Sedimentology of a vertebrate bone-bearing bed in the Triassic ...
Sedimentology of a vertebrate bone-bearing bed in the Triassic Fremouw Formation at Gordon Valley, Beardmore Glacier region, Antarctica L.A. KRISSEK and T.C. HORNER Byrd Polar Research Center
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Department of Geological Sciences Ohio State University Columbus, Ohio 43210
In the central Transantarctic Mountains, the Fremouw Formation forms a lower to middle Triassic sequence of sandstones and siltstones/mudstones that were deposited in fluvial channels and associated overbank environments (Barrett, Elliot, and Lindsay 1986; Collinson and Isbell 1986; Isbell and MacDonald,
Antarctic Journal, this issue). At several localities in the central
Transantarctic Mountains, the lower Fremouw Formation has yielded vertebrate fossils of the Lystrosaurus biostratigraphic zone (Colbert 1982; Hammer and Cosgriff 1981; Hammer et al. 1986; Hammer 1989). Late in the 1985-1986 field season, however, an additional bone-bearing horizon was discovered in the basal portion of the upper Fremouw Formation at Gordon Valley (Hammer et al. 1986); specimens recovered from that horizon were subsequently identified as characteristic of the late Early Triassic Cynognathus zone or slightly younger (Hammer, Ryan, and DeFauw 1987; Hammer 1988; Hammer, Collinson, and Ryan 1990). Because the Gordon Valley locality was not adequately collected in 1985-1986, and because it is the only known occurrence of the Cynognathus fauna in Antarctica, this locality was visited by a team of vertebrate paleontologists for further collecting during the 1990-1991 field season (see Hammer et al., Antarctic Journal, this issue for details). At the same time, we spent approximately 2 weeks (19 December 1990 to 1 January 1991) examining the depositional setting of the bone-bearing horizon in detail, to evaluate the effects of transport and deposition on the vertebrate fauna recovered at this site. Because of its position at the base of the upper Fremouw Formation, the Gordon Valley vertebrate horizon is located at
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io Figure 1. Bone-bearing conglomerate (center of photograph) overlying discontinuous, trough cross-stratified sandstone (white zone to left of top of Jacobs staff) and greenish-gray siltstone (lower right corner of photograph) in the upper Fremouw Formation at Gordon Valley. Staff is 1.5 meters long. 1991 REVIEW
17
the transition from the mudstone-dominated middle Fremouw Formation to the sandstone-dominated upper Fremouw Formation. The local stratigraphic section at the Gordon Valley site reflects this lithologic change, with the bone-bearing horizon underlain by a greenish-gray siltstone to sandy siltstone; this fine-grained facies contains moderate amounts of simple root traces, similar to those that have been described elsewhere as common to abundant in the Fremouw Formation (Barrett et al. 1986; Homer and Krissek, Antarctic Journal, this issue). The siltstone is overlain discontinuously by thin, trough cross-stratified medium sands that fill low-relief channel-forms. A thin, matrix-supported, siltstone-cobble conglomerate blankets both the discontinuous sands and the intervening siltstones above a sharp, slightly irregular contact that shows no evidence of large-scale scouring (figure 1). Clasts in the conglomerate are predominantly rounded, and composed of greenish-gray siltstone that is lithologically identical to the underlying siltstone (figure 2). The conglomerate is overlain gradationally by a thick sequence of white, medium-grained, trough cross-stratified sandstones, which are characteristic of the upper Fremouw Formation (Barrett et al. 1986; Collinson and Isbell 1986; Isbell and MacDonald, Antarctic Journal, this issue). Vertebrate bones and wood are dispersed throughout the lower sandstones and the conglomerate, but are significantly
more abundant in the latter lithology. Elongate bones and wood within the conglomerate show no preferred orientation, whereas the cross-bedding in the sands indicates the northwesterly paleoflow characteristic of the Fremouw Formation (Barrett et al. 1986; Collinson and Isbell 1986; Isbell and MacDonald, Antarctic Journal, this issue). The composition, fabric, texture, and sheet-form geometry of the bone-bearing conglomerate, the characteristics of its basal contact, and the absence of preferred orientation of its elongate clasts all indicate rapid deposition by a large, unchannelized, high viscosity, low-to-moderate velocity flow (i.e., a relatively fluid debris flow). This large flow spread laterally from an unidentified large channel across a floodplain that contained low-relief channels, transporting bones, wood, and clasts of floodplain siltstones from further upstream. This multistage transport history accounts for the mixed nature of the vertebrate fauna described by Hammer (1988). This research was supported by National Science Foundation grant DPP 88-17023 (W.R. Hammer, Augustana College) through a subcontract to J.W. Collinson (Ohio State University). We acknowledge their assistance in involving us in this project. We also acknowledge the logistical assistance of VXE-6, and of the Antarctic Services Associates and Helicopters New Zealand personnel at the Beardmore South camp.
