Coastal wetlands are prominent modern environments widely studied in geomorphology and ecology, but the term “coastal wetland” is still barely used for the sedimentological classification of ancient deposits. The depositional system studied here (Leza Formation, Cameros Basin, Early Cretaceous, N Spain) includes diverse carbonate and clastic facies deposited at the sea–land transition, and is an illustrative example of the wide array of sedimentary environments that may occur in coastal wetlands systems. The studied system was composed mainly of carbonate water bodies whose salinity ranged from fresh to brackish and near-marine, and which had variable input of clastic material due to their lateral connection with alluvial fans. In addition, the system also included carbonate water bodies with stronger marine influence, tide-influenced oolitic areas, and relatively restricted evaporative settings. The deposits of all these environments occur alternating with each other throughout a unit 30–280 m thick, and they range from continental to marine conditions in a relatively small area (approximately 10 km × 30 km). Thus, this sedimentological study of the Leza Fm provides an ideal opportunity to investigate challenging ancient deposits with both continental and marine features. Comparison with other modern and ancient coastal settings allows the conclusion that “coastal wetland” may be the most accurate sedimentological classification for the Leza Fm, since it was not part of a major coastal system (e.g., delta or estuary). A series of general sedimentological characteristics of coastal wetland deposits are gathered from the Cretaceous case study and from the modern and ancient examples examined. These characteristics include: predominance of shallow-water facies; common subaerial exposure and edaphic features; great variety of interrelated continental, transitional, and marine environments with contrasting hydrodynamic and hydrochemical conditions; and low-diversity biotic communities, including both continental and marine fossils, as well as fossils of ambiguous affinities.
Ancient coastal deposits are often sedimentologically challenging, since they show contrasting features of marine and continental affinities together with features of unclear or ambiguous affinity (e.g., Plint 1984; Tibert and Scott 1999; Azerêdo et al. 2002a; Arp and Mennerich 2008; Bennett et al. 2012). Even in modern settings, coastal zones are hard to define and delimit because their boundaries are typically transitional, including any environment from the landward limit of marine influence to the seaward limit of terrestrial influence (Carter 1988; Haslett 2000).
Coastal wetlands have been defined as systems located in coastal zones, which are partly inundated and partly emerged due to a fluctuating water table located at or near the surface, and in which the sea has a certain degree of hydrologic control (Ramsar Convention 2002; Mendelssohn and Batzer 2006; Baldwin et al. 2009; Wolanski et al. 2009). These systems are prone to include a wide spectrum of environments with contrasting hydrology, salinity, biota, and sedimentation (e.g., Frey and Basan 1978; Britton and Podlejski 1981; Wolanski et al. 2009). Furthermore, coastal wetlands may either occur as part of broader systems (e.g., deltas or estuaries), or as individual systems themselves (Ramsar Convention 2002; Perillo et al. 2009), and they may display variable patterns of sedimentation related to hydrological, chemical, and biological variations (Frey and Basan 1978; Baldwin et al. 2009; Wolanski et al. 2009; Davidson-Arnott 2010). However, the term “coastal wetland” is still rarely used in sedimentological studies of ancient deposits (see exceptions on Lacovara et al. 2003 and Greb and DiMichele 2006). Gastaldo et al. (2006) argued that unequivocal examples of coastal wetlands (coastal marshes, in particular) are rare in the stratigraphic record due to their poor preservation potential. They also noted that ancient examples might be difficult to recognize due to the subtlety of some of their characteristic features. Nonetheless, in this article we consider that “coastal wetlands” may be a very useful term for the general classification of some ancient transitional deposits that do not entirely fit within the definitions of archetypical coastal depositional systems (e.g., deltas, estuaries, tidal flats, sabkhas, lagoon–barrier systems).
Such is the case of the Leza Formation (Early Cretaceous, N Spain), a mixed carbonate–clastic unit deposited at the tectonically active margin of a rift basin, which records a remarkable variety of interrelated sedimentary environments (from alluvial fans to hypersaline settings, including fresh water bodies and tide-influenced areas) at the transition between land and sea. Therefore, the sedimentological study of the Leza Fm presented in this work provides an ideal opportunity to analyze the complex array of sedimentary environments that may characterize coastal wetlands, and to study the not commonly observed association of these systems with alluvial fans. This work also aims to compare the sedimentology of the Cretaceous example with modern analogues of coastal wetlands and with other examples of ancient coastal deposits, in order to outline the general sedimentological features that may define ancient coastal wetland deposits, and which may provide clues to further characterization of these multifaceted systems in the geological record.
