A stepped karst unconformity as an Early Silurian rocky shoreline in Guizhou Province (South China)

PALAEO ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 115 129

A stepped karst unconformity as an Early Silurian rocky shoreline in Guizhou Province (South China) R o n g J i a - y u a, M a r k e s E. J o h n s o n b a Nanjing Institute o f Geology and Palaeontology, Academia Sinica, Nanjing, China

b Department of Geology, Williams College, Williamstown, MA 01267, USA Received 11 April 1995; revised and accepted 1 September 1995

Abstract

Where succeeded by marine strata, karst unconformities signify a former rocky coastline. Such relationships may help sort out relative sea-level changes and aspects of local geography controlling facies distribution. An exceptional example of an early Silurian karst shore is well exposed near the village of Wudang in central Guizhou Province, near the capital city of Guiyang in South China. Here the Lower Silurian Kaochaitien Formation oversteps 63 m of paleotopographic relief in limestones belonging to the Llanvirn Guniutan Formation and Caradoc to early Ashgill Huanghuachong Formation (Ordovician). The corresponding rise in sea level took place coeval with tectonic uplift, as confirmed by a regionally diachronous relationship in the Ordovicia~Silurian boundary across a 250 km track from central to northern Guizhou Province. The change in sea level also fits with a global rise of sea level in late Aeronian (later Llandovery, early Silurian) time. Borings of the ichnofossil, Trypanites, are reported from the karst surface of the Huanghuachong Formation and Silurian strata fill sink holes in this unit over 5 m deep. The Silurian karst shoreline near Wudang is integrated with other regional data to construct a paleogeographic map covering the northern half of Guizhou Province.

1. Introduction Buried karst surfaces represent a distinct type of unconformity quite c o m m o n throughout the stratigraphic record. N o less than 30 examples of karst unconformities are cited by Johnson (1992) as evidence for ancient rocky shores, all where marine strata succeed carbonate rocks formerly subjected to subaerial exposure. These examples are based on detailed accounts of relationships preserved in strata ranging in age from the Precambrian (Kerans and Donaldson, 1988) to the late Cenozoic (Aigner, 1983). In some cases, the physical evidence for such relationships is subtle, as in small-scale scalloped erosion surfaces 0031-0182./96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 0 3 1 - 0 1 8 2 ( 9 5 ) 0 0 0 8 2 - 8

retained on the bedding planes of some Ordovician limestones in Virginia (Read and Grover, 1977) or some limestones in Gotland, Sweden (Cherns, 1982). In marked contrast, the scale of topographic relief exhibited by some karst unconformities may be robust, as illustrated by karst towers up to 8 m in height exhumed from beneath a Permian marine sandstone in Western Australia (Hocking et al., 1987). Or it may be dramatic as seen in the up to 100 m plus "tower and sink" relief overlain by marine strate, both of Permian age, of central East Greenland (Scholle et al., 1993). Biotas of an intertidal to shallow subtidal character have appeared through time, often with a specific preference for limestone substrates. Clionid

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R. Jia-yu et al./Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 115 129

sponges and lithophagid bivalves are especially successful examples of borers utilizing this habitat. As characterized by the ichnofossil Trypanites, a fauna probably including several different polychaet-like animals became widespread throughout much of Phanerozoic time. These animals were capable of boring small ( > 2 m m ) , mostly vertical holes in carbonate substrates. Colonization of former karst surfaces indicates their essentially intertidal to shallow subtidal affiliations. Lower-energy environments with Trypanites preserved on subtidal hardgrounds and relatively immobile cobbles are reported by Wilson (1987), but their occurrence is less common. First recorded as a distinct trace fossil by De La Beche (1846, p. 290) from a Jurassic karst surface eroded in Carboniferous strata in England, Trypanites was later confirmed by Fletcher (1988) from an equivalent karst surface in South Wales. The oldest known karst surface bearing Trypanites dates from the middle Ordovician of the Mingan Islands in Quebec (Desrochers and James, 1988). The youngest such surface with Trypanites is attributed to a Pliocene rocky shoreline in southern California (Watkins, 1990). Nearly all of the karst unconformities cited in the bibliography by Johnson (1992) are site specific. That is, their lateral extent is rarely documented in an attempt to confirm coastal relationships over any considerable distance. To some degree, this situation is controlled by limited spatial exposure of unconformities in geological profile. An exception is the work by Radwanski (1970), in which the littoral zone of a Miocene karst surface is traced in southern Poland. His map reconstruction shows a complex of peninsulas, islands, straits, and bays with a net shoreline distance of almost 100 km. Another important use for ancient rocky shores, including karst unconformities, is as a precise benchmark for the measurement of relative sea-level change. Aigner (1983, p. 320), in his attempt to relate the Messinian draw-down of the Mediterranean Sea to a carbonate coast in Egypt asserted that: "Fossil cliff lines provide the rare opportunity to place one's finger on ancient sea levels." The potential for this line of research in separating aspects of tectonic as opposed to eustatic sea-level change, however, is

