The remains of a forest of 305 million years ago allow reconstructing its history of colonisation of a coastal sand bar and its destruction by marine flooding due to faulting in a high destructive deltaic setting. Sigillarian trees snapped off just above the rooting bases, whereas woody trees (cordaitaleans?) were uprooted by the unidirectional current, which oriented the fallen logs. This record puts a new perspective on reconstructions of Carboniferous forest mires.
Les restes fossiles d’une forêt ayant existé il y a 305 millions d’années permettent de reconstituer l’histoire de son implantation sur une barre sableuse côtière et de sa destruction par une inondation marine liée à la destruction tectonique d’un système deltaïque. Les sigillaires arborescentes ont été tronquées juste au ras de leur système racinaire, alors que les arbres ligneux (
Coal seams in the Carboniferous of Europe and North America are normally found associated with fossil plant remains. Although some coals are simply enrichments in vegetable matter in lake deposits with both inorganic and organic matter, a substantial proportion of coal seams show underlying rootlet beds. These are commonly filled with appendices of
There is a substantial amount of literature dealing with standing trees in Carboniferous deposits, and also with the rooting structures of the different kinds of plant involved. One of the first authors to deal comprehensively with these structures was Grand’Eury
Grand’Eury
Although Grand’Eury highlighted the various trees with their rather substantial rooting structures, one is also aware of rootlet beds consisting of fern rootlets of a much finer texture. These are increasingly common in Permian strata.
The different elements of Carboniferous forest mires have often been depicted in reconstructions. A compendium of reconstructions was published by Jongmans
Most of the interest has been centred upon standing forests, that is, upright tree trunks in river overbank deposits, sheet flood sandstones, volcanic ash bands, etc. Less commonly, the reconstruction of Carboniferous forests is based on impressions, the reason being that plant impressions usually relate to drifted, remains with assemblages ranging from allochthonous to parautochthonous and, only very occasionally, autochthonous. With regard to the latter, only the plant associations preserved by volcanic ash fall in mires qualify as purely autochthonous. These obviously allow the reconstruction of a local, specialised floral association.
The top surfaces of rootlet beds, when exposed, which is usually due to coal mining activity, may represent the imprints of rooting structures of a local floral community. This quite often consists of stigmarian or similar structures and usually reflects the rooting of a single generation of trees constituting a forest. In the present paper, a special case is described of the imprints of rooting structures showing the successive establishment of two quite separate generations of lycopsid trees with
Small-scale opencasting for coal in steeply dipping (near vertical) strata of early Cantabrian (earliest Stephanian) age near the village of Verdeña in the mountains of northern Palencia, Cantabrian Cordillera, northwestern Spain, has exposed a wall of 180 m length and 5–12 m in height (
A full description of this basin of latest Westphalian and earliest Stephanian ages can be found in
The Palentian Basin contains the stratotypes of the Cantabrian and lower Barruelian (sub) stages, representing the Lower Stephanian. Its succession commences with Upper Asturian (ex Westphalian D
The carbonate platform split into various parts in Early Cantabrian times, producing several tilted blocks which accumulated siliciclastic successions of various different thicknesses (
The sedimentary succession in the Palentian Basin consists, on the whole, of coarsening upwards deltaic sequences. Given the high rate of sedimentation and the general evidence of tectonic mobility, it seems likely that the deltaic sequences are tectonically controlled, with the abandonment of deltaic lobes being due to seismic events (faulting) as well as delta shifting. Although there is considerable evidence of sea level changes in Pennsylvanian times, with glacial/eustatic cyclothems developing on the relatively stable continental platforms (e.g.,
The coarsening upwards deltaic sequence that terminates with a sand bar providing suitable conditions for the establishment of a coastal forest as represented in the Verdeña site is illustrated in
Among the fallen log impressions two different kinds are apparent, viz. shallow imprints and deeper imprints; the latter sufficiently deep for the miners to have scooped out the coal, which was found inside. One of the shallow imprints, a log of c. 5 m length, shows the markings of a cannelate
The sandstone surface seen on the steeply dipping wall of the abandoned opencast mine displays the upper parts of stigmarian rooting systems where they depart from the stem base. Only very occasionally does the lateral extent of the
With a 30% overlap of stigmarian rooting systems, the pioneer generation will have extensively covered the area available for colonisation on the delta lobe. Although sigillarian sporelings were undoubtedly produced at regular intervals by the pioneer generation, these could not grow into trees when the older generation occupied the total ground surface available. Indeed, the similar size of the larger imprints of rooting bases suggests that the pioneer generation grew to full size and was allowed to complete its life span. However, although the pioneer generation may have lived a full life span and thus tended to die at virtually the same time, the individual trees will have lasted longer or shorter. It may be assumed that the sporelings were only given the opportunity to establish themselves permanently when the pioneer generation had died. This is presumably the significance of finding two sharply different sizes of rooting bases. An apparent spread of sizes among the smaller, second generation tree bases imprints may be due to staggering in the demise of large trees of the pioneer generation, and a consequent recolonisation at slightly different times. For the second generation to become established, it needed to avoid the physical spaces occupied by the stigmarian rooting systems of the first generation, which were left in the soil after the trees had died. This would explain the more irregular distribution of the second generation trees (