The Cameros Basin (N Spain, Fig. 1) is the northernmost basin of the Mesozoic Iberian Rift System (Fig. 1A). It is a latest Jurassic–Early Cretaceous intraplate rift basin with up to 6500 m of sediments deposited on top of a Triassic–Jurassic substrate (Fig. 1B, C; Mas et al. 2011; Omodeo Salè et al. 2014). The sedimentary infill of the basin includes continental and transitional deposits (Mas et al. 2011; Quijada et al. 2013, in press; Suarez-Gonzalez et al. 2013) and, in general, shows a trend from proximal facies located on the W and SW, towards distal facies on the E and NE (Fig. 1C; Mas et al. 2011). The Leza Formation is a mixed carbonate–clastic unit, upper Barremian–lower Aptian in age, located at the northernmost margin of the basin (Fig. 1C, D). During this period, the SW Cameros Basin was dominated by fluvial deposits (Urbión Group, Fig. 1C) which graded towards the NE to mixed siliciclastic–carbonate fluvio-lacustrine deposits (Enciso Gr, Fig. 1C; Mas et al. 1993, 2011), renowned for their abundance of dinosaur footprints (e.g., Moratalla and Hernán 2010, and references therein). Towards the NE margin of the basin, the Enciso Group changes laterally to the Leza Fm (Fig. 1C), which was deposited in a series of small individual tectonic depressions, formed by faulting of the Jurassic substrate of the basin (Figs. 1D, 2; Alonso and Mas 1993; Suarez-Gonzalez et al. 2013). Two units were deposited in these tectonic depressions: the Jubera Fm and the Leza Fm (Figs. 1D, 2; Alonso and Mas 1993; Suarez-Gonzalez et al. 2013). The siliciclastic Jubera Fm is interpreted as alluvial-fan deposits related to the erosion of the faulted substrate (Alonso and Mas 1993; Mas et al. 2011). The Leza Fm, predominantly formed by carbonates with a variable content of clastic facies, overlies and changes laterally to the Jubera Fm (Figs. 1D, 2). This unit includes evidence of marine influence (Guiraud 1983; Alonso and Mas 1993), which is currently regarded as stronger than previously considered (Suarez-Gonzalez et al. 2013, in press). Tectonic activity caused significant variations in the thickness (from 20 to 280 m) and distribution of facies in each tectonic depression of the Leza Fm (Figs. 3, 4). This, together with the lack of biostratigraphically relevant fossils, makes accurate correlation between all the stratigraphic sections difficult. The only general trend observed throughout the Leza Fm is the upwards increase in marine influence, which has been related to the widespread early Aptian sea-level rise, and which can be used as a tentative correlation tool (see figures 6 and 13 of Suarez-Gonzalez et al. 2013).
In order to accurately characterize the general depositional system of the Leza Fm, a detailed facies analysis was conducted using 12 complete stratigraphic sections which cover all the main outcrops of the unit (Fig. 3). Sections were measured with a decimeter resolution and logged at 1∶100 scale. Figures 3 and 4 show synthetic logs of the sections. More than 750 samples were collected, and a thin section was prepared from each sample for petrographic analysis. Thin sections are 30 µm thick, polished and uncovered, and they were partially stained with Alizarin Red S and potassium ferricyanide (Dickson 1966) for accurate distinction between calcite and dolomite.
Following standard facies-analysis methodology (e.g., Collinson 1969), the Leza Fm deposits are here described and classified using a series of facies, understanding the term facies as all the lithological and paleontological characteristics of a sedimentary rock (sensuTeichert 1958), observed both macroscopically and microscopically. Facies associations are considered as “groups of facies genetically related to one another” (Collinson 1969, p. 207). Sequences of facies have been described following the original definition of “lithologic sequence” of Lombard (1956, p. 451): “a series of at least two lithologic elements forming a natural succession without relevant interruptions other than the stratification planes”. When facies of an association are interbedded with each other but do not show such a repeated characteristic sequence, they are described as showing a random distribution. The description of carbonate facies is based on their depositional texture, following the classification of Dunham (1962). However, for microbial carbonates, the terminology of Riding (2011) has been used, instead of the general term boundstone of Dunham (1962), for a more precise characterization of this type of facies. For this article, a letter code was created for each carbonate facies (Table 1). The classification and codes of clastic facies (Table 1) are based mainly on the classification originally proposed by Miall (1977), but using modifications by Waresback and Turbeville (1990), Horton and Schmitt (1996), and on our own terminology for facies not totally equivalent to the ones defined by these authors.
Sedimentology of the Leza Formation
Facies distinguished in the Leza Fm deposits are summarized in Table 1. Some facies show unchanging features throughout the unit, but other facies have a particular geographic distribution or show relevant variations through space (between stratigraphic sections) and time (vertical variations through the sections). In general, the facies show differences between the eastern sector (Fig. 3) and the western sector of the unit (Fig. 4).
Black Limestones Facies Association
This association (Figs. 5, 6) includes the most abundant facies of the Leza Fm: black limestones (bMW and bWP), sandy black limestones (SL), oncoid-rich limestones (OL), and marls (M). Facies of this association typically have black color in fresh cut, as well as a fetid smell.
Two black limestone facies are distinguished (Fig. 5): a) black mudstone–wackestones (bMW) of ostracodes and less common charophytes and vertebrate remains; b) black wackestone–packstones (bWP) of ostracodes, charophytes, dasycladales, gastropods, oncoids, black pebbles, fragments of microbial colonies, and vertebrate remains. Locally, thin beds with grainstone texture and wave and current ripples are also observed. These facies are very common both in the eastern and western sector of the unit (Figs. 3, 4). They occur as beds 0.1–2 m thick, with flat tops and bases, which are commonly arranged in thickening-upwards sequences, 1–4 m thick (Fig. 5A–C), which are laterally continuous for several hundreds of meters. Mudstone–wackestone facies typically occur at the lower part of the sequences, whereas wackestone–packstone facies are common at the upper part (Fig. 5A, B). The top surfaces of sequences typically show desiccation cracks, ferruginous surfaces, dinosaur footprints, small (∼ 5 mm wide) vertical burrows, root traces, and nodular and brecciated horizons (up to 1 m thick) with gray-ochre mottling (Fig. 5D, E).
