Lower Eocene Footprints from Northwest Washington, USA. Part 1: Reptile Tracks
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geosciences Article Lower Eocene Footprints from Northwest Washington, USA. Part 1: Reptile Tracks George E. Mustoe * Geology Department, Western Washington University, Bellingham, WA 98225, USA Received: 23 June 2019; Accepted: 19 July 2019; Published: 22 July 2019 Abstract: Lower Eocene fluvial strata in the Chuckanut Formation preserve abundant bird and mammal tracks. Reptile trace fossils include footprints from a small turtle (ichnogenus Chelonipus), and several Crocodylian trackways that consist of irregularly spaced footprints associated with linear tail drag marks. The latter trackways represent “punting” locomotion, where a submerged Crocodylian used intermittent substrate contacts to provide forward motion of their neutrally buoyant bodies. Two adjacent sandstone blocks preserve Crocodylian trace fossils that are named herein as a new ichnogenus and ichnospecies Anticusuchipes amnis. Two other Crocodylian trackways lack sufficient detail for ichnotaxonomic assignment. Keywords: Chuckanut Formation; Crocodylian; footprints; ichnology; trace fossils; tracks; turtle; Washington 1. Introduction Birds and animals are represented by a diverse variety of footprints preserved on bedding planes in gently-dipping Chuckanut Formation strata in the Mount Baker foothills in Western Whatcom County, Washington, USA (Figure1, Table1). These locations are in a forested area that has been subject to extensive commercial logging, where road construction yields new bedrock exposures. Landslides also reveal fossiliferous bedrock. The discovery of these Eocene tracks is noteworthy because evidence of vertebrate life is scarce in Paleogene continental sedimentary rocks in the Pacific Northwest. This paper describes reptile tracks; avian and mammalian trace fossils will be described in subsequent reports. These discoveries are noteworthy given the scarcity of reports of Cenozoic reptile tracks. For perspective, a comprehensive bibliography of 584 references describing Cenozoic vertebrate tracks [1] comprise 1% Amphibia, 2% Reptilia, 32.7% Aves, and 64.3% Mammalia. The only known turtle tracks are two Eocene ichnospecies, Chelonipus chardronicus and C. parvus from Texas USA [2], and two Paleocene Crocodylian ichnotaxa, Albertasuchipes russellia from Alberta, Canada, [3], and Boreaosuchipes hanksi from North Dakota, USA [4]. Our report establishes a new Crocodylian ichnotaxon, Anticusuchipes amnis, and describes a turtle trackway as Chelonipus isp. For ichnotaxonomic purposes, the term crocodyle is used include members of three families: Crocodylidae (true crocodiles), Alligatoridae (alligators and caimen), and Gavailidae (gharail and false gharail). Geosciences 2019, 9, 321; doi:10.3390/geosciences9070321 www.mdpi.com/journal/geosciences GeosciencesGeosciences 20192019,, 99,, x 321 FOR PEER REVIEW 2 2of of 18 19 Figure 1. Chuckanut Formation fossil track locations. Reptile trace fossils are known only from Western FigureWashington 1. Chuckanut University Formation (WWU)-KC-1 fossil turtle track trackway) locations. and Reptile WWU-RCS-1 trace fossils (Crocodylian are knowntracks) only sites. from Western Washington University (WWU)-KC-1 turtle trackway) and WWU-RCS-1 (Crocodylian tracks) sites. Table 1. Reptile track locations. GPS locations are from WGS 84 datum. Locality Number LocationTable 1. Reptile track locations. GPS loLatitudecations are fromLongitude WGS 84 datum.Elevation (Meters) WWU * UWBM ** Locality Number KenneyLocation Creek KC-1 C0644Latitude 48.841 ◦ NLongitude 122.107 ◦ WElevation (Meters) 627 WWU * UWBM ** Racehorse Creek RCS-1 C1481 48.871◦ N 122.118◦ W 541 Kenney Creek KC-1 C0644 48.841° N 122.107° W 627 * Western Washington University. ** University of Washington Burke Museum of Natural History and Culture. Racehorse Creek RCS-1 C1481 48.871° N 122.118° W 541 * Western Washington University. ** University of Washington Burke Museum of Natural History and Culture. 1.1. Geologic Setting 1.1. GeologicThe Chuckanut Setting Formation consists of beds of conglomerate, arkosic sandstone, siltstone, and coalThe that Chuckanut unconformably Formation overlie consists Paleozoic of and beds Mesozoic of conglomerate, metamorphic arkosic basement sandstone, rocks. siltstone, These fluvial and coalsediments that unconformably were deposited overlie on a broad Paleozoic floodplain and Mesozoic that existed metamorphic prior to the mid-Tertiarybasement rocks. uplift These of the fluvial North sedimentsCascade Range were [deposited5,6]. Isolated on exposuresa broad floodplain extend along that faultexisted zones prior to connectto the mid-Tertiary the main outcrop uplift beltof the on Norththe west Cascade side of Range the Cascade [5,6]. Isolated Range