infrequently dealt with in the literature (Johnson, 1988). The object of this paper is to introduce a locality in South China's Guizhou Province, which features a profound karst unconformity with significant applications to the paleogeographic mapping of an early Silurian (late Aeronian) shoreline and to the quantification of sea-level rise during a specific interval when both eustatic and regional tectonic factors were in play. This locality was chosen because of its unusual paleotopographic relief, as well as its likely potential for documentation of the Trypanites ichnofauna. It represents one of the best examples of an ancient karst coastline, as presently understood from many localities around the world (Johnson, 1992), and it is the first well documented rocky shoreline from the preQuaternary of China.

2. Geographical and geological setting The study site is situated on the west side of the Housuo River near the axis of an anticline plunging parallel to a narrow glen locally known by the name Huanghuachong (Yellow Flower Basin). This feature extends for a distance of 0.8 km, terminating at the junction with a smaller side glen called Yegouchong (Wild Dog Glen). The main glen gives its name to the Upper Ordovician (Caradoc) Huanghuachong Formation, which is a thick-bedded limestone unit capping the prominent ridge between the main glen and the river. The entrance to this glen is through a small water gap crossing the ridge at a point 2.6 km by unpaved road south from Wudang village. In turn, the village is located 5 km by unpaved road from the Xintianzhai New Development Zone. The zone is 8 km northeast of Guiyang, the capital of Guizhou Province, and it is well connected by a paved road (Fig. 1). Owing to the steeply dipping south limb of the anticline, the ridge capped by the Huanghuachong Formation rises sharply up to 140 m above the floor of the adjacent Housuo River valley. Stratigraphically above the Huanghuachong Formation with virtually the same attitude of bedding is the Lower Silurian (Upper Aeronian to

117

R. Jia-yu et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 115-129

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Telychian) Kaochaitien Formation, consisting almost entirely of gray-green to maroon siltstones and silty mudstones, with brecciated limestone at the base. Folding of the anticline comprising these and paraconformable Devonian to Jurassic strata took place sometime in later Jurassic to Cretaceous time, during the Yanshan (Swallow Mountain) Orogeny. Deformation took place prior to deposition of the Cretaceous, flat-lying continental conglomerate belonging to the Jiuzhou Formation. This formation directly abuts against dipping

Silurian and Devonian strata many places in the Housuo River Valley. A wider regional overview helps place the complex Ordovician-Silurian boundary relationship into better perspective (Fig. 2). Approximately 250 km farther north of the Wudang area at Xiushan (in SE Sichuan), Songtao (in NE Guizhou), and Hanjiadian near Songkan in Tongzi County (in N Guizhou), the Ordovician-Silurian succession is continuous without any recognizable stratigraphic break. The time gap between the two

118

R. dia-yu et al./Palaeogeographv Palaeoclimatology PalaeoecoloKv 121 (1996) 115-129

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Fig. 2. Regional correlation scheme across the Ordovician and Silurian boundary in central and northern Guizhou Province. systems, however, gradually expands in duration at sites nearer and nearer to Wudang (Wang, 1976; Mu et al., 1977; Ge et al., 1979). At a maximum, the gap cuts out Middle and Upper Ordovician rocks attributed to parts of the Llanvirn and all the succeeding Caradoc and Ashgill stages, as well as Lower Silurian (Llandovery) rocks attributed to the Rhuddanian and lower part of the Aeronian stages. At any one locality Ordovician and Silurian strata are discernibly in a perfectly paraconformable relationship with one another, but the regional picture indicates that some parts of Central Guizhou Land were unevenly uplifted during late Ordovician and early Silurian times. M a x i m u m uplift occurred to the present south in the Wudang