Black limestones (bMW and bWP) include a variety of fossil remains, both of continental (charophytes, Fig. 5F, G) and marine (dasycladales, Fig. 5H) affinities. Ostracodes are the most abundant fossils. Charophytes include clavatoracean-rich assemblages (Fig. 5G) and porocharacean-rich assemblages (Carles Martín-Closas, personal communication; Fig. 5F). Dasycladales are abundant but belong to a single species, Salpingoporella urladanasi (Marc Conrad, Nicolaos Carras, and Ioan Bucur, personal communication; Fig. 5H), and occur at the middle and upper parts of stratigraphic sections (Figs. 3, 4). S. urladanasi is a Barremian–Albian alga from the northern Tethys, commonly found in restricted marine facies, associated with foraminifers, ostracodes, and charophytes (Carras et al. 2006). Vertebrate remains are also varied in these facies (Pereda-Suberbiola et al. 2012). Furthermore, microbial carbonates are common components of the black limestones: in addition to oncoids (described in detail in the next paragraph), fragments of filamentous microbial colonies occur in the lower part of some of the studied stratigraphic sections, and thrombolites (sensuRiding 2011) have been observed in the upper part of the Leza River section associated with dasyclad-rich levels (Fig. 4).
Oncoids are one of the main components of the black limestone facies association. In fact, they commonly occur in rock-forming abundance, creating oncoid-rich limestones (OL) with packstone–grainstone textures (Figs. 5, 6E), which occur both in eastern and western outcrops of the unit. Beds of this facies are up to 2 m thick, and typically show irregular bases (Fig. 6A, C). Some beds show cross-bedding formed by sets < 30 cm thick (Fig. 6D). Oncoids are subspherical to ellipsoidal, and typically smaller than 4 cm in diameter (Fig. 6E), although larger oncoids, up to 15 cm in diameter, are observed. Oncoid-rich limestones also contain abundant bioclasts, mainly ostracodes, charophytes, and gastropods and less abundant dasycladales, with variable amounts of clastic material (quartz grains and Jurassic lithoclasts).
Facies of this association can be very rich in clastic material, especially in the lower half of the Leza Fm (Figs. 3, 4). When clastics dominate (mainly quartz grains and Jurassic lithoclasts), a different facies can be distinguished: sandy black limestones (SL). Sandy black limestones have the same general features as black limestones (see description above) and also occur as thickening-upwards sequences formed by beds 0.1–2 m thick (Fig. 5A). In addition, sandy black limestones are locally observed as thin layers (< 10 cm thick) with irregular bases within beds of the black limestones facies (Fig. 5A). Sandy black limestones typically alternate with the clastic facies association (described below), especially in outcrops from the western sector (Fig. 4). In the lower half of the western sections, some sequences are composed of laterally continuous levels of sandy black limestones, 10–30 cm thick, interbedded with silty sandstones (sS, described below), 1–15 cm thick (Fig. 6B, F, G). In these cases, sandy black limestones contain very abundant vertical irregular structures (up to 1 cm wide and 15 cm long) with acute lower endings, which locally show branching, and which are filled by silty sandstones of the overlying bed (Fig. 6G). These features suggest their origin as root traces (cf. Klappa 1980). Locally, in the upper part of some sections from the western sector (Fig. 4) sandy black limestones occur associated with skeletal stromatolites (sensuRiding 1977), which are stratiform, domal, or columnar, and are microscopically composed of well-preserved cyanobacterial filaments (Suarez-Gonzalez et al. in press).
Marls (M) occur mostly at the lower half of the studied sections (Figs. 3, 4). They have black-gray colors and flaky outcrop appearance, and they occur as tabular beds up to 1 m thick, which, locally, contain levels with desiccation cracks or gray carbonate nodules (Fig. 5A). Marl beds alternate with black limestones, being more abundant at the lower part of the black limestone sequences (Fig. 5A, B). Marls are composed of clay minerals, calcite, silt-size quartz grains, and bioclasts (mainly ostracodes and less common charophytes).
The thickening-upwards sequences characteristic of black limestones and of sandy black limestones (Fig. 5) are strongly similar to shallowing-upwards sequences described from modern and fossil, shallow, carbonate-dominated water bodies, such as lakes (e.g., Freytet 1973; Murphy and Wilkinson 1980; Arribas 1986; Platt and Wright 1991; Fregenal-Martínez and Meléndez 1994; Arenas and Pardo 1999) or coastal ponds and lagoons (e.g., James 1979; Strasser 1988; Pratt et al. 1992; Strasser et al. 1999). As in the case of the Leza Fm, the top surfaces of these shallowing-upwards sequences are commonly marked by root traces and mottled, nodular, and/or brecciated horizons, features indicative of edaphic alteration (e.g., Alonso-Zarza and Wright 2010b). Therefore, the black limestones facies association can overall be interpreted as deposited in shallow carbonate water bodies separated by palustrine areas. The shallow nature of the water bodies is supported by the thickness (< 4 m) of sequences, produced by the progressive infilling of the water bodies.
Water bodies had a locally important input of clastic material, especially in the western sector and during early stages (lower part of sections, Fig. 4), where sandy black limestones are more abundant. Clastic discharges into the water bodies are further indicated by the decimeter-scale alternation of laterally continuous levels of sandy black limestones and silty sandstones (Fig. 6B, F). This alternation represents the cyclic repetition of a) periods of carbonate precipitation, b) moderate edaphic alteration of the carbonate sediment, generating rootlets, and c) discharge of fine-grained clastic material which fills the rootlets. These cyclic processes suggest that sedimentation took place in very shallow and relatively wide water bodies which were easily invaded by vegetation and which underwent periodic clastic supply. Constant generation of accommodation space was necessary for preservation of the cycles.
Oncoid-rich limestones also typically include clastic material. Features of oncoid-rich limestone beds (Fig. 6A, C, D) suggest that they were deposited by channelized tractive currents. Oncoid-bearing channels are very common components of modern and ancient fluvio-lacustrine systems with carbonate-rich waters (e.g., Arenas-Abad et al. 2010, and references therein). The fact that oncoids are also commonly observed in black limestone sequences suggests that the channels drained into the water bodies of this facies association.
