Burial and preservation of a fossil forest on an early Permian (Asselian) volcano (Merangin River, Sumatra, Indonesia)

Abstract

The Early Permian (Asselian) Mengkarang Formation of the western Jambi Province, Sumatra, is well exposed in the valley of the Merangin River. No consensus regarding the depositional environment of this section had been reached hitherto. This section preserves abundant evidence of a Permian forest, named the Merangin Fossil Forest herein, which grew at the foot of an active volcano, where pyroclastic flows often made way and destroyed the vegetation and where epiclastic reworked pyroclastics rapidly entombed the vegetation. The present assessment is based on a detailed study of three localities using multiple lines of evidence, including petrography, silica typing including hot cathodoluminescence microscopy and spectroscopy, and palynofacies analysis. In situ Agathoxylon was near enough to the volcanic slope to be buried rapidly, shallow enough to avoid extreme crystallization in the lumina, and far enough from the metamorphic centre not to get recrystallized. All these combined contingencies make this a most unique find that provides significant insights into a rarely studied palaeoecological setting from the Early Permian.

Depositional settings and changing composition of the Jambi palaeoflora within the Permian Mengkarang Formation (Sumatra, Indonesia)

“…Slightly higher up, the transition from sheets to lenticular beds with uncommon coals is interpreted as indicative of the distal foot of the volcanic slope, where sheets are discontinuous (LA‐4). Tree roots in basalt flows (Matysová et al, in press) display the sinuous tapering shape without secondary roots, also observed for the tree roots in Interval II. Both in Interval II and here, in Interval V, the numerous associated Cordaites leaves suggest that Cordaithantales had colonized the volcano slope.…”

“…Moreover, the type of root systems associated to the Agathoxylon , as well as Cordaites leaves (Booi et al, ) and inflorescences, is found in gravity flows. This indicates that both the volcanic slope and its alluvially reworked base (Matysová et al, in press) were vegetated by a Cordaites /fern forest. Such a type of forest was also described by Rößler et al () where Psaronius tree ferns, calamitaleans, medullosan, and Cordaixylon gymnosperms were found in the same horizon.…”

“…The fourth pyroclastic flow exhibits a 2.5‐m‐high sigmoidal palaeorelief, formed by the abrupt change in bed thickness from pebbly tuffs to surges. A large in situ Agathoxylon and two root systems are found to have grown from this last pyroclastic flow (Matysová et al, in press).…”

“…These roots are distributed over two levels of shales. The wood and the shape of the trees were insufficiently preserved to indicate their taxonomy, but they were permineralized while they were already in a high state of oxidation (Matysová et al, in press). A large Taeniopteris sp.…”

“…The Mengkarang Formation was described as interfingering both with the overlying Telukwang Formation and with the Palepat Formation that overlies the Telukwang Formation. The Palepat Formation is composed mostly of volcanic rocks and was considered by Suwarna et al (, , ) to be the main unit forming the West Permian Volcanic Arc in the Bangko area (Matysová et al, in press, figure 1). Recent descriptions of the Mengkarang Formation emphasized epiclastic deposition in a recycled orogenic setting (Suwarna, ; Suwarna et al, ; Van Waveren et al, ), while the earlier literature pointed to a predominantly volcanic setting (Tobler, ; Zwierzycki, ).…”