area, although essentially no subaerial uplift transpired 250 k m to the present north. This situation indicates that the regional attitude of exhumed Ordovician beds must have approached 1/20 ° of slope over 2 5 0 k m , in order to account for a m a x i m u m stripping of at least 60 m of Ordovician strata in the present south. Deposition of Silurian strata thus overstepped the Upper Ordovician in a regionally diachronous, yet basically paraconformable relationship during the course of the early Silurian transgression. Although the strata above and below the Ordovician and Silurian contact surface have not been precisely dated, a relative estimate of their age can be made (Fig. 3). The upper and top parts of the Kaochaitien Formation contain an atypical

R. Jia-yu et al./Palaeogeography, Palaeoclimatology,Palaeoecology 121 (1996) 115 129

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(Wang 1976) in association with a low-diversity c o n o d o n t assemblage, indicating an age corresponding to the Monoclimacis griestoniensis to M. crenulata graptolite zones ( Z h o u et al., 1981). In the middle part o f the formation, there occur the

Nalivkinia elongata, Nucleospira calypta, N. pulchra, Eospirifer sp., Striispirifer sp. and Howellella uniplicata (Wang, 1976). This brachiopods

brachiopod assemblage is also k n o w n to occur from the upper Leijiatun to the Lower Xiushan formations, which are of latest Aeronian to early Telychian in age (Rong, 1986). The basal part of

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R. Jia-)'u et al./Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 115 129

the Kaochaitien Formation yields an aberrant, shallow-water epifauna with very low diversity, including bivalves and gastropods. They are ecologically diagnostic of restricted marine conditions, but useless for dating the rocks in which they occur. However, it seems to us that this part of the formation may be late Aeronian in age, equivalent to the Monograptus se~gwickii graptolite zone (Fig. 3). The Huanghuachong Formation below the contact surface at Huanghuachong on the eastern margin of the field area (localities A and B), yields a shelly fauna, including mainly corals, stromatoporoids, and crinoids. Yang and Wang (1980) identified the corals Plasmoporella intermedia, Heliolites orientalis, and other species belonging to these genera from the top part of the formation (their Longjing Formation) and attributed them to an early Ashgill age. The fbrmation was eroded unevenly and the age of the top part of the formation is diachronous. Based on regional stratigraphic relationships, we equate the Huanghuachong Formation with the Linhsiang Formation (Lower Ashgill) and Pagoda Formation (Caradoc) in the upper Yangtze Region (including northern Guizhou, southern Sichuan, and western Hubei provinces). This interval falls within the biostratigraphic zones Nemagraptus gracilis to Dicellograptus complanatus (Fig. 3). Exposed below the contact at Yegouchong (Wild Dog Glen) on the western margin of the field area (Locality C) is the Guniutan Formation. This unit contains the nautiloid DiderocerasMeitanoceras assemblage and brachiopod Yangtzeella kuiyangense, indicating a Llanvirn age (Zhao and Huang, 1985). In the Meitan Formation below the contact surface at Yegouchong (Locality C), there occurs Glyptograptus austrodentatus (Zhao and Huang, 1985), indicating a late Arenig age. Based on this summary of the local biostratigraphy (Fig. 3), the maximum time gap expressed by the karst unconformity extending 0.8 km between Huanghuachong (Localities A and B) and Yegouchong (Locality C) ranges from the Llanvirn (Middle Ordovician) to the Upper Aeronian (MidLlandovery, Lower Silurian).

