GEOLOGICAL NOTES “Tertiary Dinosaurs” in the Nanxiong Basin, Southern China, Are Reworked from the Cretaceous Brenda J. Buck, Andrew D. Hanson,1 Richard A. Hengst,2 and Hu Shu-sheng3 Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154-4010, U.S.A. (e-mail: buckb@unlv.nevada.edu) ABSTRACT The Nanxiong and Shanghu Formations of southeastern China span the KT boundary interval and preserve an important paleontological record. A 1285-m section across the KT boundary was measured, and lithological units were documented and sampled. Strata were deposited in alluvial fan/playa mudflat environments in a highly seasonal, semiarid climate. Previous workers placed the KT boundary at the Nanxiong/Shanghu formational contact. The uppermost Nanxiong and lowermost Shanghu Formations contain an assemblage of dinosaur egg fragments and Tertiary fossils, which led to the notion that dinosaurs survived into the Paleocene. On the basis of our results, we argue that the mixed KT assemblage is a result of debris flows reworking Cretaceous fossils. Depositional environments and paleoclimate did not change significantly across the KT boundary. Introduction 1987; Bajpai and Prasad 2000), and the Nanxiong basin in China (Sloan 1987; Rigby et al. 1993; Buck et al. 1994; Stets et al. 1996). Later, workers in Montana, New Mexico, Bolivia, and India refuted the notion of Paleocene dinosaurs by documenting evidence of reworking of Cretaceous fossils during the Paleocene (Retallack et al. 1986; Lucas et al. 1987; Eaton et al. 1989; Cappetta 1990; Lofgren et al. 1990; Courtillot et al. 2000). However, to date, no one has presented evidence refuting the presence of Tertiary dinosaurs in the Nanxiong basin. The main focus of this article is to examine paleontological data from the Nanxiong basin in light of the sedimentology and to examine evidence of climatic conditions preserved within strata that span the KT boundary. Red beds within the Nanxiong basin are divided into two formations: the Nanxiong and Shanghu Formations (fig. 1). The Nanxiong Formation lies nonconformably on Jurassic granite (Stets et al. 1996) and is conformably overlain by the Shanghu Formation. Historically, the contact between the Nanxiong and Shanghu Formations was defined by the last occurrence of dinosaur fossils, and this usage is maintained herein in order to allow comparison with previous studies. Previous workers ac- Bolide impacts and climatic change are two of the main processes implicated in major mass extinctions. Probably the best-known extinction occurred at the KT boundary. Alvarez et al. (1980) hypothesized that dinosaurs and other terrestrial and marine life-forms became extinct as the result of an extraterrestrial impact. Recognition of the Late Cretaceous Chicxulub structure near the Yucatan Peninsula of Mexico has fueled support for this theory (Hildebrand et al. 1991; Sharpton et al. 1992; Kring and Durda 2002). However, some workers have suggested that mass extinctions associated with the KT boundary were not instantaneous events but rather were prolonged events related to changes in climate and that dinosaurs survived into the Paleocene (e.g., Stets et al. 1996). “Tertiary dinosaurs” have been postulated in Montana (Sloan et al. 1986; Rigby et al. 1987), New Mexico (Fassett et al. 1987), Bolivia (Van Valen 1988), India (Mathur Manuscript received November 27, 2002; accepted May 29, 2003. 1 E-mail: andrew.hanson@ccmail.nevada.edu. 2 Department of Biological Sciences, Purdue University North Central, Westville, Indiana 46391, U.S.A. 3 Beijing Natural History Museum, 126 Tian Qiao South Street, Beijing 100050, China. [The Journal of Geology, 2004, volume 112, p. 111–118] ! 2004 by The University of Chicago. All rights reserved. 