Paleofloristic and Paleoenvironmental Information from a Late Cretaceous

Palaeogeography, Palaeoclimatology, Palaeoecology 295 (2010) 389–408 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Paleofloristic and paleoenvironmental information from a Late Cretaceous (Maastrichtian) flora of the lower Cantwell Formation near Sable Mountain, Denali National Park, Alaska Carla Susanne Tomsich a,⁎, Paul J. McCarthy a, Sarah J. Fowell a, David Sunderlin b a University of Alaska, Dept. of Geology and Geophysics, P.O. Box 755780, Fairbanks, AK 99775-5780, United States b Lafayette College, Dept. of Geology and Environmental Geosciences, Easton, PA 18042-1768, United States article info abstract Article history: The lower Cantwell Formation in Denali National Park, Alaska, is a Late Cretaceous high-latitude fluvial succession Received 28 June 2009 that contains dinosaur and bird footprints, diverse invertebrate traces, and plant fossils. New fossil discoveries Received in revised form 21 January 2010 and stratigraphic descriptions in the Sable Mountain area allow for refined paleoenvironmental reconstructions Accepted 18 February 2010 and age interpretations. The formation consists of well-indurated conglomerate, sandstone, siltstone, and shale Available online 24 February 2010 interpreted as gravelly and sandy channel, levee, crevasse splay, sheetflood, lacustrine, and floodplain sediments with interspersed debris flows. The overall depositional environment is interpreted as a braided river floodplain Keywords: that interfingered with distal alluvial fan sedimentation. Plant fossils include leaves, branches, cones and rare Late Cretaceous Lower Cantwell Formation seeds of taxodiaceous and other conifer taxa, fern fronds, rhizomes and stem segments of Equisetites,linearand High-latitude paleoenvironments broad-lanceolate monocot leaf fragments, cf. Kenella, and impressions of diverse angiosperm broad-leaf Paleobotany morphotypes likely belonging to menispermoid, platanoid and several Late Cretaceous hamamelid groups. A Polar Broad-leaved Deciduous Forest sparse palynoflora extracted from the finer-grained facies yielded the pollen taxa Taxodiaceaepollenites, Paleoclimate Aquilapollenites and Alnipollenites; monosulcate cycadophyte or ginkgophyte pollen grains; bisaccate and monosaccate conifer pollen grains and a variety of trilete fern and psilate horsetail spores. The Middle to Late Maastrichtian pollen taxon Aquilapollenites conatus provides a maximum age for the Sable Mountain deposits. Floral assemblages occur within distinct lithofacies deposited in a variety of depositional settings in the ancient floodplain. Angiosperm leaf fossils are dominant in channel and levee facies and were part of a mixed forest assemblage growing in floodplain and lake margin environments. A variety of different types of conifers— indicated by saccate pollen types and rare long, thin leaf fragments—probably inhabited distal floodplain realms and uplands. Frequent and abrupt changes in lithology and floral assemblages imply transitory habitats in a rapidly aggrading depositional setting. A Climate Leaf Analysis Multivariate Program (CLAMP) applied to a composite leaf collection yielded a mean annual temperature of 7.42+/−1.2 °C, a warmest monthly mean of 17.1+/−1.6 °C, and a coldest monthly mean of −2.3+/−1.9 °C. Growing season precipitation is estimated at 229.4+/−334.5 mm over 4.8+/−0.7 months. Results are suggestive of a cool temperate and highly seasonal paleoclimate with short, warm, dry summers and long, comparatively mild and probably wet winters. Temperature calculations are consistent with previous estimates from Alaska's North Slope and northeastern Russia suggesting a low polar temperature gradient and a slight cooling trend during the Middle to early Late Maastrichtian. Precipitation estimates for the growing season are low compared to overall wet climate predictions for the Maastrichtian. Like other Late Cretaceous Arctic floras the Cantwell flora is classified herein as a Polar Broad-leaved Deciduous Forest flora. © 2010 Elsevier B.V. All rights reserved. 1. Introduction and eggshell fragments of a diverse dinosaur fauna indicate that the Arctic was once a warm and hospitable region (Spicer, 1987; Parrish et al., 1987; Fossil and sedimentologial data from late Mesozoic and Tertiary high- Spicer and Parrish, 1990; Fiorillo and Gangloff, 2000; Golovneva, 2000; latitude terrestrial basins provide a unique glimpse into depositional Fiorillo, 2006; Godefroit et al., 2009). processes, paleolandscapes, and climate variability. Evidence for a The lower Cantwell Formation is a little-studied, isolated Late widespread broad-leaf flora, a turtle shell mold and tracks, bones, teeth, Cretaceous fluvial sedimentary unit that crops out in the foothills of the northern Alaska Range in south-central Alaska (Fig. 1). Much of the formation is located inside Denali National Park & Preserve, where ⁎ Corresponding author. numerous successions of conglomerate, sandstone, dark bluish-grey E-mail address: [email protected] (C.S. Tomsich). siltstone, mudstone, and shale can readily be seen from the park road. 