The black limestones facies association contains fossils of continental and marine affinities. In addition, Cretaceous homogeneous populations of porocharacean charophytes, as those observed in some sequences of this association, are commonly interpreted as developed in brackish-water coastal conditions (Martín-Closas and Grambast-Fessard 1986; Mojon 1989; Schudack 1993; Climent-Domenech et al. 2009; Villalba-Breva and Martín-Closas 2013). These facts indicate that the water bodies where these facies were deposited had variable influence of both freshwater and seawater, developing fresh, brackish, or near-marine salinity.
In summary, the black limestones facies association was deposited in a system of shallow water bodies separated by palustrine areas and by oncoid-rich channels. Sedimentation in this system was dominated by carbonate precipitation with contribution of microbialite accretion, and with locally significant clastic discharges. Fossil content indicates that this system of water bodies was somehow connected to the marine realm, which suggests that the system was located on a coastal plain. Overall, freshwater influence was dominant during sedimentation of the lower part of the Leza Fm and it was stronger in the western sector, whereas seawater influence increased through time, being more common at the eastern sector (Figs. 3, 4).
Well-Bedded Gray Limestone Facies Association
This association (Fig. 7) includes laminated fenestral limestones (Fen) and well-bedded gray limestones (gMW and gPG). These facies have been observed only in the western sector of the Leza Fm, especially in the middle and upper part of sections (Fig. 4), and they occur alternating with each other and with other facies of the unit with a random distribution (Fig. 7A–C). Laminated fenestral limestones are most common at the middle part of measured sections, and well-bedded gray limestones predominate in the upper part (Fig. 4).
Laminated fenestral limestones (Fen) are formed by thin (< 1 mm) micritic laminae separated by horizontally elongated fenestrae (up to 3 mm thick), which are filled by geopetal sediment and calcite cement (Fig. 7D, E). Laminae display clotted-peloidal microfabrics, dense-micritic microfabrics with rare relicts of microbial filaments, or agglutinated microfabrics with quartz grains, peloids, and bioclasts (scattered ostracodes, miliolids, and rare fragments of charophytes and dasycladales). Small vertical cracks are observed in many laminae (Fig. 7E).
Well-bedded gray limestones include two different facies: a) gray mudstone–wackestone (gMW) of ostracodes, miliolid foraminifers (Fig. 7G, H), and gastropods; b) gray packstone–grainstone (gPG) of peloids, ostracodes, miliolids (Fig. 7F), and gastropods, locally rich in silt-size quartz grains. Small-scale cross bedding has been observed in some grainstone beds, in which ostracode valves are commonly stacked in “cup-in-cup” arrangement (sensuWakefield 1995; Fig. 7F). Beds of the well-bedded gray limestones are 10–50 cm thick, with planar bases and tops (Fig. 7B). Bed tops commonly display desiccation cracks, dinosaur footprints, and/or small vertical burrows. Some beds display nodular structure and/or abundant irregular vertical sediment-filled cavities (Fig. 7C).
Laminated fenestral limestones are commonly interpreted as being the record of periodic desiccation episodes in very shallow peritidal settings (e.g., Shinn 1983; Mazzullo and Birdwell 1989; Demicco and Hardie 1994), which is supported by abundant small vertical cracks in the laminae. Clotted-peloidal microfabrics and relicts of microbial filaments suggest an origin related to successive accretion of microbial mats, as observed in modern microbial laminated fenestral deposits (Logan 1974; Hardie and Ginsburg 1977).
Gray mudstone–wackestone facies indicate deposition under relatively calm conditions dominated by benthic fauna: ostracodes, miliolids, and gastropods. Gray packstone–grainstone facies contain the same fossil assemblage, but cross-bedding and common grainstone textures suggest deposition under agitated conditions. In addition, “cup-in-cup” arrangement of ostracode valves is regarded as an indicator of very shallow environments under relatively weak but constant wave action (Guernet and Lethiers 1989; Wakefield 1995). Guernet and Lethiers (1989) indicated that this arrangement is favored by ostracode assemblages with high abundance and low diversity, typically found in lakes and coastal lagoons.
The thin, tabular, and continuous nature of beds of this facies association, together with the common presence of features indicating subaerial exposure (i.e., desiccation cracks, dinosaur footprints, nodular structures, or vertical cavities; cf. Esteban and Klappa 1983; Alonso-Zarza and Wright 2010b), suggest that these facies were deposited in relatively wide and very shallow water bodies. These water bodies hosted a marine-related biotic association with high abundance and low diversity, lacking abundant indicators of freshwater habitats, but also lacking clear indicators of normal-marine-salinity waters. These facts suggest that the water bodies were influenced by seawater but had stressful conditions limiting diversity, such as common desiccation, and anomalous or rapidly changing salinity.
Oolite–Stromatolite Facies Association
This facies association (described in further detail by Suarez-Gonzalez et al. 2014, in press) is observed only in the middle and upper parts of sections from the eastern sector of the Leza Fm (Fig. 3). Oolitic facies are formed by oolitic grainstones (OG) and gray mudstones (GM), which are commonly interfingered, forming fining-upwards sequences up to 1.5 m thick displaying flaser, wavy, and lenticular bedding (Fig. 8), and being topped by dinosaur footprints, desiccation cracks, and/or small vertical burrows (Fig. 8A). These two facies may also occur as independent beds up to 40 cm thick (Fig. 8B). Oolitic grainstones are composed of medium-coarse grain-size ooids, peloids, micritic intraclasts, ostracodes, and miliolids (Fig. 8C, D). Gray mudstones have either dense micritic or clotted textures, containing scattered peloids, ostracodes, and miliolids (Fig. 8E). At the tops of the sequences, these facies are typically associated with flat-pebble breccias (Fig. 8F, G) and with agglutinated stromatolites (Agg) sensuRiding (1991). These stromatolites have stratiform and domal morphologies (Fig. 8F), and in general they show an alternation of micrite-rich and grain-rich laminae (Suarez-Gonzalez et al. 2014). Locally, pseudomorphs after sulfates are observed within the stromatolites, deforming and replacing adjacent laminae.
