On the ostracod fauna in early tertiary lake of the liaohe fault depression basin, China

ON THE OSTRACOD

F A U N A IN E A R L Y T E R T I A R Y

LAKE

OF THE L1AOHE FAULT DEPRESSION BASIN, CHINA

by SUN ZHENCHENG *

RZSUM~ On a d~couvert dans l'Oligoc&ne du bassin du Golfe Bohai, de nombreux ostracodes fossiles appartenant aux Cyprididae et Cytheridae. La plupart de ces esp~ces, peu r~pandues, ne sont pas connues ailleurs. Cette faung, qui a suscit~ de vifs int~r~ts, fait l'objet de suggestions et de discussions quant A son origine.

une des causes principales de l'end([micit, des ostracodes. Nous avons aussi remarqu~ que los ostracodes existant dans les lacs peu profonds et les 6tangs de Chine sont pour la plupart des esp~ces/~ large distribution g~ographique, A la diff6rence des ostracodes des lacs profonds.

Le bassin consid~r~ est situ* au nord-est du bassin du Golfe Bohai. Nos recherches sur sa faune et son environnement s~dimentaire ont montr~ des similitudes entre les ostracodes de ce bassin et ceux des lacs profonds. Nous pensons que la longue p~riode d'isolement ~cologique qui caract~rise les lacs profonds est

Certaines esp~ces caract~risant le troisi*me membre de la Formation de Shahejie, dam la boue sombre/~ facies de iac profond et turbide, diff&en! fondamentaiemcnt de ceiles trouv~es dans [e d~pOt d'eau peu profondr du deuxi~me membre de cette formation.

KEY-WORDS : OSTRACODA, LAKE, TERTIARY, CHINA. MO,'I'S-CL~S : OSTRACODES, LAC, TERTIA/RE, CHINE.

* Research Instituteof LiaoheOil Field, Geobios, M6m. special n ~ 8

p. 357-363, 7 fig.

Lyon, 1984

-- 358 -As a rift valley of continental origin, the Liaohe fault depression basin is located in the Liaoning province in northeastern China, lying between the JiaoLiao uplift and the Yanshan fold-belt, adjacent to the Liaodong Bay in the south. It forms the northeastern corner of the Bohai Bay Basin, with the NE trending Tan-Lu fault zone running across it. It is an elongate fault depression basin with alternating horsts and grabens controlled by the crust fault system.

In Paleogene, the region under study was subject to faulting and downsinking along the YingkouTongerpu and the Dawa-Taian faults that separated it into 3 depression, viz., the West Depression, the East Depression, and the Damingtun Depression (fig. 1), which had received the elastic deposits about 50007000 meters in thickness, with abundant fossils. The period was also marked with several phases of volcanic eruptions.

In the Oligocene strata, the main petrolium-bearing rock series of the Bohai Bay basin, an unique fossil fauna of Ostracoda comprising of 41 genera belonging respectively to families of Cyprididae and Cytheridae has been found. 424 species have already been identified and described, among which 415 are new and a great majority of them are endemic, especially those occurring in horizons from the 3rd member of the Shahejie Formation to the Dongying Formation are sharply different not only from the Oligocene ostracod species of the Zhungeer and Chaidamu basins but also from those of the less distant Jianghan and Subei basins etc. The question of their origin has arrested general interest. There are some who have proposed that they evolved from marine stocks. But he writer puts forward the idea that a deep-lake environment seems to have played the controlling role in the occurrence and development of this unique ostracod fauna.

Fig. 1 - -

Location Map of the Liaohe Fault Depression.

THE STAGES OF DEVELOPMENT OF THE BASIN AND THE DISTRIBUTION OF THE OSTRACOD FAUNA 1 - STAGE OF TENSION F A U L T I N G In Paleogene, under the tensile force which produced tension faults, gradual and unbalanced subsidence of crust and expansion of water bodies took place in the basin, resulting in the uneven infilling of deposits. The Eocene Eucypris wutuensis assemblage in the Kondgian Formation and the Upper Eocene to Lower Oligocene Cyprinotus igneus assemblage in the middle and lower parts of the 4th member of the Shahejie Formation can only be found in the western and southern parts of the Bohai Bay basin, including

Eucypris, Limnocythere, Metacypris, Cyclopris, Cyprinotus, Cypris etc., all of them are of common occurrence in continental shallow waters. Lower Oligocene deposits represented by the upper part of the 4th member of the Shahejie Formation are more widespread. On the western slope of the West Depression of the basin, there are grey and greenish grey mudstone, calcareous mudstone and dolomitic

limestone intercalated with sandstones, conglomerates and a small amount of oolitic limestones. Fossil ostracodes of shallow water origin fauna therein comprise the Austrocypris levis assemblage with abundant

