The Iberian Basin was an intracratonic basin during the Mesozoic and experienced several extensional periods during the Mesozoic and Cenozoic [24]. The tectonic inversion of the Iberian Basin took place in several compressive phases during the Alpine orogeny, since the Latest Cretaceous to the Early Miocene [8] and [9], resulting in several folded and thrust linear structures that constituted the present-day Iberian Ranges.

As the rest of the Permian-Triassic basins in the Iberia microplate, the Iberian Basin started its development under an extensional tectonic regime, with several synrift–postrift phases that prolonged during almost 100 Myr, from the Early Permian to Late Triassic–Early Jurassic [4].

Ordovician and Silurian slates and quartzites of very low-grade metamorphism forming kilometre-size structures with NW–SE trend represent the Hercynian basement of the Iberian Basin where the Early Permian sediments were deposited [7]. Most of the extensional basin boundary faults were reactivated Hercynian or older lineaments, trending NW–SE and north–south [12] and [24].

The Iberian and the Ebro Basins were separated during the Permian and Early Triassic by a narrow and longitudinal Palaeozoic high, the so-called Ateca-Montalbán High. On the other hand, to the southwest, another ill-defined Palaeozoic high, the so-called Serranía de Cuenca High, separated the Iberian Basin from the Cuenca–Valencia Basin (Fig. 1) [4]. Thus, the present-day configuration of the Iberian Ranges presents two Palaeozoic–Mesozoic chains, the Aragonese and Castilian Branches, that roughly mimic the Permian–Triassic basin and its bounding Palaeozoic highs.

Small and isolated extensional basins were present in the Iberian Basin during the beginning of the Late Permian. Later on, a new subsidence regime allowed the connection between most of these small basins; during the Early Triassic, it had exceeded most of the highs that were separating them. During this mature phase, other different basins of the Iberian microplate, which was located close to the equatorial eastern part of Pangea [16], [21], [30], [31] and [32], were also connected.

A closer look at the Permian sedimentary record in the SE Iberian Ranges reveals a much more complex facies and large thickness variations due to differential rates of extension during the synsedimentary extensional tectonics [4], [12], [18], [21], [25], [26], [27] and [28]. The sedimentary record has been subdivided into different tectono-sedimentary cycles, not always present in every location, and separated by angular unconformities and hiatuses. For the study area, the Late Permian sediments are represented by the Boniches Conglomerates and the Alcotas Siltstone and Sandstone Formations (Fig. 2) [3] and [20], both consisting of continental alluvial sediments, but only the latter will be studied in this paper.

The Alcotas Formation consists of red siltstones and sandstones, with minor presence of conglomerate lenses. In the study area, it lies apparently conformably on the Boniches Formation or unconformably on the Hercynian basement, and it is unconformably overlain by the Cañizar Formation, although other possible contacts also occur in neighbouring areas [21]. Thickness reaches 220 m in the Montán section, although some of the studied sections are not complete. This unit crops out along the entire Iberian Ranges, except for some isolated highs.

Detailed studies on the petrological composition of the Alcotas Formation are found in López-Gómez [19] and Alonso-Azcárate et al. [1]. They showed red siltstone to be the dominant lithology (of about 70%), composed of quartz, of illite, of kaolinite, and of minor amounts of feldspars and hematite. Subrounded quartz and feldspar grains and mica flakes with clay matrix and quartz cements constitute the red to pink sandstones. A general increase in the quartz content is clearly observed towards the Southeast of the Iberian Ranges.

Five detailed sections have been used for the study of the plant assemblages of the Alcotas Formation in this work (Fig. 3). Most of these sections were previously studied from the sedimentological and petrological points of view [1], [5], [6] and [19]. The location of these sections shows a general NW–SE trend in the southeastern Castilian Branch of the Iberian Ranges (Fig. 1). Although punctual characteristics differentiate each one of the sections, a general study shows common features for all of them in the whole study area. These sections have been called: Landete, Talayuelas, Chelva, Montán, and Barranco del Rubio (referred to as 1 to 5, respectively).

The Alcotas Formation can be subdivided into three different units, referred to as 1 to 3 from base to top, respectively [5] (Fig. 3). Unit 1 is up to 150 m high and consists of lensoidal sandy and conglomerate bodies that reach 120 m in width and less than 8 m in thickness. They can be isolated or amalgamated, but in any case, they mostly show multi-storey internal structure. Siltstone facies are usually massive and they show carbonate palaeosoils of nodular and laminar horizons [6]. This unit is interpreted as the evolution of sandy and gravely braided fluvial systems with huge floodplains where carbonate palaeosoils were developed.

Unit 2 ranges from 45 to 55 m. It marks a clear change in lithofacies and fluvial style, where conglomerates disappear and sandstones represent almost 70% of the total lithology. They normally show single-storey sandstone bodies with tabular geometries and abundant lateral accretion surfaces littered by comminuted plant remains. Scarce large silicified trunks and wood fragments also appear at the base of some of the sandstone bodies [11]. This unit can be interpreted as a transition from distal sandy braided rivers to high sinuosity meandering rivers. The banks were probably vegetated by tree-like plants, sometimes uprooted and transported downwater.

Unit 3 reaches 45 m in thickness. A new drastic change is noticed with respect to the previous unit. It now consists of red siltstones (about 85%), which contain lenticular pink sandstone bodies up to 25 m wide and 1.5 m thick. No carbonate palaeosoil or macro- or microflora has been found up to now in this unit. Fining-upwards sequences with planar cross-stratification at their base constitute most of the sandstone bodies. This unit is interpreted as the development of distal very low energy braided fluvial systems within extensive floodplains.

As described previously, detailed sedimentological analysis of the differentiated zones has been recently described by Arche and López-Gómez [5]. This analysis shows abrupt changes that mainly consist in variations of the alluvial style. Zone A is represented by sandy braided fluvial systems that abruptly change to meandering systems in zones B, C, and D. Finally, a new braided fluvial style conditions is installed in the uppermost zone.

The occurrence of fossil plant remains recovered from the Alcotas Formation is significant, due to the paucity of the fossil record of megafloras of identical or similar ages in western Europe, which has made it difficult to acquire knowledge on the development of flora and vegetation in the Upper Permian of Europe.

In general terms, and due to the dominance of coniferous remains, the vegetation fits in well with Alpine Permian floras, but we have observed significant changes in the biodiversity and population density within each outcrop and defined zone. The patterns of changes and vegetation dynamics are similar to those described after the study of the palynological assemblages of the Alcotas Formation. The observed changes with respect to zone A are an increase of the megaremain levels, of the biodiversity, and of the population density in zone B, followed by a general decrease of these features in zone C. A new increase happened in zone D, and a significant decrease occurred in zone E, ushering in the lack of plant remains noticed in zone F. This pattern allows us to infer paleoecological changes for a time interval close to the biotic crisis of the end-Guadalupian.