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I Figure 2. View of large siltstone clasts, partially clast-supported, in vertebrate bone-bearing conglomerate of upper Fremouw Formation at Gordon Valley. Shading on staff is at 10-centimeter intervals. 18
ANTARCTIC JOURNAL
References Barrett, P.J., D. H. Elliot, and J.E Lindsay. 1986. The Beacon Supergroup (Devonian-Triassic) and Ferrar Group (Jurassic) in the Beardmore Glacier area, Antarctica. In M.D. Turner and J.F. Splettstoesser (Eds.), Geology of the central Transantarctic Mountains. (Antarctic Research Series, Vol. 36.) Washington, D.C.: American Geophysical Union. Colbert, E.H. 1982. Triassic vertebrates in the Transantarctic Mountains. In M.D. Turner and J.F. Splettstoesser (Eds.), Geology of the central Transantarctic Mountains. (Antarctic Research Series, Vol. 36.) Washington, D.C.: American Geophysical Union. Collinson, J.W., and J.L. Isbell. 1986. Permian-Triassic sedimentology of the Beardmore Glacier region. Antarctic Journal of the U.S., 21(5), 29-30. Hammer, W.R. 1988. The Cynognathus zone (late Early Triassic) vertebrate fauna from Antarctica. Antarctic Journal of the U.S., 23(5), 10-11. Hammer, W.R. 1989. Lystrosaurus Zone (Triassic) vertebrates from the Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S., 24(5), 40-41. Hammer, W.R., J.W. Collinson, and W.J. Ryan. 1990. A new Triassic
Therapsids, temnospondyls, and dinosaurs from the Fremouw and Falla formations, Beardmore Glacier region, Antarctica WILLIAM R. HAMMER, WILLIAM J. HICKERSON, and STEPHEN KRIPPNER Department of Geology Augustana College Rock island, Illinois 61201
JEFFREY TAMPLIN Department of Zoology Louisiana State University Baton Rouge, Louisiana 70803
Since the first discovery of Triassic fossil tetrapods at Graphite Peak in 1967 (Barrett, Baillie, and Colbert 1968), six expeditions have searched for terrestrial vertebrate remains in the Transantarctic Mountains (Hammer, Collinson, and Ryan 1990; Hammer, Ryan, and DeFauw 1987). Until the 1985-1986 season, only the lower Fremouw Formation of the Central Transantarctic Mountains near the Beardmore and Shackleton Glaciers had yielded fossils of this type (Colbert 1982; Hammer and Cosgriff 1981). During the 1985-1986 austral summer, a new, younger fauna of vertebrates was found in the upper Fremouw Forma tion of the Gordon Valley (Beardmore Glacier region). A third assemblage of even younger terrestrial animals was recently discovered in the Falla Formation on Mount Kirkpatrick near the Beardmore Glacier in 1990-1991. This new collection, which 1991 REVIEW
vertebrate fauna from Antarctica and its depositional setting. Antarctic Science, 2(2), 163-167 Hammer, W.R., and J.W. Cosgriff. 1981. Myosaurus gracilus, an anomodont reptile from the Lower Triassic of Antarctica and South America. Journal of Paleontology, 55(2), 410-424. Hammer, W.R., W.J. 1-lickerson, S. Krippner, and I. Tamplin. 1991. Therapsids, temnospondyls, and dinosaurs from the Fremouw and Falla formations, Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S., 26(5). Hammer, W.R., W.J. Ryan, and S.L. DeFauw. 1987 Comments on the vertebrate fauna from the Fremouw Formation (Triassic), Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S., 22(5), 32-33. Hammer, WR., WI. Ryan, J.W. Tamplin, and S.L. DeFauw. 1986. New vertebrates from the Fremouw Formation (Triassic), Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S., 21(5), 24-26. Homer, T.C., and L.A. Krissek. 1991. Permian and Triassic paleosols from the Beardmore Glacier region, Antarctica. Antarctic Journal of the U. S., 26(5). Isbell, J.L., and D.I.M. Macdonald. 1991. Lithofacies analysis of the Triassic Fremouw Formation at the Gordon Valley vertebrate site, Antarctica. Antarctic Journal of the U.S., 26(5).