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.02.023 390 C.S. Tomsich et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 295 (2010) 389–408 Fig. 1. Map of Alaska indicating the location of the study area in Denali National Park and the distribution of the Cantwell Formation and contemporaneous formations on the North Slope and in south-central Alaska. Recently discovered dinosaur footprints, bird footprints, and diverse Plant megafossils and fossil palynomorphs are useful in assessing the invertebrate traces and plant fossils indicate the presence of a flourishing level of floral diversity and plant community associations (Traverse, terrestrial ecosystem during the Late Cretaceous (Fiorillo et al., 2007; 1988; Pfefferkorn and Wnuk, 1992). Long transport notwithstanding, Fiorillo et al., 2009). In this paper we describe and interpret new plant intact plant litter accumulates close to the site of original growth, fossil data tied to measured stratigraphic sections of the lower Cantwell thereby offering insights into the composition of an autochthonous/ Formation at Sable Mountain in Denali National Park (Figs. 2 and 3). parautochthonous flora (Spicer and Greer, 1986; Burnham et al., 1992). Fig. 2. Distribution of exposed Cantwell Formation and the location of the study area and other sites mentioned in text. Kcs refers to the Cretaceous sedimentary lower Cantwell Formation; Tcs refers to the Early Tertiary volcanic upper Cantwell Formation. Areas left blank represent undifferentiated older metamorphic and marine sedimentary rocks, late Tertiary sedimentary rocks and Quaternary basin fill. Narrow lines mark unit contacts and the course of major rivers. Bold lines represent faults. Barbs indicate overriding thrust sheets. Faults are dotted where inferred. Letters designate fault displacement: U = up; D = down. Geology from Gilbert and Redman (1975), Csejtsey et al. (1992) and this study. C.S. Tomsich et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 295 (2010) 389–408 391 Moreover, because temperatures, rainfall and light availability are Alaska Range. Today, the upper Cantwell Formation is preserved mainly known to affect the morphology of dicotyledonous angiosperm leaves in the eastern part of the basin within local synclines or down-faulted (Wolfe, 1993), climate parameters can be quantified with the applica- blocks (Fig. 2). In the Sable Mountain area, volcanic rocks were eroded tion of leaf physiognomic analyses such as the Climate Leaf Analysis and fault-displaced below the older sedimentary rocks, exposing more Multivariate Program (CLAMP) (Spicer, 2006). CLAMP considers than 2000 m of consecutive lower Cantwell Formation strata in an area of climate-sensitive character states of angiosperm leaf fossils which are approximately 100 km2. Contained within a large monocline, beds in the scored and analyzed to output a suite of climate factors including mean study area dip consistently northwest with no internal faults apparent. annual temperatures (MAT), warmest monthly mean temperature (WMMT), coldest monthly mean temperature (CMMT), growing season 3. Methods precipitation (GSP), monthly mean growing season precipitation (MMGSP), length of growing season (LGS), relative humidity (RH) Eighteen closely spaced stratigraphic sections were measured and and enthalpy (Wolfe, 1993; Wolfe and Spicer, 1999; Spicer, 2006). described from outcrops in the study area (Fig. 3). Lithologies were In the absence of datable rocks, a palynoflora can provide reliable age denoted in terms of grain size, sedimentary structures, fossil content, information. Ridgway et al. (1997) used the Campanian to Maastrichtian bed geometry, and the nature of the bed contacts. These data were pollen genus Aquilapollenites and the Early Maastrichtian pollen genus then grouped into major lithofacies and facies associations (Table 1). Kurtzipites sp. to bracket the age of the Cantwell Formation at nearby Leaf and axis megafossils were collected from loose float material Polychrome Mountain (Fig. 2). The age of the Sable Mountain deposits, that had its clear origins in the measured stratigraphic sections. Larger or however, had not yet been determined. The Polychrome Mountain and in situ specimens were photographed in the field, while samples within Sable Mountain outcrops are spatially offset by a fault along the East smaller pieces

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