Sediments with the remains of dinosaurs near Gokwe, Rhodesia

Palaeogeography, Palaeoclimatology,Palaeoecology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

SEDIMENTS W I T H T H E REMAINS OF DINOSAURS N E A R GOKWE, RHODESIA a G E O F F R E Y B O N D AND K. B R O M L E Y

Department of Geology, University College of Rhodesia, Salisbury (Rhodesia) (Received March 10, 1970)

SUMMARY

Bones and teeth of large dinosaurs, and a single specimen of a non-marine lamellibranch, have been found in sandstones west and southwest of Gokwe, Rhodesia. These previously unknown sediments are named the Gokwe Formation, which has a maximum thickness of about 300 ft. It is divided into a lower calcareous member and an upper white sandstone member. The Gokwe Formation rests unconformably on the Karroo System a n d 4 s unconformably overlain by Kalahari Beds. The sediments represent the filling of a comparatively small take, and are thought to be either latest Jurassic or earliest Cretaceous in age. The extreme rarity of such deposits in southern Africa gives their discovery an added significance. INTRODUCTION

The country immediately west of Gokwe (18 °15'S 28 °56'E) has until recently, been uninhabited and rather inaccessible. It is shown on the latest edition (1960) of the geological map of Rhodesia (1:1,000,000) as being composed of Upper Karroo sedimentary rocks and lavas, overlain by Kalahari Beds. The geology was compiled from a few traverse records, and no detailed map of the area has previously been published. Traverses by PARSONS(1903) and MOLYNEUX (1921) passed through this area, but their accounts contain no information useful in the present context. MACGREGOR (1941) described the geology of the Mafungabusi Gold Belt and the western boundary of his map lies within about 10 miles of the eastern boundary of our map (Fig.l). MACGREGOR (1941, p.6) included a figure on which he showed four vertical sections. One of these on the road northwest of Gokwe, lies outside his mapped area and just beyond the northern boundary of ours. The youngest rocks shown on this section are the basalts at the top of the Karroo System, but in his text he refers (op. cit., p.8) to the sedimentary rocks younger than the Karroo System. We quote the last part of the relevant paragraph: 1 The essence of this paper was read at the Rhodesia Science Congress, 1st, Bulawayo, May 1967.

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"They comprise 'pipe sandstone', with silcrete nodules at the base overlying an older well-bedded reddish sandstone with a clayey matrix. Following the Rhodesian custom, the post-Karroo sediments on the map are grouped as Kalahari Beds". The area around Gokwe has been opened up in recent years for settlement, and roads have been cut radiating out from Gokwe. The second author was actively engaged in the work of planning this development, and in 1961 found fragments of fossil bone about 8 miles almost due west of Gokwe. They.were found in the area of the headwater tributaries of the Kiswiswi River (see Fig.l), on the north side of the road from Gokwe to Charama. Later, more fossil bones were found on the south side of another road which follows the watershed between the Sengwe and Mbvumbvudzi rivers, about 10 miles southwest of the first locality. Since then, many other fossiliferous areas have been located. When we visited the area together in April 1962, we were struck by the similarity between the rocks containing bone hereabouts, and the "well bedded reddish sandstone with a clayey matrix" seen by Macgregor in the roadside exposures just northwest of Gokwe. We re-examined this section and found that the sandstones were very similar. Careful search produced bone fragmentsin situ about half way down the escarpment. We have since found that these rocks are widespread and we have collected a few large bones and some teeth. They are sufficiently well preserved to show that they belonged to dinosaurs, some of which were of large size. These fossiliferous sediments, with a maximum thickness of about 300 ft., support a distinctive vegetation of scanty grass with widely scattered small trees. This vegetation differs so much from that of the Karroo System below and Kalahari Beds above that it can be clearly distinguished on air photographs. When we decided to undertake a joint investigation of these sediments, the second author did the mapping. This mapping was done by photo-interpretation, supplemented by many checks on the ground. Beginning in 1966 student parties have visited the area for three days in September each year, accompanied by one or both of the authors. These visits have allowed further extensive checking of the mapping, and the measurement and recording in detail, of several stream sections. During one of these visits Mr. B. Josiah found a single somewhat damaged lamellibranch valve, referred to in detail below. These visits have shown that bone fragments are extremely widespread particularly near the middle part of the formation. The fossil localities shown on the map (Fig.l) merely record those places where teeth and the more complete bones have been recovered. The mapping has shown that the "Kalahari Beds" of Macgregor comprise sediments of three distinct ages. The oldest beds are in the Gokwe Formation. These were later covered by the chalcedonic sandstones and sands of the earliest Kalahari Beds, distinguished on our map as "Kalahari Sand (I)". After a period of erosion "Kalahari Sand (2)" was deposited on an erosion surface at a lower Palaeogeography, Palaeoclimatol., Palaeoecol., 8 (1970) 313-327