includes dinosaur material, represents only the third terrestrial vertebrate fauna known from the Transantarctic Mountains and the antarctic mainland. Because the upper Fremouw Formation vertebrates were found near the end of the 1985-1986 field season, a field party of six (including the four authors of this paper) returned to the Gordon Valley during the 1990-1991 season to finish excavation of the remaining fossils and, in conjunction with a sedimentological team from Ohio State University, investigate the taphonomic and paleoecological setting of the upper Fremouw. During the first month of the 1990-1991 season over 100 specimens were collected from two sites in the Gordon Valley, the original locality excavated during the 1985-1986 season and a new one about 8 kilometers away at the same stratigraphic level. Taphonomic conditions have left these specimens largely scattered and fragmentary, however, excellent additional material from taxa collected in 1985-1986 (see Hammer 1990) was recovered. Fossils representing new taxa from these sites include cranial and postcranial material of smaller temnospondyls and therapsids, including at least one small dicynodont. Taphonomic data were taken at the localities and maps showing bone orientations on the bedding plane surfaces were made in the field. The Falla Formation vertebrates were first discovered late in December 1990, by a team of petrologists headed by David Elliot of Ohio State University and Richard Hanson of Texas Christian University. The fossils were mainly concentrated in one small area of siltstone exposure on Mount Kirkpatrick at about 4,500 meters above sea level. Of the over 50 bones collected, only 3 were not from this single concentration. The bones are very well preserved, large, and include much of the skeleton of one dinosaur and, perhaps, portions of a second. At least one of the three isolated postcranial bones is from yet another smaller animal. A "Megalosaurus" type theropod tooth was found among the initial bones removed. Since most of the fossils are still largely encased in rock, the number and type of animals represented is not entirely certain at this point. 19
and
Department of Geological Sciences Ohio State University Columbus, Ohio 43210
In the central Transantarctic Mountains, the Fremouw Formation forms a lower to middle Triassic sequence of sandstones and siltstones/mudstones that were deposited in fluvial channels and associated overbank environments (Barrett, Elliot, and Lindsay 1986; Collinson and Isbell 1986; Isbell and MacDonald,
Antarctic Journal, this issue). At several localities in the central
Transantarctic Mountains, the lower Fremouw Formation has yielded vertebrate fossils of the Lystrosaurus biostratigraphic zone (Colbert 1982; Hammer and Cosgriff 1981; Hammer et al. 1986; Hammer 1989). Late in the 1985-1986 field season, however, an additional bone-bearing horizon was discovered in the basal portion of the upper Fremouw Formation at Gordon Valley (Hammer et al. 1986); specimens recovered from that horizon were subsequently identified as characteristic of the late Early Triassic Cynognathus zone or slightly younger (Hammer, Ryan, and DeFauw 1987; Hammer 1988; Hammer, Collinson, and Ryan 1990). Because the Gordon Valley locality was not adequately collected in 1985-1986, and because it is the only known occurrence of the Cynognathus fauna in Antarctica, this locality was visited by a team of vertebrate paleontologists for further collecting during the 1990-1991 field season (see Hammer et al., Antarctic Journal, this issue for details). At the same time, we spent approximately 2 weeks (19 December 1990 to 1 January 1991) examining the depositional setting of the bone-bearing horizon in detail, to evaluate the effects of transport and deposition on the vertebrate fauna recovered at this site. Because of its position at the base of the upper Fremouw Formation, the Gordon Valley vertebrate horizon is located at
-' IALI,
*
S lot
7 p
)
-
V
I
1'g
'%'N ak ARAIN
io Figure 1. Bone-bearing conglomerate (center of photograph) overlying discontinuous, trough cross-stratified sandstone (white zone to left of top of Jacobs staff) and greenish-gray siltstone (lower right corner of photograph) in the upper Fremouw Formation at Gordon Valley. Staff is 1.5 meters long. 1991 REVIEW
17