3. Program of field work

Geological field mapping in the Wudang area was previously undertaken during the 1970s by the Guizhou Geological Surveying Team (Yang and Wang, 1980). Their work established the regional pattern of structural control and provided detailed stratigraphic logs. The nature of the karst unconformity was first recognized by Gao (1962) and subsequently confirmed by Zhao (1979) and Yang and Wang (1980), who used the term "karst parallel unconformity." Our restudy of the karst unconformity in the Wudang area was made in June 1994, with field objectives in three broad categories. First was a determination of the local paleorelief on the Ordovician karst surface transgressed by Silurian marine sediments. The vertical extent of original topographic relief may be used to directly assess the magnitude of relative sealevel change which took place during late Aeronian (early Silurian) time, In addition to evidence of Ordovician strata having been significantly overstepped by Silurian strata, as implied by the diachronous nature of the contact surface, features on a smaller scale typical of karst erosion also were checked for. These aspects were studied by directly tracing out the contact surface exposed in profile along the cliff face at the water gap (Locality A, Fig. 1) and along the north side of the 0.8-kin long glen at Huanghuachong (from Localities A and B to C, Fig. 1). Our second objective was a careful search of the karst surface, itself, for signs of boring activity or encrustations by Silurian marine organisms. This was attempted in places where recent erosion of Silurian strata resulted in the exhumation of the upper-most bedding planes belonging to Ordovician strata. Our third objective was to improve the resolution of biostratigraphic control for the basal Silurian in this area by collecting samples for possible microfossils such as chitinozoans and acritarchs.

4. Results

Fig. 4 shows a scaled longitudinal profile tracing the Ordovician-Silurian unconformity from the entrance to Huanghuachong (Locality A) to

R. Jia-yu et aL/Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 115-129

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Fig. 4. Long profile of Ordovician Silurian strata parallel to the plunge of an anticline formed during the Yanshan Orogeny. The north limb of this anticline exposes the stepped unconformity at the Ordovician-Silurian boundary. The uneven double line represents a foot path running approximately0.8 km from the mouth of Huanghuachong (Locality A) to Yegouchong(Locality C). Yegouchong (Locality C). The profile trends parallel to the axis of the east plunging anticline. It is also perpendicular to the actual direction of dip in the south limb of the anticline. Thus, the eastern dipping beds demonstrated in this profile reflect only the approximate plunge dip (see Fig. 4), not the true dip which would appear even higher in a north-south profile. The Silurian and Ordovician strata are essentially paraconformable, however, and preserved in an overstepping relationship. The magnitude of relative rise in sea level during late Aeronian (early Silurian) time may be directly calculated by measuring the cumulative thicknesses of the fully lithified Ordovician units buried by the Silurian during a transgression. At this locality, the thickness of the lowest step is 23 m, conforming to the Llanvirnian Guniutan Formation. Several other steps are comprised of beds belonging to the Caradocian Huanghuachong Formation, which has an uneven thickness no less than 40 m. Thus, the minimum thickness of carbonate cliffs drowned by rising sea level in this area during late Aeronian (early Silurian) time is 63 m. The karst surface locally exhibits considerable relief related to sink holes. The fact that Silurian strata are preserved within sink holes confirms their formation during pre-Silurian time. The best

example of a karst depression filled with Silurian strata is located at the entrance to Huanghuachong (Fig. 5A), where more than 7 m of Silurian strata are exposed on the east wall of the water gap along the trackway crossing through the ridge. Of these strata, the lower 5.4 m fill a well-defined depression in the Ordovician limestone. The basal fill includes 2.2 m of brecciated limestone, which are succeeded by beds more characteristic of the Kaochaitien Formation with siltstones and mudstones incorporating thin intervals of reworked carbonate clasts. The fill shows distinct signs of compaction characterized by sagging layers (Fig. 5B,C). Contemporary erosion of the water gap took advantage of the diminished thickness of the carbonate ridge at this site, but this also resulted in removal of most of the Silurian fill (Fig. 5B). A reconstruction of the Silurian karst fill is given in Fig. 6, indicating that the original Ordovician depression at Locality A was 14 m across in its present east-west axis. It is not possible, however, to determine the extent of the depression in the present north-south axis. It is important to note, however, that the reworked carbonate clasts in the Kaochaitien Formation are angular in shape and that their size generally diminishes through the

122

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Fig. 5. Paleokarst features at H u a n g h u a c h o n g (Yellow Flower Basin). A. Water gap cutting across the Upper Ordovician H u a n g h u a c h o n g Formation to form the basin's entrance (view from the m o u t h of the basin looking out across the Housuo River valley). B. View of pathway on the water gap, which cuts through a Silurian sink hole eroded in the Ordovician limestone (outlined in white). C. Detail of the Silurian sink hole.