The cyclic interfingering of oolitic grainstone and gray mudstone observed in this association, and the sedimentary structures it produces (flaser, wavy, and lenticular bedding), together with the common presence of flat-pebble breccias and agglutinated stromatolites, partially formed by trapping and binding of particles, have led to interpretation as deposition in tide-influenced environments (Suarez-Gonzalez et al. 2014, in press). The presence of desiccation cracks and dinosaur footprints at the tops of sequences indicates that sediments were commonly desiccated, which is consistent with the common presence of flat-pebble breccias, typically interpreted as the result of erosion of a previously desiccated and indurated muddy layer (Demicco and Hardie 1994).
Therefore, this association can be interpreted as deposited at the marginal areas of shallow water bodies that underwent periodic and cyclic changes in water agitation, due to tidal influence (Suarez-Gonzalez et al. 2014, in press). Marine-related fossil content further supports that water bodies were connected with the marine realm, allowing tidal currents to influence sedimentation and hindering the development of freshwater biotic associations, which occur in other facies of the Leza Fm.
Evaporite–Dolomite Facies Association
This association (Fig. 9) is characterized by dolomites (D) with pseudomorphs after evaporites (Ev) and less abundant dolomite breccias (Dbr). These facies have been observed only in the eastern sector of the Leza Fm, where they occur mostly at the upper half of sections (Fig. 3). Dolomites (D) are thin-bedded to laminated, show gray to ochre colors (Fig. 9), and under the microscope they display dense micritic or peloidal textures, the latter commonly including silt-size quartz grains. Dolomites commonly show wavy lamination (Fig. 9B) formed by the alternation of thin irregular layers of dense micritic texture and peloidal–silty texture. Dolomites include variable proportions of pseudomorphs after evaporites (Ev; Fig. 9), and their fossil content is scarce, with only scattered ostracodes and miliolids. The top surfaces of dolomite beds commonly display desiccation cracks, ferruginous surfaces, and/or small vertical burrows. Pseudomorphs after evaporites (EV) are currently composed of quartz, calcite, and dolomite, but they preserve lenticular and tabular habits (Fig. 9C, D), characteristic of gypsum and anhydrite, respectively (e.g., Warren 2006, and references therein). They are typically submillimetric to millimetric, scattered in the dolomites (Fig. 9E) or grouped in laterally continuous horizontal layers (Fig. 9F). Some beds include big lenses, up to 35 cm long, and irregular nodules, up to 10 cm across (Fig. 9G), of evaporite pseudomorphs that locally coalesce forming enterolithic structures. Pseudomorphs deform the adjacent sediment and also incorporate small dolomicrite fragments (Fig. 9C–E). Locally, pseudomorph-rich beds show dolomite breccias (Dbr), up to 50 cm thick, composed of poorly sorted angular fragments of dolomite surrounded by a crystalline mass of quartz, calcite, and dolomite cements (Fig. 9H). The contact between fragments and cements shows euhedral tabular and lenticular habits, suggesting that the crystalline mass that surrounds the fragments was originally formed by sulfates, which were subsequently dissolved and/or replaced by quartz, calcite, and dolomite.
Dolomite beds are very thin and show common features related with subaerial exposure (desiccation cracks and ferruginous surfaces), suggesting that this facies was deposited under very shallow conditions. Common wavy lamination indicates alternation of agitated and calm conditions. Pseudomorphs after sulfates deforming and including the adjacent carbonate sediment indicate intrasedimentary displacive and replacive sulfate growth, formed by progressive increase in saturation of interstitial water due to capillary evaporation, as commonly observed in continental and marginal marine evaporative environments (Schreiber and El Tabakh 2000; Warren 2006; Ortí 2010).
These features suggest that this association was deposited in very shallow and relatively restricted, high-salinity water bodies. Conditions of high salinity explain the scarce biotic content of the evaporite-dolomite facies, composed of ostracodes and less abundant miliolids. However scarce, this biotic content and the lack of freshwater fossils point to influence of seawater during the sedimentation of these facies, which would also provide a likely source of sulfate to the water bodies. Furthermore, this facies association is typically interbedded with the oolite–stromatolite facies association (Fig. 3), which suggests that their sedimentary environments may have been laterally associated. Similarly, modern tide-influenced associations of oolitic sands and agglutinated stromatolites from Shark Bay (Australia) are laterally related to restricted areas where gypsum precipitates (Hagan and Logan 1974; Jahnert and Collins 2012).
Clastic Facies Association
Clastic facies of the Leza Fm include conglomerates, sandstones, and less common siliciclastic mudstones (Figs. 10, 11). These facies are observed in most measured sections, but they are more abundant in the western sector of the unit, especially in the lower part, because abundance and grain size of the clastic facies typically decreases upwards in the sections (Figs. 3, 4). Clastic facies typically show a random distribution (Fig. 10A), and they are commonly interbedded with the carbonate facies described above (Figs. 3, 4).
