerly, to Nektoralik K-59 on the right,

the most northerly, are represented at approximately their actual distance apart. Depths are represented in 100 foot steps below sea level.

The tolerant assemblage occurs in all five wells and is the most prominent component in Adgo

F-28. The marine assemblage increases in importance from Adgo F-28, where it is sparsely represented, to Tarsiut A-25 and Nektoralik K-59, in the latter of which it is the predominant component

of the ostracod fauna. Conversely, the fresh water assemblage is present between 600 and 2000

feet in the three inshore wells, occurs sparsely between 500 and 1400 feet in Tarsiut A-25, and is

absent from Nektoralik K-59, the furthest well from land.

PLATE1-Fig. 1. Candona ruwsoni Tressler, female left valve, GSC 68784, length 1220 pm, NetserkB-44, 1230 ft.

below K.B. Fig. 2. Cytherissa Iacustris (Sars), right valve, GSC 68785, length 860 pm,Netserk B-44, 1050 ft.

below K.B. Fig. 3. Limnocythere reticulatu Sharpe, right valve, GSC 68786, length 580 pm, Kiggavik A-43,

1246 ft. below K.B. Fig. 4. Ilyocypris bradyi Sars, right valve, GSC 68787, length 890 pm, Netserk B-44,

1230 ft. below K.B. Fig. 5. Cytheromorpha macchensneyi Brady and Crosskey, male carapace, right GSC

68788, length 580 pm, Netserk B-44, 470 ft. below K.B. Fig. 6. Pteroloxa curnuloidea Swain, male left valve,

GSC 68789, length 530 pm, Netserk B-44, 230 ft. below K.B. Fig. 7. Cytheretta teshekpukensis Swain,

right valve, GSC 68790, length 1110 pm, Netserk B-44, 590 ft. below K.B. Fig. 8. Heterocyprideissorbyana

(Jones), female right valve, GSC 68791, length 980 pm, Netserk B-44,590 ft. below K.B.

(The bars on the figures represent 100 microns).

Ostracod Assemblages in Eastern Beaufort Sea 539

The coastal assemblage is present in all five wells; it is not found abundantly anywhere, and is

least well represented in Nektoralik K-59.

Tarsiut A-25 is the most difficult well to interpret. Perhaps an unusual amount of reworking has

occurred in this area, for the occurrence of coastal and fresh water species is discontinuous.

However, at 2500-2600 feet Tarsiut A-25 contains a very few speciments of an outer shelf marine

assemblage (D in Table 1) characterised by Henryhowella sp. and Muellerinu ubyssicolu, which

may represent an older deposition. If so, the disconcerting appearance of a single valve of Henryhowelfa sp. at loo0 feet may be due to reworking; it is impossible to say. Another well in this

area, Kiggavik A-43, has a few specimens of assemblage D at 2800, 3100 and 3900 feet. None of

this assemblage has been found in Nektoralik K-59in spite of the depth of that well; if it does

represent an older fauna, its absence from Nektoralik K-59 might be explained by the much

greater thickness of the Iperk Sequence at that site.

Although the material is scanty and has been reworked, it seems that the depositional environment of the Iperk Sequence can still be deduced from the assemblages of ostracods found there. If

so, the distribution records of ostracods in the other wells from which samples have been obtained

should combine to enhance understanding of this period considerably.

The specimens illustrated in this paper are deposited in the collection of the Geological Survey

of Canada, Ottawa.


The ostracod material which formed the basis of this study was provided by Esso Resources

Canada Ltd., Institute of Sedimentary and PetroIeum Geology and Scott Geological Services, all

from Calgary, Alberta. The author was fortunate to discuss this material with W. M. Briggs, Jr.,

E. M. Brouwers, and R. M. Forester. D. H. McNeil provided information on the stratigraphy of

this area. U. M. Grigg read the manuscript.


1979. Bivalve mollusks of the western Beaufort Sea. Contrib. Sci. Natur. Hist. Mus. Los Angeles county,

313, 1-180.

DIETRICH, J.R., DIXON, J. and MCNEIL, D.H. 1985. Sequence analysis and nomenclature of Upper Cretaceous to Holocene strata in the Beaufort-MacKenzie Basin; In Current Research. Part A. Geol. Survey Canada, Paper 85-IA,


YOUNG, F.G. and MCNEIL, D.H. 1984. Cenozoic stratigraphy of the MacKenzie Delta, Northwest Territories. Geol.


Survey Canada, Bull. 336.

2-Fig. 1. ParacyprideispseudopunctillataSwain, male right valve, GSC 68792, length 880 pm. Netserk B-44,

350 ft. below K.B. Fig. 2. Rabilimis septentrionalis (Brady). female right valve, GSC 68793, length 1310 pm,

Netserk B-44.540ft. below K.B. Fig. 3. Sarsicytheridea bradii (Norman), female right valve, GSC 68794,

length 910 pm, Netserk B-44, 590 ft. below K.B. Fig. 4. Cytheropteron montrosiense Brady, Crosskey and

Robertson, right valve, GSC 68795, length 490pm. Netserk B-44, 350 ft. below K.B. Fig. 5. Krithe

glacialisBrady, Crosskey and Robertson, female left valve, GSC 68796,length 910 rcm, Nektoralik K-50, 1400

ft. below K.B. Fig. 6. Rabilimis mirabilis (Brady), carapace right, GSC 68797, length 1170 pm, Nektoralik

K-59,1770 ft. below K.B. Fig. 7. Rabilimis paramirabilis Swain, male carapace, right GSC 68798, length 1220

pm, Netserk B-44, 1650 ft. below K.B. Fig. 8. Henryhowella sp, left valve, GSC 68799, length 880 pm,

Kiggavik A-43, 2772-2788 ft. below K.B.