Austrocypris, llyocypris, Cyprinotus, Candona, Limnocythere, Cyprois, Cypris and charophytes etc.,

which is quite similar to that occurring in the Oligocene strata of the Chaidamu basin in west China. As Austrocypris levis BOJIE is a predominant member in Oligocene ostracod fauna in many Cenozoic inland basins of west China, there is no way to prove the hypothesis that Austrocypris is of marine origin. Moreover, most of the ostracodes contained in the 4th member of the Shahejie Formation still have their extant forms in the present ponds and paddy fields in Liaoning and also in other nonmarine water bodies elsewhere in China. 2. STAGE OF DEEP SUBSIDENCE

The tectonic movement at the time of deposition of the 3rd member of the Shahejie Formation was

- - 359

primarily a vertical movement sliding along the tension fault plane. Sharp settling of the basin was accompanied by the deepening and expansion of its water bodies. A very thick sequence of dark grey and brownish grey mudstones and a large suits of'massive conglomerates were deposited in the West Depression, with visible graded bedding, being the deep water turbidites deposited in non-compensating lake basins under deep-fault tectonic environment. At the time of deposition of the 3rd member of the Shahejie Formation, Austrocypris levis BOJIE which flourished at the close of deposition of the 4th member of the Shahejie Formation had already been extinct, Ilyocypris and charophytes had mostly disappeared, Cyprinotus had receded to live in marginal shallow water on the western slope, Huabeinia and Fusocandona predominated the region, and in the deeper water, Candona of the Baikalian type preponderated over the others (fig. 2). In the later period of depositioN

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moides and other shallow water taxa in increasing abundance in the northeasterly direction. In addition, a small amount of Camarocypris rectispinata BoJm and C. ovata commonly occurring in such shallow water deposits as oolithic limestones and calcareous sandstones etc. has been found. The depositional environment was widely different from that of the West Depression, while the difference of ostracod populations between the two depressions is also conspicuous. A

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Biofacies m a p of E3s 3 in Liaohe Fault Depression.

of the 3rd member of the Shahejie Formation, a part of the western slope uplifted. During the middle and later periods of deposition of this horizon, the water bodies in the East Depression and the Damingtun Depression became shallower, in which interbeds of sandstones, conglomerates and grey mudstones intercalated with more carbonaceous mudstones were deposited. Swampy environment prevailed, and fossils are scanty. Here, the Fusocandona and Candona of the Baikalian type disappeared earlier than those in the West Depression, and Huabeinia suddenly decreased in number or totally disappeared, and in their place came with Cyprois palustris BOJIIE, Ilyocypris, elongated Candona as well as charophytes, Toluto-

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Biofacies area of E3s 3 of Qijia-Shuangtaizi in West Depression.

Four areas of ostracod distribution are proposed for the middle-lower parts of the 3rd member of the Shahejie Formation (fig. 3) : (1) area of Cyprinotus Huabeinia; (2) area of Huabeinia;(3) area of Huabeinia-Fusocandona ; (4) area with scanty ostracod fossils, except a small amount of Fusocandona, Huabeinia and Candona. Analyses of oil source indices reveal that areas (2), (3) and (4) are the main oil sources of the region. On the whole, this tallies with the change of thickness of dark mudstones in the region, and obviously the distribution of ostracodes was controlled by water depth. Candona of the Baikalian type is largely distributed in areas (2) and (3), also in members E and C or even in D of the Bouma sequence of turbidites. For instance, abundant and well-preserved Candona posticancava B O J I E , C. dorsicurta B O J I E , C. minitrapezoidea BOJIE, accompanied by Huabeinia and Fusocandona are found in the thin intercalations of mudstone or arenaceous mudstones, several cm to dozens of cm in thickness, above the member A (with graded bedding) in drilling well Jing n ~ 2-8-011. Abundant ostracodes have also been reported from the mudstone intercalations in the deltaic sandstones and conglomerates underlying the 3rd member of the Shahejie Formation around drilling well Qi n ~ 45 on the western slope, mostly elongated Candona and Cyprinotus, with very few Huabeinia. In the Gaosheng trough where the lake was relatively deep, and turbidite sandstone bodies filled in the