Conglomerate beds (up to 3 m thick) are tabular and laterally very continuous, and they have planar to slightly irregular bases. Conglomerates are typically poorly sorted, and they are composed of subrounded to subangular pebbles and cobbles (Figs. 10, 11). Locally, scattered boulders up to 40 cm in diameter are observed. The clasts are mainly lithoclasts of Jurassic limestone and sandstone, and quartzite pebbles (Fig. 10B, C). Conglomerates from the western sector of the unit are generally dominated by carbonate lithoclasts (pebbles and cobbles), whereas quartzite pebbles dominate in conglomerates from the eastern sector. Two main conglomerate facies have been distinguished: a) Muddy conglomerates (mG) are clast-rich, matrix-supported, typically massive and ungraded, but locally showing outsized cobbles and boulders at the top, producing inverse grading (Fig. 10D). Their matrix is formed mainly by clay- and silt-size particles with minor sand grains. b) Sandy conglomerates (sG) are matrix-supported with a coarse- to very coarse-grained sandy matrix and a variable content of pebbles and cobbles. Sandy conglomerates are typically ungraded, but normal grading locally occurs, and they are either massive, horizontally stratified, or cross-stratified. Cross-bed sets are up to 1.5 m thick, and they are formed by foresets that thin and flatten downdip and updip, generating tangential bottomsets and topsets (Fig. 10E). Vertical and lateral transition between sandy conglomerates and sandstones is commonly observed, even in the same bed (Fig. 10F, G).
Sandstone beds are up to 1.5 m thick, they are laterally and vertically associated with conglomerate beds (Figs. 10, 11) and commonly interbedded with carbonate facies (Figs. 3, 4). Several sandstone facies are observed: a) Sm: ungraded, massive, poorly sorted, medium- to very coarse-grained sandstones with scattered pebbles, commonly interfingered with and grading into conglomerates (Fig. 10G). b) Sh: fine- to medium-grained, well sorted sandstones with parallel lamination, interbedded with conglomerates (Figs. 10G, 11B). c) Sc: cross-bedded, medium- to very coarse-grained, poorly sorted sandstones with pebbles (Fig. 10F). They are vertically and laterally associated with conglomerates, and are formed by cross-bed sets with tangential bottomsets and topsets, similar to those described for the sG facies (Figs. 10F, 11C). Cross-bed sets commonly display sigmoid-like morphologies, convex-up geometries at the topsets (Fig. 11C), and laterally offset stacking patterns (sensuTurner and Tester 2006). Some Sc beds have very abundant bioturbation at their top surfaces, formed by horizontal, vertical, and inclined burrows, which are broadly cylindrical, 0.5–1.5 cm wide, with irregular borders and with locally Y-shaped branching (Fig. 11D). Burrows are filled with sandstone of different grain size than the surrounding sediment, and meniscate structures have not been observed. d) Sr: fine- to coarse-grained, moderately sorted sandstone with trough cross-bedding sets up to 4 cm thick and with current ripples (Fig. 11E). Climbing-ripple geometries also occur. Sr facies are commonly interbedded with carbonate facies. e) sS: poorly sorted, massive silty sandstones, observed in the lower part of stratigraphic sections from the western sector, as laterally continuous thin levels (< 15 cm) interbedded with sandy black limestones with rootlets (see description above; Fig. 6).
Petrographically, sandstones are composed mainly of quartz grains and Jurassic lithoclasts (Fig. 11F, G) with rare white mica and feldspar grains. Individual ooids and bioclasts from the Jurassic limestone lithoclasts are commonly included as grains in the sandstones, as well as in the sandy matrix of conglomerates (Fig. 11G). Locally, Jurassic lithoclasts and individual Jurassic ooids show subrounded sac-like borings, 50–250 µm in diameter, on their surfaces (Fig. 11F). Size and shape of these borings are consistent with those of the Entobia ichnogenus, a typically marine bioerosion, generally interpreted as produced by sponges (e.g., Bromley 1994; de Gibert et al. 2012).
The top surfaces of many conglomerate and sandstone beds show a thin ferruginous surface and/or orange-red mottling (Fig. 11H, I). Strongly mottled beds typically contain abundant white carbonate nodules, up to 15 cm thick, either subrounded or vertically elongated, up to 1 m long and locally showing downwards branching (Fig. 11H, I).
Siliciclastic mudstones (F), composed of clays, silt grains, and variable amounts of carbonate (Alonso-Azcárate et al. 2005), are much less common than conglomerates and sandstones, and are typically interbedded with sandstone beds. Siliciclastic mudstones form beds up to 0.5 m thick, with centimeter-scale horizontal stratification and reddish or greenish colors. Stratification surfaces commonly show desiccation cracks and small (less than 4 mm wide) vertical burrows. Ostracodes and charophytes have locally been observed.
The main features of clastic deposits of the Leza Fm indicate that their origin is related to the erosion of the Jurassic substrate of the Cameros Basin, and that their transport was relatively limited, being deposited in alluvial systems close to their source area. In fact, compositional differences observed in the conglomerate clasts between the eastern sector (mainly quartzite pebbles) and the western sector (mainly carbonate lithoclasts) of the Leza Fm can be explained by the lithological variations of the source areas, the Middle and Upper Jurassic deposits of the northern Cameros basin substrate. This substrate is mainly composed of limestones on the western sector, and of sandstones and conglomerates on the eastern sector (Alonso and Mas 1990; Wilde 1990). Conglomerates of the Leza Fm are typically matrix-supported, indicating en masse deposition by sediment-rich flows (e.g., Benvenuti and Martini 2002), but differences in the matrix compositions are observed. Sandy conglomerates are the most abundant conglomerate facies and show sandy matrix, whereas muddy conglomerates have a clay- and silt-size matrix. The low mud content of sandy conglomerates suggests that they were transported and deposited by hyperconcentrated flows, characterized by the transport of large quantities of suspended sandy–gravelly sediment with very little (< 3–10%) fine fraction (Costa 1988; DeCelles et al. 1991; Mulder and Alexander 2001; Pierson 2005). In contrast, matrix composition of muddy conglomerates suggests that they were transported and deposited by debris flows (Costa 1988; Mulder and Alexander 2001; Pierson 2005). In fact, inverse grading, locally observed in mG facies, is a common feature of debris-flow deposits (e.g., Fisher 1971).