(The bars on the figures represent 100 microns.).





Cronin : What are the oldest occurrences of Heterocyprideis sorbyana and Cytheromorpha


Siddiqui: Heterocyprideis sorbyana is one of the commonest ostracods in the Iperk Sequence.

In several wells it occurs all the way down to the bottom at more than 3000 feet. The actual age of

the Sequence at this depth is not known, but is most likely to be Pliocene (Dietrich et al., 1985).

H. sorbyana is present from the fist appearance of this fauna.

Cytheromorpha macchesneyi only occurs infrequently, always in the top thousand feet in a few


Hazel: Do you not find Rabilimis paramirabilis occurring before the other Rabilimis in the see

tion? My report of it in the Recent off Alaska in 1967 was a clerical error. Its first and last occurrences are probably both good biostratigraphic data, although we don’t know what the relationship of these are to the true Pliocene-Pleistoceneboundary. Rabilimis paramirabilis may be the

ancestor of both the shallow water form R. septentrionalis and the deeper form. R. mirabilis.

Siddiqui: No,I do not find Rabilimis paramirabilis occurring before the other Rabilimis species

in the section. I have no evidence to suggest that it is ancestral to the other Rabilimis species; however, the Iperk Sequence is not perhaps the best formation for elucidating these relationships.

Plio-Pleistocene Ostracods from the Sogwipo

Formation, Cheju Island, Korea




Korea University, Seoul, Korea


As a unique uplifted outcrop in the Yellow Sea and northern East China Sea, the Sogwipo

Formation provides an unusual opportunity to study the palaeoceanographic history in this region

during late Cenozoic time. It measures about 54 metres in thickness under the overlying volcanic

complex and is characterised by near-shore, lagoon, barrier bar, offshore, tuffaceous sand, and

volcanic conglomerate facies in ascending order. From nine samples taken from the lower half of

the formation, 124 ostracod species belonging to 58 genera were recovered. The vertical occurrence

data show that the palaeoenvironment of the formation varied from bay to open sea, was strongly

influenced by a warm current and finally filled with volcanic sediments. The geological age of the

formation is tentatively regarded as Plio-Pleistocene.


The Sogwipo Formation, which is distributed in a narrow belt along the southern coast of

Sogwipo City, Cheju Island is one of the important fossiliferous outcrops in Korea (Text-fig. 1).

Cheju Island is located at the centre of the East Asian Continental Margin and at a point where

the Kuroshio Current bifurcates, one branch flowing northward into the eastern Yellow Sea

and the other through the Korean and Tsushima straits between the Korean Peninsula and Japan.

The palaeontological study of the formation is, therefore, of great value in understanding the

late Tertiary and Quaternary palaeoceanographical and tectonic history of the area.

Palaeontological studies of the formation have been made by several workers; in 1923, Yokoyama first reported Pliocene molluscan fossils from the formation. Haraguchi (1931) collected a

prolific fauna of molluscs, brachiopods, echinoids, corals and fish teeth and considered its age to be

Pleistocene. Recently, Kim (1972) recovered 72 species of benthonic and 18 species of planktonic

foraminifers and concluded that the formation was Pliocene in age. No systematic study on its

ostracod fauna has yet been made.

The purpose of the present study is firstly to record the fossil ostracod fauna of the formation

and secondly to deduce the palaeo-environment and geological age of the formation.

54 1




1-Map showing the location of Cheju Island and the Sogwipo Formation.


All of the samples used in this study were collected at the type locality in February, 1982 and in

February, 1985 by the writers. A stratigraphical section was measured and 9 samples containing

abundant ostracods were selected for the study (Text-fig. 2). In general 40-100 grams of dry material

were taken from each rock sample and processed for the ostracod analysis. The gasoline method of

Ferguson (1968) was modified by the writers and employed to disintegrate the samples. In washing,

a 60-mesh screen (250 pm opening) and a 230-mesh screen (63 pm opening) were used, Ostracod

species were identified from the published literature and from collections made from the sea floor

around the Korean peninsula. Species counts were made for each sample, but adult and juvenile

specimens were not differentiated.



The Sogwipo Formation is*exposedin a sea cliff, west of Sogwipo harbour. The extent of the

formation is restricted to a narrow 1-km long belt along the coast. It overlies a volcanic lava flow

and is overlain by a thick Quaternary basalt flow which forms the basement of Mt. Halla on the

volcanic island of Cheju-Do. Unfortunately, the lowermost part of the formation is hidden under

the sea. Thus the writers measured only the 50 metre thick section from sea level to the top of the

sedimentary beds.

The formation strikes N 10"W and dips 8" to the west. It consists mainly of fine to coarse,

light-grey sandstone but has occasional intercalations of siltstone and shale. Its characteristic

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Chapter 40. The Iperk sequence (Plio-Pleistocene) and its ostracod assemblages in the eastern Beaufort Sea

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