0022-1376/2004/11201-0007$15.00 111 112 B. J. BUCK ET AL. Figure 1. Location and distribution of Late Mesozoic and Early Tertiary formations in the Nanxiong basin (adapted from Stets et al. 1996). cepted the contact between the Nanxiong and Shanghu Formations as the KT boundary. The published reports of Tertiary dinosaurs are based on conflicting pollen, vertebrate, ostracod, and magnetostratigraphic data (fig. 2; Sloan 1987; Rigby et al. 1993; Buck et al. 1994; Stets et al. 1996). Tertiary fossils have been found in the upper 101 m of the Nanxiong Formation (Stets et al. 1996), and we therefore consider this part of the Nanxiong to be Tertiary in age. We measured a 1285-m section across the KT boundary northeast of Nanxiong (fig. 2). In particular, we studied the sedimentology and depositional environments in order to address issues of possible reworking. We also examined the paleosol record and the compositional maturity of clastic sand in order to address issues related to paleoclimate. KT Boundary Relationships Nanxiong Formation. On the basis of dinosaur, ostracod, and charophyte fossils, pollen analysis, and magnetostratigraphy, most of the Nanxiong Formation is Late Cretaceous (Chen 1986; Zhang 1992; Stets et al. 1996). In addition, three 40Ar/39Ar whole-rock dates from separate basalt flows that crop out stratigraphically below our measured section yield a maximum age of 66.7 " 0.3 Ma (Rigby et al. 1993; Buck et al. 1994). The preservational quality of fossil bones within the Nanxiong red beds is relatively poor, and many bones appear to have been splintered and desiccated before burial. However, dinosaur eggshells and trackways are common and are found throughout much of the Nanxiong Formation (Zhao et al. 1991, 2002; Rigby et al. 1993; Hansen et al. 1994; Stets et al. 1996). The first Tertiary palynomorphs, which are mixed with Cretaceous palynomorphs, appear 101 m below the top of the Nanxiong Formation (fig. 2; Stets et al. 1996). This mixed assemblage changes to one composed entirely of Tertiary palynomorphs 80 m below the Nanxiong/Shanghu contact (fig. 2). Zhao et al. (2002) measured Iridium (Ir) anomalies in the Nanxiong Formation and identified maximum concentrations of Ir that coincide with the first appearance of exclusively Tertiary pollen and suggested moving the KT boundary to this lower position. In addition, a Tertiary Asian lizard occurs in the upper 80 m of the Nanxiong Formation (Sloan 1987) and coincides with the occurrence of the exclusively Tertiary pollen assemblage mentioned earlier (Stets et al. 1996). Dinosaur eggshells and fragments, as well as Cretaceous ostracods, continue throughout the upper 80 m of the Nanxiong Formation and have been used to suggest that this portion of the Nanxiong Formation is Cretaceous (Zhang 1992). These data require one of the following interpretations: (1) Tertiary age determinations based on the pollen and lizard species in the Nanxiong Formation are incorrect, and all of the Nanxiong Formation is Cretaceous in age; (2) pollen and lizard ages in the Nanxiong Formation are correctly identified as Tertiary, and the Cretaceous ostracod and dinosaur faunas are actually Tertiary in age; or (3) Cretaceous ostracod and dinosaur fossils found in association with Tertiary fossils are reworked, and the KT boundary should be moved to a stratigraphic position coincident with the first appearance of Tertiary pollen. Shanghu Formation. The Shanghu Formation is Early Paleocene in age on the basis of the presence of fossil mammals (Sloan 1987; Lucas and Williamson 1995; Wang et al. 1998), insects (Wang and Zhai 1995), conchostracans and charophytes (Chen 1986; Huang 1988), and pollen (Stets et al. 1996). In addition, a 206Pb/238U date of 62.2 " 0.8 Ma obtained on zircons from an air-fall tuff high in the formation supports the Paleocene age derived from fossils in the formation (Rigby et al. 1993). Ostracods in the lowermost 76 m of the Shanghu Formation are composed of mixed Cretaceous and Tertiary fauna, and dinosaur egg fragments occur in the lowermost 55 m of the Shanghu Formation (fig. 2; Zhang 1992; Buck et al. 1994). Lithofacies Interpretations The Nanxiong and Shanghu Formations are composed of massive mudstone and siltstone, debris Journal of Geology “TERTIARY DINOSAURS” flow and mudflow deposits, palustrine limestone, clay-pebble conglomerates, and sandy braided stream deposits. Nanxiong Formation. On the basis of our fieldwork, we divided the Nanxiong Formation into the following facies. Massive Mudstone and Siltstone Facies. The massive mudstone and siltstone facies comprises 40% of the Nanxiong Formation. Color gradually changes from reddish brown (2.5YR 4/3, 4/4, 5/3, 5/4) to weak red near the top of the formation (10R 5/4, 4/3, 5/2). Individual beds in this facies vary from 3–5 m in thickness and are highly bioturbated and commonly mottled (5Y 7/1 light gray mottles). Welldeveloped paleosols are not common in the Nanxiong Formation as a whole but are