Sandy conglomerates commonly show gradual lateral and vertical transitions with pebbly sandstones (Sm and Sc), without sharp contacts between them, which suggests that the three facies were deposited by the same hyperconcentrated-flow mechanism, differentiated only by a lower gravel content, probably due to decreasing flow velocity (Pierson 2005). This predominance of tabular and laterally extensive hyperconcentrated-flow deposits, together with their common alternation with parallel-laminated sandstones (Sh), are characteristic features of alluvial-fan systems dominated by unconfined episodic sediment-rich floods, as opposed to streamflow-dominated alluvial fans (Costa 1988; Blair and McPherson 1994; Committee on Alluvial Fan Flooding 1996; Mutti et al. 1996).
The common occurrence of siliciclastic mudstones and carbonate facies (mostly deposited by subaqueous suspension fallout) interbedded with conglomerates and sandstones suggests that at least part of the clastic facies of the Leza Fm may have been deposited subaqueously, as interpreted for similar alternations by Horton and Schmitt (1996). The fact that the interbedded carbonates (mainly sandy black limestones) contain the same clastic material as that which composes the clastic facies supports the hypothesis of a lateral connection between alluvial fans and the water bodies where carbonate facies were deposited. In fact, the cross-bed sets of some conglomerate and sandstone facies (sG and Sc) are similar to “nested lenticular sets” of Turner and Tester (2006), interpreted as lobate mouth bars within shallow interdistributary lakes, and to the “flood-generated sigmoidal bars” of Mutti et al. (1996), formed when hyperconcentrated flows enter shallow ephemeral lakes. Thus, it is interpreted here that most of the sG and Sc deposits (and also Sr, which occur interbedded with carbonates) were deposited subaqueously by the progradation of lobes of clastic material in shallow water bodies. This interpretation is further confirmed by the presence of borings at the surface of some sand-size carbonate lithoclasts and by the presence of strongly burrowed Sc facies, features that require a certain time to develop under water (Blair and McPherson 2008).
In summary, clastic facies were formed as alluvial fans sourced mainly in the faulted Jurassic substrate of the Cameros Basin. These fans were distally connected to the water bodies in which carbonate facies were deposited. The eroded material was transported down-fan by gravity mainly as unconfined hyperconcentrated flows, and deposited on the fan slopes and in the water bodies distal to the fans. The orange-red mottling and white carbonate nodules overprinted on some beds of the clastic facies are typical of pedogenic calcretes (e.g., Alonso-Zarza and Wright 2010a, and references therein), and represent prolonged periods of no deposition during which vegetation developed over alluvial-fan deposits, probably at the palustrine fringes of the water bodies.
General Depositional System of the Leza FM
The Leza Fm depositional system included a significant variety of paleoenvironments, located at the sea-land transition, and which covered virtually all of the range from proximal continental to marginal marine settings, including: alluvial fans; fresh and brackish water bodies; water bodies with stronger seawater influence; palustrine areas; tide-influenced environments; and relatively restricted marine-influenced evaporative environments. Furthermore, facies associations occur intercalated with each other throughout the stratigraphic sections (Figs. 3, 4), suggesting that sedimentary environments were closely interrelated and commonly changed through time. For example, continuous interbedding of clastic and carbonate facies, especially in the western sector, indicates that proximal areas of the system were dominated by alluvial fans coming down from the faulted Jurassic substrate of the basin and discharging into shallow water bodies (Fig. 12). Therefore, Jurassic carbonates may have been an important source of carbonate-saturated water to the rest of the system. Carbonate facies also occur intercalated with each other (Figs. 3, 4). This fact, together with the ubiquitous features of subaerial exposure and edaphic alteration, and the common evidence of marine influence, suggests that the general depositional system was an extensive low-gradient area, very prone to both flooding and desiccation, located in a coastal setting (i.e., coastal plain). This area was mainly covered by shallow water bodies surrounded by palustrine areas.
Given this general setting, relatively small variations in any of the factors controlling sedimentation (e.g., freshwater supply, tectonics, eustasy; see Suarez-Gonzalez et al. 2013 for further details) could have easily triggered important spatial changes in the sedimentary environments of the system, which, if repeated through time, would produce the continuous interbedding of contrasting facies observed in the stratigraphic sections (Figs. 3, 4). Therefore, the general depositional system of the Leza Fm was probably configured as a complex mosaic of laterally related sedimentary environments, as it is observed in other low-gradient coastal systems (e.g., Laporte 1967; Martín-Chivelet and Giménez, 1992; Lacovara et al. 2003; Wilkinson and Drummond 2004; Maloof and Grotzinger 2012). In addition, the facies mosaic of the Leza Fm shows further complication by the presence of alluvial fans proximal to the carbonate system.
This general system was divided in two subsystems: a) the western sector, where alluvial fans, clastic-influenced water bodies, and fresh-water bodies were especially abundant (Figs. 4, 12C); and b) the eastern sector, with less clastic influence, and where tide-influenced water bodies were associated with evaporitic areas, probably due to a weaker freshwater input than in the western sector, and to a location closer to the marine realm (Fig. 3, 12C). Therefore, the Leza Fm not only represents a system with a wide spectrum of environments, from alluvial fans to marginal marine, but also a complex coastal system where all of those paleoenvironments were closely interrelated with each other.
Looking For Analogues And Names For the Leza FM System: Coastal Wetlands
Such a complex system with many contrasting paleoenvironments and located on the continental-marine transition is not easily classified from a sedimentological point of view because many different names are applied to similar systems in the literature, and because contrasting classifications could be applied to different sectors or environments of the same system. Thus, terminology commonly applied to modern and ancient examples similar to the Leza Fm is here reviewed.