360 trough valley between two opposite faults, fossil remains are scarce, only some Huabeinia and a lot of well- preserved Fusocandona xinglongtaiensis BOjIE with thin shells of separated bivalves occur in the argillaceous siltstones of member C (with laminae), which is 2 cm thick and intercalated in the sandstones and conglomerates. They are instances of deep water ostracodes buried in situ on the deep lake bottom (fig. 4). B

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Fig. 4 -- Biofaciesarea of E3s3 of Gaoshengin WestDepression. (3) STAGEOF CONTRACTION The large scaled contraction of the basin took place during the time of deposition of the 2nd member of the Shahejie Formation. Lakes were only confined to the narrow region in the southern part of the West Depression where greyish massive sandstones and conglomerates intercalated with purple mudstones and a small amount of oolitic limestones were deposited, overlying unconformably, at places, the 3rd member of the Shahejie Formation. A noticeable change in the aspect of biota took place during the period from the deposition of the 3rd to that of the 2nd member of the Shahejie Formation. The Fusocandona, Candona of the Baikalian type, Huabeinia etc. that flourished during the time of deposition of the 3rd member completely disappeared or extincted and replaced by Camarocypris alliptica Bojm with thick, strong and smooth carapace during the time of deposition of the 2nd member. The later is common especially in the calcareous mudstones, sandstones and oolitic limestones, usually forming the nuclei of oolitoids, indicating a turbulent shallow water depositional environment. Besides, there: are large amounts of ostracodes with small and smooth carapace, like Pseudocandona, Potamocyprella, Phacocypris, Candona, Candoniella and the shallow water Cyproispalustris BOJIE. The gastropod Tulotomoides found in association with them is also an inhabitant in rivers or other turbulent shallow water bodies. This stage represents the typical developmental period of deltaic deposits. (4) STAGE OF EXPANSION The second water encroachment took place in the course of deposition of the 1st member of the Shahejie Formation during the further subsidence of the

basin. As a result, the lake was greatly extended as compared with the area it covered during the foregoing period. The small and smooth Phacocypris, Pseudocandona, Candoniella etc. of the family Cyprididae flourished wide spread in all the three depressions. With the increase of water depth, dark mudstones intercalated with oil shales were deposited in the center of the lake in the southern part of the basin, representing another oil source horizon. Xiyingia and Guangbeinia with larger carapace as well as Candona dorsalta BOJIE, C. sinensis BOJIE, C. directa BRONST. etc., and Candona of the Baikalian type were abundant in the deep and relatively deep waters in the southern Part of the basin, where Chinocythere flourished, the more so farther southward (fig. 5). In the 1st member of the Shahejie Formation in the western slope of the West Depression where oolitic limestones are well-developed, with the exception of a small amount of Gastropoda, there are plenty of Eucypris leligensis BOJIE, E. fida BOJIE and Limnocythere armata BOJIE, but Xiyingia and Candona of the Baikalian type are rare. Evidently, the distribution of fauna and flora there is controlled by water depth (Sun Zhencheng & Zhao Ou, 1981).

Fig. 5 -- Biofaciesmap of E3sI in Liaohc Fault Depression. On the other hand, in the Damingtun Depression and in the northern parts of the West Depression and East Depression where the water is shalllower, Eucypris, Limnocythere, Glenocypris, Candona of great diversity and abundance as well as some gastropods have been found in the grey sandstones with dark grey mudstone intercalations. Gastropods are particularly

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abundant in the 1st member of the Shahejie Formation in the whole Damingtun Depression in which llyocypris and charophytes and other shallow water organisms are also found (fig. 5). The lake. at that time was in its old age, with shallow water and rich nutrients.

(5)

STAGE OF REGRESSION

During the time of deposition of the Dongying Formation, the rate of subsidence of the basin gradually lowered, and the basin itself retreated southward continuously to its final extinction. When the 3rd member of the Dongying Formation was deposited, the lake basin retreated to the southern part of the West Depression on a large scale, resulting in the deposition of an interbed of greenish-grey mudstones and siltstones, intercalated with thin beds of calcareous mudstones. Fossil ostracodes that predominate here include such aboriginal species as Phacocypris lepida BOJIE, Dongyingia spongiformis BOJIE, D. dorsinodosa BOJ1E etc. which can only be found in the Liaohe fault depression, while Chinocythere unicuspidata BOJIE and other species that spread northward from the south of the Bohai Bay basin only reached the southwestern corner of the West Depression (fig. 6).