most common in this facies. Most paleosol horizons exhibit high degrees of bioturbation, with few other pedogenic features. Mature paleosols are Calcic Vertisols containing stage 2 nodules, wedge-shaped peds, vertical cracks (on average, 12 cm long, 5 cm wide, and filled with overlying mudflow deposits), slickensides, brecciated mottles, and manganese and carbonaceous stains (Gile et al. 1966; Mack et al. 1993). Fine- to medium-grained sand is highly dispersed throughout the mudstone. In thin sections, detrital sand grains are mainly monocrystalline quartz and white micas with lesser amounts of plagioclase and microcline feldspar, biotite, and metamorphic and sedimentary lithic fragments. Plutonic rock fragments and rare grains of amphibole also occur. Feldspar grains exhibit only minor alteration; mica grains are relatively fresh with only minor parting along cleavage planes. Debris Flow and Mudflow Facies. This facies is the most common facies in the upper portion of the Nanxiong Formation (56% of the bedding) and is concentrated near the KT boundary (fig. 2). These beds are characterized by matrix-supported clasts, including dinosaur fossils (fig. 3). Bed thickness ranges from 20 cm to several meters. Clast concentrations are greater toward the top of individual beds. Soft-sediment deformation features are common. Rare clast-supported lenses at the top of individual flows indicate waning flow deposits. Overall, debris flow deposits exhibit inverse grading and coarsen upward with clasts ranging from 2 mm up to 8 cm in diameter. This facies gradually changes from reddish brown in color (2.5YR 4/3, 4/4, 5/3, 5/4) to weak red near the boundary (10R 5/4, 4/3, 5/2). Dinosaur egg fragments are most commonly associated with this facies. Granite, quartzite, chert, and quartz sandstone dominate clast composition, but schist, phyllite, and amphibolite were also recognized. Feldspar and 113 mica within all clasts are pristine and unaltered. Detrital sand grains observed in thin section are similar to those described from the massive mudstone and siltstone facies. Palustrine Limestone Facies. Palustrine limestone is lacustrine limestone that has been altered by soil-forming processes (Zarza et al. 1992). This facies comprises less than 1% of the Nanxiong Formation. The limestone occurs as discontinuous, thin (0.5–6 cm, locally up to 30 cm thick), finely laminated, light gray (5Y 7/1) carbonate lenses. Desiccation cracks, evaporite halite casts, burrows, and root traces are extremely common. Soilforming processes have obliterated most sedimentary structures. In the lower part of the Nanxiong Formation, these beds contain ostracod and charophyte fossils. Only one bed in the upper 101 m of the Nanxiong is palustrine limestone. Ostracods reported from this portion of the Nanxiong were not recovered from this bed but rather from a mudflow bed that is higher in the section. In thin sections, these intervals are silty to sandy micritic limestone. Sand grains include monocrystalline quartz, plagioclase and potassium feldspar, metamorphic lithic grains, and white mica. Similar to the first two facies, detrital grains show little to no evidence of alteration. Clay-Pebble Conglomerate and Sandy Braided Stream Deposits. These deposits comprise 4% of the Nanxiong Formation. Discontinuous lenses of mottled, weak red (10R 5/4), reddish brown (2.5YR 5/4), and light gray (5Y 7/1) coarse to fine sandstone and clay-pebble conglomerate are interbedded with the palustrine limestone facies. The clay-pebble conglomerate is composed of sand and granulesized mud aggregates. Sandstones vary from fine to coarse grained, are well sorted, contain vertical burrows, and are either cross- or horizontal laminated. In thin section, sand grains are mainly monocrystalline quartz with smaller amounts of sedimentary lithic grains and white mica. Lesser amounts of metamorphic lithic grains, biotite, polycrystalline quartz, and rare amphibole grains were observed. Grains show little evidence of alteration. Shanghu Formation. The lithological facies within the Shanghu Formation are the same as those in the Nanxiong Formation, and the contact between these formations represents only a subtle facies change. Overall, the percentage of mudstone/ siltstone increases in the Shanghu Formation to 73%. Twenty percent of the measured beds in the Shanghu Formation