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DINOSAUR REMAINSNEAR GOKWE (RHODESIA)

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altitude, being derived from "Kalahari Sand (1)". The present river system is incised into this surface. STRATIGRAPHIC TERMINOLOGY

We have attempted to follow the recommendations of the Subcommittee set up by the Geological Society of London as has been suggested in its report (GEORGE et al., 1967). We propose the name Gokwe Formation for these strata. Since the main settlement of Gokwe does not lie directly on the outcrop of this formation, being on the overlying Kalahari Beds, it is not strictly appropriate, but the name is also used for the district, and furthermore, there are no other well known place names in the area. We subdivide the Gokwe Formation into two members. We propose that the lower sub-division should be called the calcareous member, since it includes thin limestones, and the sandstones have a calcareous cement. The upper sub-division is named the white sandstone member, which is self-explanatory. This scheme of nomenclature leaves room for further sub-division, should this later prove necessary. THE STRAT1GRAPH1C POSITION OF THE GOKWE FORMATION

The full geographical extent of this formation is greater than is shown on the area of our map (Fig. 1). Enough is shown, however, to establish beyond reasonable doubt the relations of the Gokwe Formation to the Kalahari Beds above and the Karroo System below. In the road section just northwest of Gokwe, the formation overlies Karroo Basalts, which in a borehole 7 miles to the north are 270 ft. thick. Since these basalts are known to have a much greater thickness elsewhere, this small thickness suggests that considerable erosion took place before the Gokwe Formation was deposited. In the headwater tributaries of the Kiswiswi River 8 miles west of Gokwe, the formation rests on the Forest Sandstone, and the basalts are absent. Still further west, 25 miles west-southwest of Gokwe, on the north flank of the Mbvumbvudzi River valley, the formation rests on the pebbly arkose, with abundant fossil wood. The base of the formation, therefore, transgresses on to progressively older divisions of the Upper Karroo System in a westerly direction, indicating an unconformity between the Karroo System and the Gokwe Formation. The relationship between the Gokwe Formation and the overlying Kalahari Beds is not quite so easily defined. We have divided the Gokwe Formation into two members on the map. Kalahari Sand (1) seems to transgress over the eroded surface of the white sandstone member, indicating a slight tilt, and a period of Palaeogeography, PalaeoclimatoL, PalaeoecoL, 8