361

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water invaded the central and southern parts of the East and West Depressions, hut the invasion was much less extended than that occurring at the time of deposition of the 1st member of the Shahejie Formation. A sequence of greyish and green-grey mudstones intercalated with grey-white sandstones was deposited, in which ostracodes such as Phacocypris guangraoensis BOJIE and Chinocythere cornuta BOJ1E with small carapace were widespread while Chinocythere Iongispinata BOJIE, Dongyingia influxicostata BOJIE, D. labiaticostata BOJIE and D. biglobicostata BOJIE with large, nodulose and spinose carapace occupied in succession, the deeper parts of the lake, resulting in ring-like belts of distribution, generally in line with the changing trend of deposition shown, in the isopach (fig. 7). From the margin of the lake toward its center, the increasing size of ostracodes the growing complexity of their shell ornamentation and enlarging of the ventral flat in contact with the substratum suggest that ostracodes well-adapted to deeper bottom environments are not easily to get stuck in the mud.

Fig. 7 --

(6)

Fig. 6 - -

Biofacies map of E3d 3 in Liaohe Fault Depression.

Reappearance of such shallow water biota as charophytes and Ilyocypris and flourishing of gastropods are the other features of the time. At the time of deposition of the 2nd member of the Dongying Formation, the southern part of the central uplift was submerged, and the later transient northward advance of lake

Biofacies map of E3d 2 in Liaohe Fault Depression.

STAGE OF EXTINCTION

In the Neogene, the three above-mentioned local depressions of the basin became unified into a depression basin, with the Neogene strata predominantly of greyish-white sandstones and conglomerates lying unconformably over the older rocks. In the Quaternary, following the development of the Bohai Sea, resulting from connecting of the Bohai Bay basin with the open sea, three important transgressive strata were deposited, in which large amounts of shallowrater to littoral ostracodes and foraminifers have been found.

-- 362 - ORIGIN OF THE OSTRACOD FAUNAS IN HORIZONS FROM THE 3RD MEMBER OF THE SHAHEJIE FORMATION TO THE DONGYING FORMATION

Like the Bohai Bay basin, the ostracod fauna in the 3rd member of the Shahejie Formation are noticeably different from that in the 4th member in the Liaohe fault depression basin in having many taxa with nodulose and spinose carapace. Some geologists and paleontologists have hypothesized that they are of marine origin. In the opinion of the author, such a general supposition is not justified from the point of view of taxonomy, because a part of species with nodulose and spinose carapace belong to the family Cyprididae (viz., Huabeinia, Camarocypris, Tuozhuangia, Pseudocandona, Dongyingia) ; while in contrast, many of the ostracodes with this kind of ornamentation in modern seas belong to an entirely different family Cytheridae. Besides, the carapace of different marine genera of Cyprididae is quite smooth, or sometimes ornamented with indistinct network or pitted. Therefore, it is illfounded, in taxonomy, to suppose that these ostracod faunas came from the sea just because they bear strong ornamentation. As a matter of fact, the appearance of cypridids ornamented with nodes and spines in fresh water is nothing new. For instance, in the middle part of the carapace of Tuberocypris sp. 2 found in Middle Esmeralda Formation (Swain, Becker & Dickinson, 1971), a Tertiary nonmarine sequence in the state of Utah, U.S.A., a central spine and some small scattered nodes similar to those found on Huabeinia obscura BOJIE from the region under discussion have been noticed. Also, in the middle of both valves of Tuozhuangia alispinata BOJIE from the 3rd member of the Shahejie Formation in the Bohai Bay basin, there is a big spine protruding backward. The same shell ornamentation has been found on the existing ostracodes of continental facies, e.g., on Cypris bispinosa LUAS now living in the fresh to brackish water along the Mediterranean coast. The author has collected and studied a series of ostracod fossils from the Cenozoic continental deposits of fresh-saline water transitional facies in northwest China, as well as ostracodes from the presentday salted lakes. It has been found that with the exception of some Ilyocypris belonging to the subfamily of Ilyocyprinae that possess nodes and spines on the carapace, there is no taxa of the family Cyprididae developing nodes and spines on the carapace due to the change of habitat caused by salinization. On the