The impressions of fallen tree trunks are also quite common at the Verdeña site. Most of these trunks were transported as is evident from the general lack of attached rooting structures and a preferred orientation. However, in one case (
Fallen (transported) logs are only partly of the kind that produced shallow impressions, and of lycopsid origin. Indeed, one specimen shows the markings of a cannelate
Fallen tree trunks are commonly present on the sandstone surface. Both shallow and deeper impressions are found, as mentioned before. These show a clear orientation, with a spread of about 30°. Occasional, shorter logs, corresponding to woody trees are found at right angles to the preferred direction. Length is variable, up to 5–7 m for the shallow imprints. Only one of these imprints, representing a cannelate
The width of the sigillarian stem imprints accords well with the diameter of the second generation rooting bases. When the lycopsid forest was destroyed, only the second generation trees were found standing, the inference being that the first generation had died earlier. The deeper stem imprints of a woody tree (
It is observed that small plant debris is notably absent. This is regarded as significant. Whatever agency provoked the catastrophic demise of the Verdeña forest, it cleared the site of litter. It is also observed that both the uprooted woody tree (
The woody trees may have come from a more distant area. Not only is there an absence of imprints of rooting structures other than the lycopsid ones, but woody trees may float more easily and suffer longer transport without breaking up entirely. On the other hand, the absence of branches attached to the logs of woody trees suggests a certain amount of breakage as a result of transportation. Perhaps, these trees were cordaitaleans living on the seashore in a mangrove setting, as Cridland
Downfaulting of part of a coastal area would explain a unidirectional current with trees oriented in the direction of the current. A tsunami is out of the question since the backwash would have disorganised the stem remains.
With this scenario the overlying coal bed is likely to have originated in a lagoon, which became established on the site where the lycopsid (sigillarian) forest (and the presumed cordaitaleans on the sea front) were destroyed by a sudden influx of seawater. A forested area would still exist landwards, and this would provide plant debris for an accumulation as represented by the overlying thin, layered coal with a high ash content. Fully marine sediments with crinoid debris and other marine fossils are found overlying the coal. These start off another deltaic sequence. Although not every deltaic sequence in the upper Asturian and lower Cantabrian succession of the Palentian Basin ends with a sandstone colonised by plants, sequences similar to that present in the opencast site near Verdeña are found repeatedly. Even though this site is not unique in the Palentian Basin succession, it is a well-exposed, fully representative example. This splendid example is being conserved under the protection of the Consejería de Medio Ambiente of Castilla-León.
This example of a fossil forest established in a high-destructive deltaic setting that led to its undoing is quite unique in the geological literature. Descriptions of palaeosols with the imprints of lycopsid tree bases do reflect forest dynamics (e.g.
The Verdeña forest impression site is a unique example of coastal lycopsid (sigillarian) colonisation followed by complete destruction as a result of marine flooding in the context of a high-destructive delta. Two sharply delimited lycopsid generations are recognised, as well as “nurseries” of sporelings. The first generation completed a full life span. The second generation, established after the first generation died, was destroyed by the marine flooding event. This catastrophic event snapped off the lycopsid stems above the rooting bases, but uprooted woody trees believed to represent a cordaitalean. The woody trees ( This is the first Carboniferous forest community known thus far from a tectonically mobile area in an overall marine deltaic setting.
Rosa Calvo and Carmen Álvarez-Vázquez are thanked for aid in fieldwork and help with the production of this paper. R.C. drew the reconstruction of Sigillaria (
Composite showing steeply dipping sandstone (rootlet bed) in abandoned opencast site (
Fig. 1. Planche montrant un front de grès escarpé dans une carrière à ciel ouvert abandonnée (
Geological map showing the distribution of upper Westphalian and lower Stephanian strata of the Palentian Basin in northeastern Palencia province (Spain). Upper Silurian to lower Westphalian strata, deformed during the Palentian tectonic phase (late Langsettian), are shown without detail as pre-Palentian “basement”. A small outlier of unconformable Stephanian B deposits at Peña Cildá is shown as well as the Triassic cover (underlain in places by Lower Permian).
Fig. 2. Carte géologique montrant la distribution des couches du Westphalien supérieur et du Stéphanien inférieur du bassin Palentien, du Nord-Est de la province de Palencia. Les strates du Silurien supérieur et du Westphalien inférieur, déformées pendant la phase tectonique palentienne (Langsettien supérieur) sont figurées globalement comme le « socle » pré-Palentien. Une petite butte témoin (« Peña Cildá »), du Stéphanien B discordant, est figurée, ainsi que la couverture triasique (reposant par endroits sur du Permien inférieur).
Generalised stratigraphic column showing the deltaic San Salvador Coal-Bearing Formation in between the entirely marine Verdeña and Brañosera formations. The rootlet bed with imprints of rooting bases and fallen trunks is in the middle part of the San Salvador Formation.
Fig. 3. Colonne stratigraphique composite, montrant la formation deltaïque houillère de San Salvador, encadrée par les formations marines Verdeña et Brañosera. Le niveau à radicelles, avec empreintes de bases racinaires et troncs tombés, se situe dans la partie médiane de la formation San Salvador.
Imprints of
Fig. 4. Empreinte d’une