are debris flow and mudflow facies. Mudstone beds increase in thickness and commonly contain interspersed sand-sized grains. Journal of Geology “TERTIARY DINOSAURS” Paleosol development is poor and consists of burrows, burrow-fill concretions, root traces, manganese stains, few slickensides, some mottling, angular blocky peds, and very indistinct horizonation. Palustrine limestone beds comprise less than 1% of the Shanghu Formation and are more variable in thickness. Clay-pebble conglomerate and sandstone comprise 6% of the Shanghu Formation (a slight increase relative to the Nanxiong Formation). Color is similar to the upper Nanxiong Formation, mostly weak reds (10R 5/4, 4/3, 5/2), with gradual changes to reddish browns (2.5YR 4/3, 4/4, 5/3, 5/4) higher in the section. Petrographically, grain types are similar to those in the Nanxiong Formation and are nearly pristine with little evidence of weathering. Debris flow and mudflow facies decrease and are finer grained in the Shanghu Formation than in the Nanxiong Formation. Discussion and Conclusions Paleoclimate and Depositional Environment Interpretations. The Nanxiong and Shanghu Formations were deposited in comparable alluvial fan/playa mudflat environments in a highly seasonal semiarid rift basin. Paleosols throughout the section are poorly developed, which is indicative of both slow weathering rates associated with the semiarid climate and high rates of sediment accumulation common in rift valleys with highly seasonal precipitation. The presence of Vertisols containing calcium carbonate nodules indicates a semiarid paleoclimate with distinct episodes of wetting and drying. Today, pedogenic calcic nodules form in environments that generally receive fewer than 80 cm of precipitation per year (Mack and James 1994). This interpretation is supported by the presence of pristine feldspars and micas both as isolated grains and within clasts. The widespread occurrence of Vertisols with cracks that formed during the dry season and that were subsequently filled by mudflows initiated at the onset of the rainy season is evidence of a highly seasonal climate. Palustrine limestone was deposited in shallow playa environments that were greatly affected by seasonal rainfall and bioturbation/rooting. The massive mudstone and siltstone facies accumulated along the distal portion of alluvial fans and proximal playas 115 and represent overbank deposits with varying degrees of soil development. The presence of highly dispersed fine- to medium-grained sand in the mudstone and siltstone facies is most likely of eolian origin. These grains originated in ephemeral braided streams but were blown across and adhered to the moist surface of the playa. After eolian deposition, bioturbation and the shrink/swell behavior of expandable clays resulted in the mixing of the sand into the mud. The clay-pebble conglomerate and sandstone units were probably deposited in ephemeral braided streams and as sheetflood deposits along the distal portion of alluvial fans. The granule-sized mud aggregates that comprise the clay-pebble conglomerates were eroded from Vertisols forming along the distal portion of alluvial fans and playas (Rust and Nanson 1989). Flash floods moving across alluvial fans deposited the mudflow and debris flow units. Debris flows and mudflows are the most common facies within the controversial interval of the upper Nanxiong and lower Shanghu Formations (fig. 2). The KT boundary section is well preserved within the stratigraphic section we examined, and no evidence of faulting was observed. Individual beds can be traced along strike for several hundreds of meters, although the discontinuous nature of the red beds and modern-day rice paddies limit exposure. In addition, the most highly developed soil profile near the KT boundary represents, at most, a few tens of thousands of years of nondeposition on the basis of its morphology (Machette 1985). Therefore, there is no indication of a significant unconformity anywhere within this section. The consistency of facies, the occurrence of characteristic paleosols, and the presence of labile sand grains throughout the entire section indicate no significant climatic shifts within the section. We infer that the climate was semiarid throughout the entire time when sediments were deposited, and we have seen no evidence to support