contrary, we have discovered in a deep flesh water lake in Yunnan province some Herpetocyprella bearing nodes on their carapace. In the well-known deep fresh water Baikalian Lake, Pseudocandona tuberculata BRONST. and P. gajenskaijae BRONST. ornementated with nodes and fin-like protrusions are known. Similar species have also been found in the Liaohe fault depression basin, viz., Pseudocandona unipapulatg BOJIE and P. tripapulata BOJIE etc. in the 3rd and the younger members of the Shahejie Formation. As for the origin of Ostracoda in Lake Baikal, Bronstein pointed out that of the total 36 species of ostracodes known from this lake, 35 are indigenous of limited distribution, which suggest that Lake Baikal was ecologically isolated. In a review of 82 species of ostracodes in Lake Tanganyika, a deep rift-valley lake in East Africa, Sars reported that all are endemic species, except one which can also be found in other water bodies. The same is true to ostracodes so far found in the 3rd member of the Shahejie Formation. Among the 154 known species of Ostracoda, 153 are new species. And, among the 38 species of Gastropoda, 35 are new species. Rift-valley lakes or deep in the fault depression basins are mostly longevity-lakes, with physical and biochemical conditions generally different from those in the shallow lakes and having sufficient time to permit the formation and development of a biota peculiar to these deep water bodies. Most of the extant Ostracoda in the deep lakes in Yunnan province, are aboriginal forms never seen elsewhere. The author himself has also collected and studied many living ostracodes now inhabiting in shallow lakes, ponds and ditches. He has found that most of them are common types of widespread distribution. A comparative study of the species from the Cenozoic continental shallow- water deposits north and south of Mts. Tienshan, Chilienshan and Kunlunshan reveals that despite the separation by big mountain ranges hundreds to nearly a thousand kilometers apart, they are very much alike and are mostly of widespread distribution. They are essentially different from those found in deep lakes which are of localized and restricted distribution. Between the Candona of the ladder-type (including Candona exacuta BOJIE, C. dorsicurta BOJIE, C. posticancava BOJIE, C. minitrapezoedea BOJIE, C. exigua BOJIE etc.) and the Candona found by Bronstein in the Lake Baikal (including Candona dorsocancava BRONST., C. directa BRONST., C. sensibilis BRONST.,

363 - -

C. semilunaris BRONST., C. rupestris BRONST. ), there is one feature in common, that is they are both bilaterally asymmetrical, with the left valve overlapping the right one along the dorsal hinge line. As the Candona in the Lake Baikal were collected at water depth from 10 to dozens or sometimes as deep as a hundred meters, there is every reason to believe that they are forms peculiar to deep lakes. Out o f the 36 species o f Ostracoda found by Bronstein in Lake Baikal, there are 18 species of Cytherissa, which, like Chinocythere, belong to the family Cytheridae, and a part of them have the same morphology as the type of Chinocythere with nodulose carapace as represented by Chinocythere huiminensis BOJIE found in the 3rd member of the Shahejie Formation. The author has collected, from the coastal region of Lake Dalainoir in Inner Mongolie, a large amount of specimen o f fossils Ostracoda including a lot of Cytherissa from the Pleistocene to Holocene strata and discovered that the general carapace ornamentation of some young forms is similar to that of the Chinocythere found in the basin under review (not inclu-

ding Chinocythere longispinata BOJIE). And recently, he has got a batch of ostracod specimens from some fresh water lakes in Yunnan which bearing close resemblance to Chinocythere. Such being the case, it seems justified to say that we can not take the occurrence of Chinocythere in p r o o f of the existence o f a sea water or saline water environment in a general way. H. Junde has also found a large amount of Chinocythere fossils from the Yongning Formation of Paleogene in the Nanning basin o f Guangsi province, and after trace elemental analysis, he has concluded that these forms come neither from fresh water lakes nor from the sea. They represent a population of the brackish water type rarely known in our country (H. Junde, 1981). It is quite clear from the above data that we can not put down, in general terms, that the above mentioned ostracod faunas appearing in the Bohai Bay basin (including the Liaohe fault depression basin) are of marine origin. They are mainly forms o f restricted distribution developed in deep lakes isolated ecologically for a long geologic period.

REFERENCES

JUNDE H. (1981) - Early Tertiary Ostracods from the Nanning Basin in Guangxi (in Chinese). Bull. Nanjing Inst.

Geol. Palaeont., Acad. Sinica, 211-240. SWAIN F.M., BECKER J. & DICKINSON A. (1971) Paleoecology of Tertiary and fossil Quaternary nonmarine Ostracoda from the Western Interior United States. Colloquium on the Paleoecology of Ostracodes, 461488. YOUTANG H., YINGPEI L. & alii (1978)- Early Tertiary Ostracoda Fauna from the Coastal Region of Bohai (in Chinese) Science Press, Beijing.

ZHENCHENG S. & OU Z. (1981) - Distribution of lower Tertiary fossil assemblages in the lower Liaohe Plain and its significance. The selected Papers on the 1st Convention of Micropaleontologicai Society (in Chinese), Science Press', Beijing, 37-45. - gpontuTenn

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