sequence preserved in this structure. In the basal layer, H1 (Fig. 6), the largest observed clasts are 45 cm across their long axis, but most are within the range of 3 cm to 8 cm. Those clasts closer to the east wall of the contact surface are consistently larger in size. Beginning from the top of layer H3, an interval 25 cm in thickness contains smaller clasts mainly in the size range of 1-3 cm. In the middle part of unit H4, however, there appear a few larger clasts in the range of 3 14 cm but the majority of clasts are less than 2 cm in size. Finally, the top part of unit H5 includes some clasts in the range of 2-5 cm, but the majority remain less than

2 c m in size. Above this level 5.4 m from the bottom of the karst depression, no other carbonate clasts are present. An even larger depression 24 m across is preserved on the Ordovician karst surface at Locality B about 70 m to the west up the slope of the carbonate ridge (Figs. 1 and 4). This structure is distinctly asymmetrical in outline on its east-west axis, demonstrating a steep east wall contrasted with a gently climbing west wall. The depression retains up to 5 m of fill by Silurian strata. In comparison to the smaller feature at Locality A, there are no brecciated limestone cobbles preserved

R. Jia-yu et al./Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 115-129

L O C A L I T Y

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Fig. 6. Reconstruction of a transverse section across the paleokarst hole at Locality A, with Silurian sediments and reworked Ordovician clasts filling the depression within the Ordovician limestone (Huanghuachong Formation).

at the base of the Silurian sequence at this locality. Only scattered limestone pebbles are present in the basal layer of the Kaochaitien Formation at this spot, however, at higher intervals there are influxes of limestone granules in a matrix of siltstone separated by intervals of mudstone with no granules. Farther to the west, on the south side of the ridge top about one-third of the way between Localities A and C (Fig. 4), scattered limestone granules were observed in the basal Silurian layers. At Locality C, where the Kaochaitien Formation (Silurian) rests directly on the Meitan Formation (Late Arenig, Ordovician), no reworked limestone clasts were observed. Considerable effort was made to find Silurian encrusting fossils or boring trace fossils on the Ordovician karst surface. Very little of the exhumed contact surface could be examined at

Locality A, but here it was possible to dig out reworked limestone clasts from the lower intervals in the Silurian. All such clasts were barren of encrustations or borings. At Locality B, however, Trypanites borings were identified on the upper surface of a 75-cm high step corresponding to a single, thick limestone bed in the Huanghuachong Formation. As shown by the perspective in Fig. 7A, this bed is barely exhumed from the Silurian siltstone now remaining as a thin bed on its northern margin. What looks like a simple step in the photograph is actually a corner step. Silurian strata are present not only on the top back margin of this limestone block, but also immediately behind it (out of view). A near view of the flat carbonate surface with the Trypanites borings is shown in Fig. 7B. The borings are perfectly circular in planar cross section and about 2 mm in diame-

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R. Jia-yu et al./Palaeogeography, Palaeoclimatology,Palaeoecology121 (1996) 115 129

Fig. 7. Karst surfaces penetrated by Trypanites borings. A. Karst unconformity at locality B showing Ordovician limestone to the right of the field bag and Silurian siltstone to the left. B. Detail of worn but bored surface. C. Karst surface on Ordovician limestone about one third of the distance between Localities A and C. D. Detail of borings (black pen cap is 4 cm in length). ter, but quite shallow in vertical cross section due to planation o f the karst surface. Care was taken not to confuse these features with possible contemp o r a r y pitting o f the karst surface. The m o d e r n karst surface is eroded below the level o f the Ordovician Silurian contact only a few meters to the south. This surface was minutely examined to determine if anything resembling the small, circular holes could be f o u n d there. Only larger-scale, irregular patterns o f surface pitting could be f o u n d on the m o d e r n surface, but nothing which resembled well defined circular holes. A n o t h e r concentration o f Trypanites borings (Fig. 7C,D) was discovered farther up the ridge slope a b o u t one fourth o f the distance between Localities A and C (Fig. 4). In b o t h cases, the circular traces can be f o u n d only in close proximity to the Silurian overburden. N o Silurian encrusting fossils were f o u n d in these spots. Silurian macrofossils are especially u n c o m m o n in the lower part o f the succession. N o macrofossils were observed anywhere in the Silurian sequence at Locality A. At Locality B, the first macrofossils