the transition from the mudstone-dominated middle Fremouw Formation to the sandstone-dominated upper Fremouw Formation. The local stratigraphic section at the Gordon Valley site reflects this lithologic change, with the bone-bearing horizon underlain by a greenish-gray siltstone to sandy siltstone; this fine-grained facies contains moderate amounts of simple root traces, similar to those that have been described elsewhere as common to abundant in the Fremouw Formation (Barrett et al. 1986; Homer and Krissek, Antarctic Journal, this issue). The siltstone is overlain discontinuously by thin, trough cross-stratified medium sands that fill low-relief channel-forms. A thin, matrix-supported, siltstone-cobble conglomerate blankets both the discontinuous sands and the intervening siltstones above a sharp, slightly irregular contact that shows no evidence of large-scale scouring (figure 1). Clasts in the conglomerate are predominantly rounded, and composed of greenish-gray siltstone that is lithologically identical to the underlying siltstone (figure 2). The conglomerate is overlain gradationally by a thick sequence of white, medium-grained, trough cross-stratified sandstones, which are characteristic of the upper Fremouw Formation (Barrett et al. 1986; Collinson and Isbell 1986; Isbell and MacDonald, Antarctic Journal, this issue). Vertebrate bones and wood are dispersed throughout the lower sandstones and the conglomerate, but are significantly
more abundant in the latter lithology. Elongate bones and wood within the conglomerate show no preferred orientation, whereas the cross-bedding in the sands indicates the northwesterly paleoflow characteristic of the Fremouw Formation (Barrett et al. 1986; Collinson and Isbell 1986; Isbell and MacDonald, Antarctic Journal, this issue). The composition, fabric, texture, and sheet-form geometry of the bone-bearing conglomerate, the characteristics of its basal contact, and the absence of preferred orientation of its elongate clasts all indicate rapid deposition by a large, unchannelized, high viscosity, low-to-moderate velocity flow (i.e., a relatively fluid debris flow). This large flow spread laterally from an unidentified large channel across a floodplain that contained low-relief channels, transporting bones, wood, and clasts of floodplain siltstones from further upstream. This multistage transport history accounts for the mixed nature of the vertebrate fauna described by Hammer (1988). This research was supported by National Science Foundation grant DPP 88-17023 (W.R. Hammer, Augustana College) through a subcontract to J.W. Collinson (Ohio State University). We acknowledge their assistance in involving us in this project. We also acknowledge the logistical assistance of VXE-6, and of the Antarctic Services Associates and Helicopters New Zealand personnel at the Beardmore South camp.
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I Figure 2. View of large siltstone clasts, partially clast-supported, in vertebrate bone-bearing conglomerate of upper Fremouw Formation at Gordon Valley. Shading on staff is at 10-centimeter intervals. 18
ANTARCTIC JOURNAL
References Barrett, P.J., D. H. Elliot, and J.E Lindsay. 1986. The Beacon Supergroup (Devonian-Triassic) and Ferrar Group (Jurassic) in the Beardmore Glacier area, Antarctica. In M.D. Turner and J.F. Splettstoesser (Eds.), Geology of the central Transantarctic Mountains. (Antarctic Research Series, Vol. 36.) Washington, D.C.: American Geophysical Union. Colbert, E.H. 1982. Triassic vertebrates in the Transantarctic Mountains. In M.D. Turner and J.F. Splettstoesser (Eds.), Geology of the central Transantarctic Mountains. (Antarctic Research Series, Vol. 36.) Washington, D.C.: American Geophysical Union. Collinson, J.W., and J.L. Isbell. 1986. Permian-Triassic sedimentology of the Beardmore Glacier region. Antarctic Journal of the U.S., 21(5), 29-30. Hammer, W.R. 1988. The Cynognathus zone (late Early Triassic) vertebrate fauna from Antarctica. Antarctic Journal of the U.S., 23(5), 10-11. Hammer, W.R. 1989. Lystrosaurus Zone (Triassic) vertebrates from the Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S., 24(5), 40-41. Hammer, W.R., J.W. Collinson, and W.J. Ryan. 1990. A new Triassic
Therapsids, temnospondyls, and dinosaurs from the Fremouw and Falla formations, Beardmore Glacier region, Antarctica WILLIAM R. HAMMER, WILLIAM J. HICKERSON, and STEPHEN KRIPPNER Department of Geology Augustana College Rock island, Illinois 61201
