Alpine glacial geology of the Tablelands, Gros Morne National Park, Newfoundland

Abstract

Controversy persists in western Newfoundland regarding Pleistocene, particularly Late Wisconsinan, glacial ice volumes. Independently, a set of alpine glacial deposits on the flanks of the Tablelands in Gros Morne National Park has attracted much attention but little scrutiny. In this study, cosmogenic nuclide dating of the alpine deposits places some limits on post-late glacial maximum (LGM) ice dynamics in the vicinity of the Tablelands, a plateau bounded on the northeast by Trout River Gulch. Small valleys incised into the flanks of the Tablelands are floored with a diamict that contains both till and ice-contact deposits. Rock glaciers rest on the diamict, and rock glacierization also has affected talus lining the south wall of Trout River Gulch. A small moraine rests in the Devil's Punchbowl cirque. The cirque moraine, lobate deposits below the cirque moraine, rock glaciers, and a colluvial veneer overlying the till in the small valleys have cosmogenic 36 Cl ages as old as either ca. 20 or 15 ka, depending on what erosion rate is assumed, indicating that these bodies are Late Wisconsinan in age but post-date the local LGM. Trout River Gulch was deglaciated early and perhaps did not contain active ice even at the LGM, but previous work shows that ice was streaming seaward both north of Trout River Gulch and south of the Tablelands even as the gulch lay relatively ice free.Resumé: Il subsiste des controverses dans l'Ouest de Terre-Neuve quant au volume des glaces au Pléistocène, surtout au Wisconsinien tardif. Indépendamment, un ensemble de dépôts de glaciers alpins sur les flancs des Tablelands dans le parc national Gros Morne a attiré beaucoup d'attention mais peu d'analyse. Dans cette étude, la datation de dépôts alpins ayant des nucléides cosmogéniques établit certaines contraintes sur la dynamique des glaces après le dernier maximum glaciaire aux environs des Tablelands, un plateau limité au nord-est par le ravin de la rivière Trout. Les fonds de petites vallées coupant les flancs des Tablelands sont tapissés d'un diamicton qui contient du till et des dépôts de contact glaciaire. Des glaciers rocheux reposent sur le diamicton et l'englaciation des roches a aussi affecté les talus qui tapissent le mur sud du ravin de la rivière Trout. Une petite moraine réside dans le cirque Devil's Punchbowl. La moraine de cirque, des dépôts en forme de lobes sous la moraine de cirque, des glaciers de roches et un placage de sédiments colluviaux par-dessus le till dans les petites vallées ont des âges cosmogéniques 36 Cl aussi anciens que 20 ou 15 ka, selon le taux d'érosion assumé, indiquant que ces amas datent du Wisconsinien tardif mais sont plus tardifs que le dernier maximum glaciaire local. Le ravin de la rivière Trout a été déglacé tôt et il ne contenait peut-être pas de glace active même au dernier maximum glaciaire; cependant, des études antérieures ont démontré que la glace s'écoulait vers la mer à la fois au nord du ravin de la rivière Trout et au sud des Tablelands alors même que le ravin demeurait relativemen...

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“…As Late Wisconsinan ice flow in the area was directed northward 118 , 123 , this westward flow through Trout River Gulch ceased prior to the most recent glacial maximum. The combination of 36 Cl ages and glacial erratics indicate that Trout River Gulch was likely not glaciated during the Last Glacial Maximum 118 , 122 . Finally, two sampling spots were located on diamict deposits in Winterhouse Gulch, a previously glaciated tributary valley that feeds into the Trout River Gulch.…”

“…The Bay of Islands ophiolite was emplaced onto the passive continental margin of eastern North America during the Ordovician age Taconian Orogeny 119 , 121 . The plateau edges are defined by sharp u-shaped formerly glaciated valleys with associated glacial landforms, including moraines, rock glacier deposits, pro-talus lobes, and diamict 118 , 122 . Vegetation on the Tablelands is sparse, composed primarily of small ground-hugging shrubs with stunted pine trees at lower elevations.…”

“…One sampling site was located just up-slope from and within the protruding lobe of a terminal moraine within the Devil’s Punchbowl cirque. The terminal moraine was dated to 17.6 ± 0.3 ka using chlorine-36 isotope analyses 122 . One sampling site was located on a terrace overlooking Highway 471 in Trout River Gulch.…”

“…One sampling site was located on a terrace overlooking Highway 471 in Trout River Gulch. Debris benches in the Trout River Gulch valley were cosmogenically dated using 36 Cl isotopes to between 15 and 20 ka 122 , necessitating the establishment of an ice-free valley prior to that point. Quartzite erratics sourced from over 20 km east of Bonne Bay necessitate westward ice flow at the time of deposition 118 .…”

“…One sampling site was located at the mouth of Winterhouse Gulch, and one halfway up the valley between the mouth and the cirque. Though cosmogenic ages have not been determined for glacial material in Winterhouse Gulch Canyon, the late Wisconsinan age ranges for other glacial deposits on the Tablelands, suggest a similar age range is likely for the presence of a valley glacier in Winterhouse Gulch 122 , and thus for initiation of soil development following glacial retreat. Reconstructions of Late Wisconsinan ice in Atlantic Canada suggest that the icesheet covering Newfoundland reached a most recent maximum extent around 20 ka 123 , 124 , followed by deglaciation that resulted in glaciers retreating away from the coast by ~12 ka 123 , 124 .…”