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erosion before the Kalahari Beds were deposited. If this is so, an unconformity is also present at this contact. Kalahari Sand (2) represents a late redistribution of the older Kalahari Sand (1) and therefore its relationship to the Gokwe Formation is immaterial. On the basis of these field relations the Gokwe Formation is significantly younger than the Stormberg Basalts of the Karroo System, but older than the Kalahari Beds. Thus it is younger than the early part of the Jurassic and older than the Mid-Tertiary. This is a wide bracket, which by palaeontological means, may be narrowed considerably. The presence of remains of dinosaurs rules out any age later than the end of the Mesozoic. None of the bones have yet been identified, but the single lamellibranch shell (Plate I B, C) has been examined by Mr. N. Morris of the Department of Palaeontology in the Natural History section of the British Museum. He reported as follows: "The specimen is a thick-shelled bivalve with a heterodont style hinge. While it is not very well preserved and difficult to identify with certainty, it is apparently the first invertebrate remains found in this formation and therefore of some interest, particularly if it could furnish a reliable date to those beds. The teeth seem to place this specimen in the Arcticacea resembling those of Pronoella but the characteristic posterior carina of that genus is not present. Certain similarities to other groups may be discussed when all the characters are considered. Sphaeriola (Mactromyidae) has similar teeth but the shape is dissimilar. Superficial resemblances to the Triassic genera Trigonodus and Pachycardium cannot be substantiated in detail. The specimen cannot be identified as similar to any described species of these genera. The specimen most closely resembles various species of the Jurassic-Cretaceous genus Anisocardia. What remains of the teeth matches well with those of Anisocardia, the radial striations on the shell surface are to be found in some species of that genus, for instance A. elegans (Upper Jurassic of Europe), and finally, the shell shape is much like that of Anisocardia. The species of Anisocardia that are of most closely comparable shape are of Upper Jurassic and Lower Cretaceous age. The specimen also shows great similarity to the genus Saharella (MONGIN, 1963) from the continental intercalaire of the Hoggar Massif, said to be of Lower Cretaceous Age. Mademoiselle Mongin includes this genus in the Unionidae but because of the dentition I regard this as rather unlikely. Saharella could well be a non-marine derivative of the Arcticidae, possibly closely related to Anisocardia." His conclusion that this one shell could be Upper Jurassic or Lower Cretaceous is consistent with the field relationships of the Gokwe Formation, and with the occurrence in them of bones of large reptiles. More than a broad correlation with other post-Karroo-pre-Kalahari formations in Rhodesia is not yet possible. Two areas of "Cretaceous" rocks are shown on the 1960 edition of the 1:1,000,000 geological map of Rhodesia. One of

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PLATE I

A. Reptilian teeth of two types from the Gokwe Formation. Specimens in collection of Geology Department, University College of Rhodesia. B. Exterior of? Anisocardia sp. from the calcareous member of the Gokwe Formation. C. Interior of? Anisocardia sp. Same specimen as in B.

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them is in the extreme southeast and the other in the northeast of the country. Neither has yet yielded any fossils. The post-Karroo sediments of the Kadsi River area in the Zambezi Valley described by ARMSTRONO et al. (1967), which yielded the bones of large sauropods recorded by RAATH (1967), may well be of approximately the same age as the Gokwe Formation. The rarity of fossiliferous strata of this age in the whole of southern Africa is shown by the reference in HAUGHTON (1969) to a single occurrence in Namaqualand. This rarity of inland deposits is in marked contrast to their abundance in the coastal region. Marine Cretaceous rocks occur along much of the southern Africa coast. DETAILS OF THE SUCCESSION

In a measured section at the southwestern corner of the map (Fig.l) 120 ft. of the calcareous member is almost continuously exposed. At the base a few feet of conglomerate occur in a sandy matrix with a calcite cement. The pebbles include agates derived from the Karroo Basalts. The rest of the section is predominantly calcareous coarse sandstones, some with pebbles ranging up to 0.5 inch diameter scattered through the rock. Some beds are conspicuously current bedded. At intervals throughout the section there are thin pisolitic impure limestones in beds from 6 inches to 6 ft. in thickness. At least seven such beds occur. They can be traced horizontally for some distance, but none seems to persist for more than a mile or two. In thin section these rocks consist of scattered "pisolites" set in a coarsely crystalline clear calcite matrix, in which numerous well rounded grains of quartz and fresh feldspar occur. The "pisolites" themselves have no radial or concentric structure and consist of a brown-stained mosaic of carbonate minerals. They have no recognisable nucleus, but contain small scattered quartz grains. Most of the "pisolites" have darkened rims, from which the clear calcite cement often radiates (Plate II A). The absence of internal structure and the presence of the dark brown rims suggest that these are rolled clasts and not true pisolites, having been formed from the break up by current action of a previously deposited limestone. Seven miles to the east of this measured section the top part of the calcareous member consists of about 50 ft. of red sandstone with a calcareous cement. These rocks are conspicuous and extend over the greater part of the area mapped. They are well bedded and predominantly fine grained. Near the base is a hard band of pink sandstone with vertical tubes some 6 inches long and 0.25 inch in diameter. The tubes are lined with black material with white crystalline calcite in the centre. They appear to be burrows, and the bedding plane upon which they open is covered in tracks, some with meniscus-like fillings which appear to have been made by worms. No other fossils have been found in this bed. The white sandstone member, with a maximum thickness of 130 ft. follows and it is the basal portion of this member which contains the most bone. Palaeogeography, Palaeoclimatol., Palaeoecol., 8 (1970) 313-327