JEFFREY TAMPLIN Department of Zoology Louisiana State University Baton Rouge, Louisiana 70803
Since the first discovery of Triassic fossil tetrapods at Graphite Peak in 1967 (Barrett, Baillie, and Colbert 1968), six expeditions have searched for terrestrial vertebrate remains in the Transantarctic Mountains (Hammer, Collinson, and Ryan 1990; Hammer, Ryan, and DeFauw 1987). Until the 1985-1986 season, only the lower Fremouw Formation of the Central Transantarctic Mountains near the Beardmore and Shackleton Glaciers had yielded fossils of this type (Colbert 1982; Hammer and Cosgriff 1981). During the 1985-1986 austral summer, a new, younger fauna of vertebrates was found in the upper Fremouw Forma tion of the Gordon Valley (Beardmore Glacier region). A third assemblage of even younger terrestrial animals was recently discovered in the Falla Formation on Mount Kirkpatrick near the Beardmore Glacier in 1990-1991. This new collection, which 1991 REVIEW
vertebrate fauna from Antarctica and its depositional setting. Antarctic Science, 2(2), 163-167 Hammer, W.R., and J.W. Cosgriff. 1981. Myosaurus gracilus, an anomodont reptile from the Lower Triassic of Antarctica and South America. Journal of Paleontology, 55(2), 410-424. Hammer, W.R., W.J. 1-lickerson, S. Krippner, and I. Tamplin. 1991. Therapsids, temnospondyls, and dinosaurs from the Fremouw and Falla formations, Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S., 26(5). Hammer, W.R., W.J. Ryan, and S.L. DeFauw. 1987 Comments on the vertebrate fauna from the Fremouw Formation (Triassic), Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S., 22(5), 32-33. Hammer, WR., WI. Ryan, J.W. Tamplin, and S.L. DeFauw. 1986. New vertebrates from the Fremouw Formation (Triassic), Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S., 21(5), 24-26. Homer, T.C., and L.A. Krissek. 1991. Permian and Triassic paleosols from the Beardmore Glacier region, Antarctica. Antarctic Journal of the U. S., 26(5). Isbell, J.L., and D.I.M. Macdonald. 1991. Lithofacies analysis of the Triassic Fremouw Formation at the Gordon Valley vertebrate site, Antarctica. Antarctic Journal of the U.S., 26(5).
includes dinosaur material, represents only the third terrestrial vertebrate fauna known from the Transantarctic Mountains and the antarctic mainland. Because the upper Fremouw Formation vertebrates were found near the end of the 1985-1986 field season, a field party of six (including the four authors of this paper) returned to the Gordon Valley during the 1990-1991 season to finish excavation of the remaining fossils and, in conjunction with a sedimentological team from Ohio State University, investigate the taphonomic and paleoecological setting of the upper Fremouw. During the first month of the 1990-1991 season over 100 specimens were collected from two sites in the Gordon Valley, the original locality excavated during the 1985-1986 season and a new one about 8 kilometers away at the same stratigraphic level. Taphonomic conditions have left these specimens largely scattered and fragmentary, however, excellent additional material from taxa collected in 1985-1986 (see Hammer 1990) was recovered. Fossils representing new taxa from these sites include cranial and postcranial material of smaller temnospondyls and therapsids, including at least one small dicynodont. Taphonomic data were taken at the localities and maps showing bone orientations on the bedding plane surfaces were made in the field. The Falla Formation vertebrates were first discovered late in December 1990, by a team of petrologists headed by David Elliot of Ohio State University and Richard Hanson of Texas Christian University. The fossils were mainly concentrated in one small area of siltstone exposure on Mount Kirkpatrick at about 4,500 meters above sea level. Of the over 50 bones collected, only 3 were not from this single concentration. The bones are very well preserved, large, and include much of the skeleton of one dinosaur and, perhaps, portions of a second. At least one of the three isolated postcranial bones is from yet another smaller animal. A "Megalosaurus" type theropod tooth was found among the initial bones removed. Since most of the fossils are still largely encased in rock, the number and type of animals represented is not entirely certain at this point. 19
