Fossil trees, tree moulds and tree casts in the Palaeocene Mull Lava Field, NW Scotland: context, formation and implications for lava emplacement

Abstract

Megafossils and macrofossils of terrestrial plants (trees, leaves, fruiting bodies, etc.) are found in sedimentary and pyroclastic units interbedded with lavas in many ancient lava fields worldwide, attesting to subaerial environments of eruption and the establishment of viable plant communities during periods of volcanic quiescence. Preservation within lava is relatively rare and generally confined to the more robust woody tissues of trees, which are then revealed in the form of charcoal, mineralised tissue or as trace fossil moulds (tree moulds) and casts of igneous rock (tree casts, s.s.).In this contribution, we document several such fossil trees (s.l.), and the lavas with which they are associated, from the Palaeocene Mull Lava Field (MLF) on the Isle of Mull, NW Scotland. We present the first detailed geological account of a unique site within the Mull Plateau Lava Formation (MPLF) at Quinish in the north of the island and provide an appraisal of the famous upright fossil tree – MacCulloch's Tree – remotely located on the Ardmeanach Peninsula on the west coast of the island, and another large upright tree (the Carsaig Tree) near Malcolm's Point in the district of Brolass, SW Mull; both occurring within the earlier Staffa Lava Formation (SLF). The taphonomy of these megafossils, along with palynological and lithofacies assessments of associated strata, allows speculation of likely taxonomic affinity and the duration of hiatuses supporting the establishment of forest/woodland communities. The Ardmeanach and Carsaig specimens, because of their size and preservation as upright (? in situ) casts enveloped by spectacularly columnar-jointed basaltic lava, appear to be unique. The aspect of these trees, the thickness of the enveloping lavas and the arrangement of cooling joints adjacent to the trees, implies rapid emplacement, ponding and slow, static cooling of voluminous and highly fluid basaltic magma. The specimens from Quinish include two prostrate casts and several prostrate moulds that collectively have a preferred orientation, aligning approximately perpendicular to that of the regional Mull Dyke Swarm, the putative fissure source of the lavas, suggesting local palaeo-flow was directed towards the WSW. The Quinish Lava is an excellent example of a classic pāhoehoe (compound-braided) type, preserving some of the best examples of surface and internal features so far noted from the Hebridean Igneous Province (HIP) lava fields.These Mull megafossils are some of the oldest recorded examples, remarkably well preserved, and form a significant feature of the island's geotourism industry.

Robert McCormick's geological collections from Antarctica and the Southern Ocean, 1839–1843

“…This may describe a tree encased in basalt lava, but still preserved in its original, upright growth position. Though rare, such a phenomenon has been reported from other volcanic terrains (Bell and Williamson 2017) although not, to the author's knowledge, in the more recent literature from Kerguelen.…”

Volcano Geology Applications to Ancient Volcanism-Influenced Terrains: Paleovolcanism

“…Volcano Geology Applications to Ancient Volcanism-Influenced Terrains: Paleovolcanism DOI: http://dx.doi.org/10.5772/intechopen.108770 presence of fossils in the interbedded epiclastic, but also in the pyroclastic deposits, can provide information about the age of the rocks and their depositional environment, although it is not always easy to know whether the fossils were deposited in situ or were transported and redeposited. The presence of fossils (vertebrate, invertebrate and plants) within pyroclastic deposits (e.g., [19]) and lavas (e.g., [20]) is common. In addition to the stratigraphic, paleoenvironmental, and paleoclimatological information that this represents (e.g., [21,22]), we can also obtain information on the emplacement temperature of the deposit and the direction and sense in which it was emplaced (e.g., [23,24]).…”

Chemical, Textural and Thermal Analyses of Local Interactions Between Lava Flow and a Tree – Case Study From Pāhoa, Hawai’i

“…Many effusive centers (e.g., Hawaii, Etna, the Virunga region, La Réunion) are densely vegetated and in climatic settings that favor rapid vegetation regrowth after inundation by lava (e.g., Ziegler, 2002;Smathers and Mueller-Dombois, 2007). Forest inundation by lava flows is thus a common phenomenon, which has been frequently described during eruptions at, for example, Etna (Andronico et al, 2005;Carveni et al, 2011), Piton de la Fournaise (Bertile, 1987) and Kilauea (Moore and Richter, 1962;Lockwood and Williams, 1978;Bell and Williamson, 2017;Chevrel et al, 2019). As a result, it is well known that when lava invades a forest, it causes widespread combustion of the trees, shrubs and undergrowth comprising the biotic system as, for example, during the eruption of Kilauea Iki (Kilauea, Hawaii) in 1959 (Richter et al, 1970).…”