P L A T E II

A. "Pisolitic" limestone from the calcareous m e m b e r of the G o k w e F o r m a t i o n . Crossed polars; x 17. B. R o u n d e d a n d s u b r o u n d e d q u a r t z grains with frosted surface textures, from insoluble residue of "pisolitic" limestone, G o k w e F o r m a t i o n . Oblique incident light; grains with no m o u n t i n g med i u m ; x 17. C. S a n d s t o n e f r o m the white s a n d s t o n e m e m b e r of the G o k w e F o r m a t i o n . Crossed polars; ~ 17. D. S a n d s t o n e f r o m the calcareous m e m b e r of the G o k w e F o r m a t i o n . Crossed polars; > 17.

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Coarse clay pellet sandstones with current bedding in units up to 2 ft. thick are common near the base. Thin sections show no sign of calcite cement. The rocks consist of quartz, flesh microcline, rare zircon and tourmaline, with a few muscovite flakes. The matrix is white clay. The sorting is good and the grains are subrounded to rounded. Red clay pellets are abundant. The top 70 ft. consists of white sandstone of similar kind without conspicuous clay pellets (Plate II D). Bone fragments become rarer towards the top. CONDITIONS OF DEPOSITION AND FOSSILIZATION

The original extent of this formation has not yet been determined, but enough is known on which to base some reasonable deductions about its mode of deposition. The sediments seem to have been laid down in a basin of restricted size, under a subaqueous environment. The sandstones are arkosic and the feldspars extremely fresh. The rocks are well bedded, and many of the sandstones are current bedded. W o r m casts and tracks have been seen on a bedding plane about 130 ft. above the base of the formation. They probably were laid down under shallow water lacustrine conditions; each bed was rapidly deposited. The well-marked bedding planes and frequent lithological changes indicate many diastems. Frequent clay pellet conglomerates suggest drying out of the surface of sedimentation. Bones occur more or less throughout the succession, but they are most abundant in the middle portion. The scattered and fragmentary nature of much of the bone allows further speculation about the manner of sedimentation. Small fragments are extremely abundant in most areas, but nowhere have we found even two vertebrae articulated together, and the chances of finding articulated limbs or complete skeletons seem poor. Carcases therefore probably were not transported whole into the lake and deposited with the sediment, or if they were, then sedimentation was intermittent and depended upon rare flash floods into the basin. More likely the animals died on a dry lake bed, and their bones were exposed to a period of desiccation. This situation would result in the cracking of the bone into the rectangular fragments which are so common. Later floods would scatter the fragments, roll them slightly, and incorporate them in sediment. A few large bones have been found. Some of them are shown on Plate I I I A, B. Most of their articular ends are badly weathered, and their surfaces are often veined with a network of rectangular cracks, making them fragile and PLATE III A. Distal end of femur of large sauropod from the lower part of the white sandstone member of the Gokwe Formation. (By courtesy of M.A. Raath, Curator of the Queen Victoria Museum, Salisbury.) B. Proximal end of femur of probably the same individual as A. (By courtesy of M.A. Raath.) Palaeogeography, Palaeoclimatol., Palaeoecol., 8 (1970) 313-327