exposures with the Swauk extend Formation along fault in zones Central to connect Washington. the main Correlative outcrop beltstrata on also the extendwest side north of into the BritishCascade Columbia Range wi whereth the they Swauk are called Formation the Huntingdon in Central FormationWashington. [7]. CorrelativeMustoe and strata Gannaway also extend [8] suggested north into that British this far-flung Columbia outcrop where distribution they are called is evidence the Huntingdon of a large Formationdepositional [7]. basin Mustoe that wasand dissectedGannaway by strike-slip[8] suggested faulting. that this far-flung outcrop distribution is evidenceThe of Chuckanut a large depositional Formation basin is one th ofat thewas thickest dissected nonmarine by strike-slip sedimentary faulting. formations in North America,The Chuckanut but extensive Formation forest coveris one restrictsof the thickest measurement nonmarine of continuoussedimentary stratigraphic formations in sections. North America,Estimates but of the extensive total thickness forest of cover the formation restricts inme theasurement main outcrop of continuous belt in northwest stratigraphic Washington sections. range Estimatesfrom 3000 of m the [9] tototal 8300 thickness m [10]. Recentof the formation mapping suggestsin the main a minimum outcrop belt thickness in northwest of 4000–5000 Washington m [11]. rangeSeveral from 3000 schemes m [9] haveto 8300 been m proposed[10]. Recent for ma dividingpping suggests the Chuckanut a minimum Formation thickness into of stratigraphic 4000–5000 mmembers [11]. [5,8,11]. The basic architecture is a three-component system composed of the basal Late Paleocene-EarlySeveral schemes Eocene have Bellingham been proposed Bay Member, for dividi overlainng the inChuckanut the Mount Formation Baker foothills into stratigraphic by the Early membersEocene Slide [5,8,11]. Member The basic (Figure architecture2). The third is a component three-component is the middle system Eocene-Latecomposed of Eocene the basal Padden Late Paleocene-EarlyMember, which Eocene is perhaps Bellingham separated Bay from Member, underlying overlain strata in the by anMount unconformity. Baker foothills The stratigraphicby the Early Eocenepositions Slide of two Member minor (Figure members 2). remainThe third uncertain component [11]. is the middle Eocene-Late Eocene Padden Member,All trackwhich fossils is perhaps that have separated so far from been underlying discovered strata have comeby an fromunconformity. Slide Member The stratigraphic strata in the positionsMount Baker of two foothills. minor memb Theseers discoveries remain uncertain are related [11]. to the gentle dip of Chuckanut strata in this part of the main outcrop belt, where outcrops commonly expose bedding plane surfaces where tracks are Geosciences 2019, 9, x FOR PEER REVIEW 3 of 18 Geosciences 2019, 9, 321 3 of 19 likely to be preserved. In contrast, other stratigraphic members largely comprise steeply-dipping Geosciences 2019, 9, x FOR PEER REVIEW 3 of 18 strata, where bedrock exposures show bedding edges rather than bedding planes. Figure 2. Stratigraphy of the Chuckanut Formation. All track fossils that have so far been discovered have come from Slide Member strata in the Mount Baker foothills. These discoveries are related to the gentle dip of Chuckanut strata in this part of the main outcrop belt, where outcrops commonly expose bedding plane surfaces where tracks are likely to be preserved. In Figurecontrast, 2. Stratigraphyother strati ofgraphic the Chuckanut members Formation. largely comprise steeply-dipping strata, where bedrock exposuresFigure show 2. Stratigraphy bedding ofedges the Chuckanut rather than Formation bedding. planes. Locations areare shown shown in Tablein Table1. Reptile 1. Reptile trace fossilstrace havefossils found have only found at two only locations, at two WWU-KC-1 locations, (turtleWWU-KC-1All trackway) track (turtle fossils and trackway) that WWU-RCS-1 have and so WWU-RCS-1far (Crocodylian been discover tracks).(Crocodylianed have The WWU-KC-1cometracks). from The Slide WWU-KC-1 site hasMember been site describedstrata has inbeen the in describeddetailMount [12 Baker]. in The detail foothills. WWU-RCS-1 [12]. Thes The eWWU-RCS-1 site discoveries is at a major are site landsliderelated is at a majorto thatthe occurredgentlelandslide dip 9 that of January Chuckanut occurred 2009 in9 strata January the headwaters in this 2009 part in theofof Racehorsethe headwaters main outcrop Creek. of Racehorse Thebelt, landslide where Creek. outcrops (Figure The3 landslidecommonly) extends over(F exposeigure 1 km 3) bedding inextends length, plane over with surfaces1 an km estimated in wherelength, 5 tracks with105 marean3 × estimatedoflikely displaced to be5 × rockpreserved. 105 m distributed3 of displaced In contrast, over