appear f r o m a layer o f reddish silty m u d s t o n e 5.5 m above the base o f the unconformity, including a single specimen o f the gastropod Planitrochus sp. In other places farther up the ridge crest to the west, specimens o f the same gastropod together with another gastropod, Perneritrochus sp. and the bivalves Eurymyella sp., Amphicoelia sp., and Modiolopsis sp. are m o r e a b u n d a n t at a somewhat higher level. O u r attempt to extract microfossils f r o m Silurian strata in the sink hold at Locality A yielded no results. Six samples collected over a vertical succession o f 6 m proved to be barren o f chitinozoans and acritarchs.

5. Discussion The ichnofossil, Trypanites, is well d o c u m e n t e d from older karst unconformities which developed in Ordovician time (Desrochers and James, 1988) as well as f r o m a y o u n g e r karst u n c o n f o r m i t y which developed in D e v o n i a n time on a Silurian

R. Jia-yu et al./Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 115 129

substrate (Pemberton et al., 1980). Heretofore, the Silurian record has been poorly represented in this regard. The only other report of Trypanites in a Silurian coastal environment is from altered carbonate cobbles in the Kidney Seam of the Red Mountain Formation (Lower Silurian, Llandovery Series) of Alabama (Baarli et al.). The presence of Trypanites borings from the karst unconformity at the top of the Huanghuachong Formation near Wudang is not an unexpected feature, and it represents the first report of these trace fossils from a karst coast drowned in Silurian time. The importance of the trace fossils is in their confirmation of a littoral to shallow submarine incursion against the karst coastline. An encrusting fauna of some sort might also be expected on this drowned karst surface, but the shallowness of the Trypanites borings suggests that any fauna encrusted on the surface would have been eroded away. Encrusting corals, as known from other localities (Johnson and Baarli, 1987) would be unexpected on the Huanghuachong karst surface due to the turbidity of the Silurian transgression. In any case, the actual unconformity surface is very limited in the size of area which may be searched.

5.1. Application to sea-level studies The circumstances under which the transgression took place during late Aeronian (early Silurian) time in central Guizhou Province are unusual, because they involve elements of an eustatic sealevel rise against the backdrop of regional tectonic uplift. Tectonic uplift is evident in the regional stratigraphic relationships summarized in Fig. 2, where younger and younger Silurian strata sit on older and older Ordovician strata along a n o r t h south gradient. The time gap at the OrdovicianSilurian boundary is at its maximum in the central Guizhou area around Wudang. Much the same situation occurred in New York State, along the axis of the Mohawk River, where younger and younger Silurian strata rest disconformably on older and older Ordovician strata. Maximum uplift took place to the present east on the margin of Taconic highlands, whereas the most complete stratigraphic succession through Ordovician and

125

Silurian time took place to the present west in the vicinity of Niagara Falls (Rickard, 1975). During late Aeronian time (correlative to the Monograptus sedgwickii graptolite zone), a rise in sea level took place in epicontinental waters on several different paleocontinents (Johnson et al., 1991). On the stable carbonate platform of Laurentia in present-day eastern Iowa, the rise in sea level resulted in a change from a coral-stromatoporoid community (Benthic Assemblage Zone 2) to a community dominated by the brachiopod Stricklandia lens progressa (BA Zone 4). The magnitude of this event is estimated by Johnson (1987) to have been on the order of 50-60 m. In New York State, there is a widespread pattern involving the replacement of an Eocoelia-like brachiopod community (BA2) in clastic-rich rocks by a Pentamerus community (BA3) in a thin limestone. At Hanjiadian near Songkan, about 35 km north of Tongzi, northern Guizhou, a comparable coralstromatoporoid community (BA2) is succeeded by an Apopentamerus muchuanensis community (BA3) in the sequence incorporated by beds 7-9 of the Shihniulan Formation (Rong et al., 1984, p. 678). This transition, estimated to have involved a change in sea level on the order of 20-30 m, was originally dated as late Aeronian to early Telychian in age. Now based on new interregional correlations (Rong et al., 1990), the Shihniulan Formation can be correlated with the upper Xiangshuyuan Formation and Leijiatun Formation, in which there occur the chitinozoans Conochitina cf. iklaensis, Ancyrochitina shiqianensis and others (Geng, 1990) indicating a late Aeronian age (correlative to the Monograptus sedgwickii zone). While the magnitude of Silurian sea-level rise during this time interval may be estimated on the basis of community replacements in northern Guizhou Province, eastern Iowa, and other places on other paleocontinents, the Wudang area described in this report has the advantage of providing a direct measure of sea-level change on the basis of stratigraphic overstep across a fixed rocky shoreline. The figure of 63 m arrived at by this means, agrees very well with the bathymetric range in habitat estimated for Silurian communi-