PLATE III

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difficult to handle. This condition is reminiscent of the effect of exposure on bones of dead wildebeest in the Makarikari Lake basin of Botswana. The parallel may extend even to the intermittent mode of sedimentation in this basin of internal drainage, the high evaporation rate which results in long periods of drying out before the next flood brings its load of sediments, and a pH well above 7.0 which would favour the preservation of bone, but not plants. The almost perfect rounding of many of the larger quartz and quartzite grains of the Gokwe Sandstones could have been inherited from material derived from older sediments. The freshness and rounding of the feldspars is, however, a suggestion that they are first cycle grains which have been through aeolian transport before reaching their final resting place in this basin. The insoluble residues from the limestones contain abundant millet seed quartz grains with lightly frosted surfaces (Plate I I B ) . Such grains could well have been blown into the lake from arid surroundings. Sediments of the Gokwe Formation may have been laid down in a small shallow lake, carried by intermittent floods from a land surface surrounding the lake on which aeolian transport of sand took place. The climate at the time must have been predominantly semi-arid, and the lake subject to long periods of drying out. Limestones of the lower member may indicate periods of more continuous flooding. A single determination of carbon and oxygen isotope ratios has been made for us by Professor Mackenzie L. Keith of Pennsylvania State University. The results were: 613C: 7.86~o; 6180:5.61 ~o (relative to the Chicago P.D.B. standards). A firm interpretation based on a single determination of this kind would not be wise. Deficiency in a3C is due to fractionation by plants. Hence it can be used to supplement other evidence in determining whether a sediment is marine or nonmarine, and to some extent in indicating different non-marine environments. The information is consistent with a non-marine origin on the border between lacustrine and fluvial conditions. Inferences of much plant life and of wind transport of sand are not necessarily in conflict, since the vegetation may have been mainly swamp living plants. A shallow lake with abundant swamp plants might well be the ideal habitat for herbivorous sauropods, even though the surrounding country was arid and inhospitable. The northern limit of the lake has not been determined, and the deep erosion of the country north and northwest of Gokwe has probably destroyed evidence of its former presence. The southern and western limits of sedimentation are not yet known, though the second author has seen similar sandstones in the Lupani Valley a few miles east of Lake Alice, about 70 miles southwest of Gokwe. Small hills, capped by conglomerate, are near the road from Que Que to Gokwe a few miles before the drift across the Ifafe River 30 miles southeast of Gokwe. The conglomerate is poorly cemented, is only a few feet thick and its base lies at an altitude near 3,700 ft. The underlying rock is the fine red marly sandstone Palaeogeography, Palaeoclimatol., Palaeoeco[., 8 (1970) 313-327

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of the Upper Karroo, which here rests directly on granite of the Basement Complex. The conglomerate consists of well-rounded, potato-sized pebbles of quartzite, vein quartz, banded ironstone and agate. The agate can only have come from the Karroo Basalts, and the conglomerate cannot therefore belong to the Karroo System. It must be younger by at least one cycle of erosion. These conglomerates possibly are the shore-line facies of the Gokwe Formation, and may represent the eastern limit of the basin of deposition. These scattered observations suggest that the shallow lake may have had an east-west extent of at least 80 miles. In the north-south direction the indications are that it was greater than 70 miles. Dimple-like depressions of the same order of magnitude exist today on otherwise stable shield areas in central Africa, and are presently active depositories. Examples are Lake Bangweolu in Zambia, and Makarikari in Botswana. ACKNOWLEDGMENTS

We wish to thank Dr. J. W. Wiles, Director of the Geological Survey of Rhodesia, for having our map redrawn in the Survey Drawing Office, and Mr. J. D. Morgan, Chief Technician in the Department of Geology in the University College of Rhodesia, for taking most of the photographs which illustrate this paper. REFERENCES ARMSTRONG, D. C.. CASTELIN, C. A. and LOCKETT, N., 1967. Notes on the geology of the Kadsi River Area, mid-Zambezi Valley. Detritus, 2:11-17. BOND, G. and BROMLEV, K., 1967. Sediments with remains of large dinosaurs near Gokwe (Rhodesia). Proc. Rhodesian Sci. Congr., 1st, Bulawayo, 1967, Abstr., p.5. GEORGE, T. N. et al., 1967. Report of the Stratigraphical Code Sub-Committee. Proc. Geol. Soc. (London), 1638: 75-87. HAUGHTON, S. H., 1969. Geological History of Southern Africa. Cape and Transvaal Printers, Cape Town, 535 pp. HEr)BERG, H. D. (Editor), 1961. Stratigraphic classification and terminology, hTtern. Geol. Congr., 1st, Copenhagen, 1960, Abstr., p.25. MACGREGOR, A. M., 1941. Geology of the Mafungabusi Gold Belt. Geol. Surv. S.R., 35: 1-26. MOLYNEUX, A. J. C., 1921. Karroo rocks in the Mafungabusi, southern Rhodesia. S. AJ?iea J. Sci., 18(2): 1-10. PARSONS, C. E., 1903. Notes on a geological section from Gwelo to the Zambesi River. Proc. Rhodesian Sci. Assoc., 4: 48-52. RAATH, M. A., 1967. Notes on the occurrence of fossil bone in the Sipolilo-Musengezi area, Rhodesia. Detritus, 2: 18-20.

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