hypotheses that link extinctions at the KT boundary in the Nanxiong basin in southeast China to climate. Factors Related to Tertiary Dinosaurs. We believe the mixed KT assemblages in the controversial interval (the upper 101 m of the Nanxiong Formation and the lower 76 m of the Shanghu Formation) are the result of reworked Cretaceous fossils that were Figure 2. Distribution of fossil vertebrates, pollen, ostracods, and magnetostatigraphic data in relationship to the currently accepted KT boundary and our measured section of the Nanxiong and Shanghu Formations (data are from Chen 1986; Sloan 1987; Zhang 1992; Rigby et al. 1993; Buck et al. 1994; Stets et al. 1996). 116 B. J. BUCK ET AL. Figure 3. Dinosaur bone fossils (white) contained within a matrix-supported (red mudstone) debris flow deposit in the upper part of the Nanxiong Formation. The dark brown–black areas are due to desert varnish on the surface of the outcrop, the bluish-green areas are reduction spots, and the white material (excluding the dinosaur bones) is granules and pebbles that are supported within red mudstone. redeposited in previously unrecognized Paleocene debris flows and mudflows (figs. 2, 3). Although some workers have noted minor debris flow deposits in the Nanxiong Formation (Liu and Wang 1990; Rigby et al. 1993), previous studies apparently failed to recognize the extent of the debris flow deposits that occur in the controversial interval. Zhao et al. (2002) reported measurable amounts of Ir at different stratigraphic positions within the controversial portion of the Nanxiong Formation, and they attribute these to multiple Ir-delivering events. However, a simpler interpretation of these data in light of the sedimentological evidence is that these anomalies are the result of reworking of a single end-Cretaceous impact deposit rather than the result of multiple events. Zhao et al. (2002) cite the presence of intact egg nests below the controversial interval. They also state that two clutches of eggs occur in the controversial interval. The presence of intact dinosaur egg nests in this interval would weaken our interpretation. However, detailed descriptions of stratigraphic location, egg geometries, and the sedimentology of encasing materials are lacking. Zhao et al. (2002) state that eggs and fragments occur as heaps throughout the Nanxiong Formation. We, too, observed concentrations of eggshells within the mudflow deposits in the controversial interval but saw no intact nests or eggs. Therefore, until more detailed descriptions are published, all of these egg occurrences can more easily be ascribed to debris flow and mudflow processes than they can to Tertiary dinosaurs. We believe that eggs that have been cited as being Tertiary are, in fact, reworked Cretaceous fossils now contained within Tertiary pollen-bearing mud and debris flows. Magnetostratigraphic data show a pattern of normal polarity changing to reversed polarity in the upper part of the Nanxiong and lower Shanghu Formations (fig. 2; Stets et al. 1996). These intervals were assigned to chrons 30N and 29R because of the presence of dinosaur eggshells in the upper Nanxiong Formation (Stets et al. 1996). However, the magnetostratigraphic data are incomplete in two areas of the section, which we think results in an incorrect interpretation. Specifically, the assignment of the lower part of the Shanghu Formation to chron 29R is in conflict with the presence of Baemalambda, a mammal fossil correlative with a younger magnetozone (Sloan 1987). We believe that the upper 101 m of the Nanxiong Formation is Tertiary in age. Reworking of Creta- Journal of Geology “TERTIARY DINOSAURS” ceous fossils carried in debris and mudflows deposited during the Tertiary can account for the mixed Cretaceous and Tertiary fossils. On the basis of previous palontological data and our sedimentological data, we conclude that controversy regarding the presence of dinosaur fossils in Tertiary rocks is the result of sedimentological processes not previously recognized. 117 ACKNOWLEDGMENTS Financial support was provided by K. Cash. R. Howley provided valuable suggestions and drafted figures. We thank P. Sheehan and an anonymous reviewer for helpful suggestions. We thank the field crews of 1993–1995 and R. Song, J. K. Rigby, Jr., Y. Wang, and J. Guan. 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