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R. Jia-yu et al./Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 115 129

ties by Rong et al. (1985) and Johnson (1987), but it is somewhat higher than the estimates suggested by Brett et al. (1993). This result does not imply that the coastal waters along the rocky shoreline near Wudang were anything approaching 63 m deep during the maximum peak of Aeronian transgression. Indeed, quite the opposite was true, because the area was rapidly filled in with fine clastics derived from the erosion of the exposed Ordovician Meitang Formation. The upper 7 m of this formation consist of thinly interbedded fine-grained sandstones, silty shales, together with argillaceous and bioclastic limestone. When the Silurian transgression reached the Wudang area in Aeronian time, much clastic material was locally recycled into the lower part of the Kaochaitien Formation. A very similar process took place in eastern Iowa, when valleys 45 m deep eroded into the Ordovician Maquoketa Shale were flooded by a rise in sea level at the beginning of the Silurian Period (Johnson, 1975). As rising waters lapped against the shale hills of the Maquoketa Formation, much clastic material was recycled into the accumulating Silurian sediments. From the time the first Silurian waters entered the pre-existing Ordovician topography in eastern Iowa until the time they shoaled over the tops of the shale hills, water depth remained very shallow despite the 45 m rise in sea level. The valley walls of the Maquoketa Formation were not resistant enough to form actual sea cliffs, but the limestones of the Guniutan and Huanghuachong formations were.

5.2. Application to paleogeographic studies An even more obvious use for ancient rocky shores relates to paleogeography. Although the overstepped karst shore near Wudang may be traced for less than a kilometer in surface exposures, it provides an important key to the early Silurian geography of central and northern Guizhou Province. With their limited gastropod and bivalve fauna, the siltstones and mudstones of the Lower Kaochaitien Formation represent a restricted facies much in contrast with coeval carbonates probably belonging to the Leijiatun Formation in the northern part of the province

(Fig. 2). This fauna has not been found associated with normal marine, shallow-water faunas elsewhere in the Yangtze region. The striking emdemism of the Lower Kaochaitien fauna is thought to reflect an abnormal environment (Xu Jun-tao, pers. comm., 1994). The underlying Ordovician sequence, which dipped imperceptibly to the north, had its highest rate of uplift in central Guizhou. The karst unconformity traced diagrammatically in Fig. 4 signifies a south-facing escarpment, where these Ordovician units were truncated by erosion in late Ordovician and earliest Silurian times. A perspective view of this escarpment is interpreted in Fig. 8A, representing a very early Silurian (Rhuddanian) geography when probably a small river valley crossed the Wudang area. One line of evidence that such a valley was actually eroded and drained by a river manifests itself in the silty facies which can be correlated from Zunyi to Shiqian (Fig. 8A, localities 2 and 4). A more detailed geography representing late Aeronian time is offered in Fig. 8B. This reconstruction features an embayment with a narrow opening, which resulted from the drowning of the former river valley and its prominent escarpment in the Wudang area. Bordered to the north by the peninsula of Mid-Guizhou Land, the configuration of the embayment helps account for the restricted muds and silts which comprise the Silurian Kaochaitien Formation, and this configuration can be expected to have formed restricted conditions. Coeval facies outside the bay to the north are much less affected by clastics and include carbonates with normal marine faunas of corals and brachiopods. At Wengxiang in Kaili County (Fig. 8B, locality 6), the corals Palaeophyllum, Amplexoides, Trgidssonites and others occur in a medium-thick limestone only 8 m above the Ordovician Silurian contact. This coral assemblage indicates a normal marine environment with no muddy component and it clearly marks the entrance of the embayment. With the formation of the bay surrounding the Wudang area, it may be said that the 63 m rise in sea level which occurred in late Aeronian time as part of a global event, simultaneously transgressed both the inner escarpment of MidGuizhou Land (from the present south) and its

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6.

