[ Biostratigraphy of the Permian Standard Section ]
JOHN M. MILLER, Ph.D.
University and Jepson HerbariaRoom 1001, Valley Life Sciences Building 2465University of California, BerkeleyBerkeley, California, USA 94720-2465
Folded and overthrust mountain belts of southwestern North America yield classic exposures of a nearly complete sequence of Paleozoic rocks. Exposures consist of uplifted and overthrusted Devonian and Pennsylvanian rocks of the Marathon Fold Belt; and Permian sediments of the Del Norte and Glass Mountains. Sedimentary beds in the Del Norte and Glass Mountains comprise the standard North American type section through rocks of Permian age.
While the invertebrate fauna of the Glass Mountains is well known from detailed studies by Cooper and Grant (1972) and Olszewski and D. H. Erwin (2009); the transitional (deltaic) and marginally marine depositional environments of the Del Norte Mountains, which contain Permian (Leonardian) gigantopterid plant and bellerophontacean gastropod megafossils assignable to Cymatospira or Patellilabia (Mamay et al. 1984), are less well understood.
There are significant similarities of the Permian Del Norte Mountains flora with South American paleofloras of the Permian Palmarito Formation (Ricardi et al. 2004), the Venezuelan Carache Formation (Ricardi-Branco 2008), and Leonardian florules of Mexico (Weber 1997). The Del Norte Mountains Leonardian florule while low in biodiversity and otherwise similar to an older coastal Permian flora from the Abo Formation (DiMichele et al. 2007), is not dominated by walchian conifers and Supaia-like peltaspermaleans.
The late Sergius H. Mamay, Ph.D. of the United States National Museum is pictured above standing on the fossiliferous upper members of the Leonardian Cathedral Mountains Formation of the Del Norte Mountains of southwestern North America.
Cretaceous limestones overtopping a 200 m thick bed of limestone pebbles, dolomite, quartz, and shales comprising the Triassic Bissett Conglomerate (Jurassic rocks are eroded away); together with underlying Permian conglomerates, limestones, shales, and siltstones; and Paleogene volcanic intrusions, are prevalent in the rugged Del Norte Mountains of North America, including significant deposits of lead and hematite that have been mined in the last century (Barnes 1982).
Permian rocks of the Del Norte Mountains include marine and transitional, deltaic sediments of the Permian Wolfcampian, Leonardian, Wordian, Guadalupian, and Ochoan Ages (Wardlaw 2000).
Glass and Del Norte Mountains rocks yield gymnospermous fossils in several areas including Units 5 and 6 of Section IV, uppermost Cathedral Mountain Formation (Rohr et al. 1987, Wardlaw et al. 1990), and in several other isolated stations in this mountainous region (C. N. Miller and Brown 1973, Mamay et al. 1988). Dating of the layers is supported by micropaleontological evidence from conodonts (Wardlaw et al. 1990, Wardlaw 2000) and fusulinids (Yang and Yancey 2000).
The image to the left is a rock slab exfoliating from the Upper Members of the Cathedral Mountain Formation. A private mapping party in 1981 discovered this slab along a wild game trail in the Del Norte Mountains. The in situ slab pictured to the left shows overlapping leaf compressions of Delnortea abbottiae and Taeniopteris. The kodachrome below to the left is of two fossilized seeds, assignable to Cordaicarpus (Mamay et al. 1984), which are virtually identical to platyspermic seeds from the South American Palmarito Formation, also associated with Delnortea leaves (page 82, Figure 6, Ricardi et al. 2004).
Pictured above and the right is a nearly complete fossil leaf of the holotype of Delnortea abbottiae (USNM 364416), photographed by the author a few days after the fossil was unearthed from beds of the Lower Permian Cathedral Mountain Formation, Del Norte Mountains, southwestern North America.
Large leaf compressions and permineralizations of Lower Permian (Leonardian) plants were described about 20 years ago (Mamay et al. 1986, 1988). A preliminary biostratigraphic study of the fossiliferous layers by Rohr et al. (1987) resulted in mapping of Section IV, Units 5 and 6, which were later assigned to the uppermost Cathedral Mountains Formation by Wardlaw (1990) based on his detailed conodont studies of the North American Permian type section.
Mamay's suggestion that the stratigraphic occurrence of Delnortea in Upper Leonardian rocks of the Cathedral Mountain Formation may lead to a better understanding of Permian floral zones is supported by discovery of Delnortea from the Artinskian of northwestern South America (Ricardi et al. 1999).
A scanning electron micrograph on this page shows the arrangement of permineralized tracheids and ray parenchyma cells of a fragment from the Dadoxylon log (see image to the left) found exfoliating from graded conglomerates in Unit 6. Distribution of rounded chert pebbles up to two centimeters in diameter, sands, and fines in the conglomerate, suggest a much higher energy depositional environment than the fine-grained mudstone comprising Unit 5 (Rohr et al. 1987).
The notion of Delnortea as a widespread and common Pangaean floristic element of the Lower Permian of North America (Mamay et al. 1984) is supported by a startling discovery of Delnortea leaf fragments and other plant fossils in core samples of the Permian, Leonardian, Clear Fork Dolomite recovered from three wells drilled more than 2000 meters deep through the thick sediments of southwestern North America (DiMichele et al. 2000).
Tropical, summer wet, terrestrial biomes of the early Permian Period contained innovative and unusual xeromorphic seed plant assemblages (DiMichele et al. 2004). The widely separated Delnortea-dominated floras reported by Mamay et al. (1984) and Ricardi et al. (1999) are examples of pervasive seed plant associations that might reflect long-term stasis in Permian terrestrial paleoenvironments.
Taphonomic studies of the delnortea beds (Unit 5) and log bed (Unit 6) of Section IV are needed to better understand paleoenvironments of the Cathedral Mountains Formation in relation to western Pangaean subtropical coastal landscapes (DiMichele et al. 2007, Ricardi-Branco 2008) that existed during the Permian Period more than 260 million years ago.
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DiMichele, W. A., D. S. Chaney, W. J. Nelson, S. G. Lucas, C. V. Looy, K. Quick, and W. Jun. 2007. A low diversity, seasonal tropical landscape dominated by conifers and peltasperms: early Permian Abo Formation, New Mexico. Review of Palaeobotany and Palynology 145(3-4): 249-273.
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Yang, Z. and T. E. Yancey. 2000. Fusulinid biostratigraphy and paleontology of the Middle Permian (Guadalupian) strata of the Glass Mountains and Del Norte Mountains, West Texas. Pp. 185-259 In: B. R. Wardlaw, R. E. Grant, and D. M. Rohr, (eds.), The Guadalupian Symposium. Smithsonian Contributions to Earth Sciences Number 32. Washington D. C.: Smithsonian Institution Press.
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