Conclusions

This report represents the first applications of a Silurian rocky shoreline to study of sea-level changes and regional paleogeography. The following events are concluded to have taken place from late Ordovician to early Silurian time in Guizhou Province on the South China platform. Uneven uplift of Llanvirn to lower-Ashgill

strata, including the Meitan, Guniutan, and Huanghuachong formations, resulted in a gentle regional dip along a 250 k m S W - N E axis, such that sedimentation across the Ordovician-Silurian boundary was uninterrupted in the more northerly parts of the province but m a x i m u m uplift occurred in the central part of the province. The elevated Ordovician strata were truncated in post early Ashgill (late Ordovician) and pre late Aeronian (early Silurian) time, probably by normal stream dissection, which resulted in the erosion of a valley more than 100 km long (Fig. 7) trending on a present east west axis. This activity resulted

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R. Jia-yu et al./Palaeogeography, Palaeoclimatology, Palaeoecology 121 (1996) 115 129

in the development o f a 63 m high escarpment at H u a n g h u a c h o n g near Wudang, northeast o f Guiyang. A l t h o u g h gradual uplift continued t h r o u g h the ensuing Silurian Period ( R o n g et al., 1985, p. 693: J o h n s o n et al., 1989, p. 49), a rise in global sea level reached the W u d a n g area in the central part o f the province in late A e r o n i a n (early Silurian) time. The local effect o f this transgression is exhibited by stratigraphic over step o f the Ordovician escarpment by Silurian strata belonging to the Kaochaitien Formation. Early signs o f this Silurian transgression are represented on the karst unconformity by borings o f the trace fossil Trypanites in the H u a n g h u a c h o n g Formation. The siltstones and mudstones o f the overlying Kaochaitien F o r m a t i o n p r o b a b l y were recycled f r o m the clastic-rich Meitan F o r m a t i o n forming part o f the former valley floor. At H u a n g h u a c h o n g near W u d a n g , the local rise in sea level was 63 m and the sediments comprising the Silurian Kaochaitien F o r m a t i o n completely filled the former valley. Regional data collected on facies patterns in Silurian strata d o c u m e n t the b r o a d relationships o f paleogeographic maps reflecting changes in G u i z h o u Province t h r o u g h early Silurian time (Fig. 7). The peninsula forming m u c h o f MidG u i z h o u L a n d acts as a prominent focal point in this history. To the present south o f the peninsula, the Ordovician escarpment and former valley are precisely defined by p a l e o t o p o g r a p h i c relief. To the present n o r t h o f the peninsula, the adjacent silty facies o f the " L u n g m a c h i F o r m a t i o n " atypically reflect a pro-delta environment, as well as general proximity to M i d - G u i z h o u L a n d (Fig. 7a). The fine clastic lithology and molluscan fossils o f the lower Kaochaitien F o r m a t i o n are appropriately restricted by the boundaries o f the submerged valley, where low-salinity conditions prevailed due to an influx o f fresh water. Outcrop-scale observations incorporating the relationships o f ancient rocky shorelines have valuable roles to play in solving issues o f great interest to geologists. It is a direct and practical a p p r o a c h to questions o f tectonics, sea-level change, and regional paleogeography.

Acknowledgements This research was undertaken with the joint support of the U.S. National Science F o u n d a t i o n (International Division) and the Chinese A c a d e m y o f Sciences (Academica Sinica). The authors are very grateful to Prof. Z h a o Yuan-long (Guizhou Engineering College in G u i y a n g ) for assisting us in the field during our work at H u a n g h u a c h o n g . Silurian bivalues were stratified by Dr. X u Ju-tao and Silurian gastropods were identified by Dr. Pan H u a - a h o n g , both at the Nanjing Institute o f G e o l o g y and Paleontology.

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