Tidal flats (developed in estuaries and tide-dominated deltas, or as open-coast tidal flats; Dalrymple 2010) are settings favorable to the development of low-gradient sedimentary systems with abundant shallow water bodies, and relatively vegetated areas (e.g., Bird 2008; Gao 2009; Dalrymple 2010). In addition, if tidal flats have significant input of meteoric water (e.g., Bahamas; Hardie and Garrett 1977; Maloof and Grotzinger 2012) they are especially suitable to develop a complex mosaic of environments with variable salinities, like that of the Leza Fm. Similar ancient deposits have, in fact, been interpreted as associated with tidal flats (e.g., West 1975). The studied unit includes tide-influenced deposits, but it lacks other typical facies associations of tidal systems (e.g., sand bars, sand or mud flats, tidal channels; Dalrymple 2010), suggesting that this unit was not part of an estuary or other tide-dominated system.
Systems equivalent to that of the Leza Fm may occur on modern delta plains (e.g., Sasser et al. 2009; Bhattacharya 2010; Flaux et al. 2012), as proposed for similar ancient deposits (e.g., Águeda et al. 1991) and even for some areas of the Leza Fm (Guiraud 1983). However, the absence of major distributary-channel deposits associated with the unit (as noted by Guiraud 1983 himself) discourages classification as part of a deltaic system. Moreover, the general evolution of the unit shows a retrograding trend, which contrasts with the characteristic prograding pattern of delta systems (e.g., Bhattacharya 2010).
The abundance of coarse-grained clastic deposits observed in some areas of the system, and the fact that parts of them were deposited in fresh and marine-influenced water bodies, could prompt a classification of the clastic facies as fan deltas, which are sediments delivered by alluvial fans, and deposited entirely or mainly subaqueously within a standing body of water (Nemec and Steel 1988). Although part of the clastic facies of the Leza Fm are interpreted here as deposited subaqueously, water bodies of the system were shallow and relatively ephemeral. In fact, the abundant edaphic features observed in the clastic facies shows that even when deposited subaqueously, clastic deposits were subject to common variation of the water table. In contrast, fan deltas are typically deposited in the sea or relatively perennial water bodies (Nemec and Steel 1988).
Water bodies of the Leza Fm could be considered lagoons or coastal lakes. In fact, many similar ancient deposits have been interpreted as lagoons or coastal-lake–lagoon systems (e.g., Dini et al. 1998; Azerêdo et al. 2002b; Batten 2002; López-Martínez et al. 2006; Climent-Domenech et al. 2009; Radley and Allen 2012; Díez-Canseco et al. 2014). However, the term “lagoon” is generally restricted to water bodies partially isolated from the sea by sandy spits or barrier islands (Barnes 1980; Bird 2008), which is not the case for the studied unit. The term “coastal lake” is more suitable for the studied water-body deposits since it just describes lakes located close to the sea. Coastal lakes may have variable salinities (Fregenal-Martínez and Meléndez 2010; Díez-Canseco et al. 2014), as is the case of the studied system. However, usage of the term “coastal lake” in present-day settings commonly refers to fresh water bodies (e.g., Barnes 1980), and in ancient examples it is commonly used to describe lacustrine deposits with sporadic marine influence (e.g., Meléndez et al. 2000; Surlyk et al. 2008). Therefore, the term “coastal lake” may not be totally accurate for the studied system since, besides water bodies with varied salinities, it also included other environments.
The term “paralic” (from the Greek, “close to the sea”) is also applied to ancient sedimentary systems similar to the Leza Fm (e.g., Lacovara et al. 2003). However, it was originally used for clastic coal-bearing coastal deposits by Naumann (1854), and its usage in sedimentology is still generally restricted to such deposits (e.g., Ielpi 2013). Moreover, “paralic” is used in modern settings to describe coastal environments as varied as small sabkhas or the Baltic Sea (Guerloget and Perthuisot 1992), and, therefore, it may be too general a term to constrain the studied system.
Ancient coastal-plain deposits with variable salinities and abundant edaphic alteration are also often classified as coastal marshes (e.g., Plint 1984; Martín-Chivelet and Giménez 1992; Wright 1994; Tibert and Scott 1999; Gastaldo et al. 2006; MacNeil and Jones 2006; Armenteros and Edwards 2012) or coastal swamps (e.g., Hudson 1980; DiMichele et al. 2006; Bennett et al. 2012). Both “marsh” and “swamp” are suitable terms for the studied system, since they indicate vegetated, low-gradient, wet areas. However, they are ecological terms based on the type of vegetation (Brinson 2011), and therefore they are not always easy to differentiate in fossil examples. The edaphic features of the studied deposits suggest a predominance of herbaceous vegetation, indicating marsh environments, but woody vegetation, typical of swamp environments, probably also occurred, as suggested by the wide and long carbonate nodules of some calcretes (Fig. 11I). Thus, a broader term including both coastal marshes and swamps, as well as various types of water bodies, is needed for the Leza Fm depositional system.
Such a broader term may be “coastal wetlands.” “Wetland” is a geomorphological and ecological term widely used in sedimentology to describe ancient vegetation-rich and coal-bearing deposits (e.g., Greb and DiMichele 2006). Moreover, modern freshwater wetlands are regarded as the most appropriate analogues for ancient palustrine deposits (e.g., Weedman 1994; Valero Garcés and Gierlowski-Kordesch 1994; Wright and Platt 1995; Liutkus and Ashley 2003; Dunagan and Turner 2004; Alonso-Zarza et al. 2006; Marty and Meyer 2006; Reuter et al. 2009; Buscalioni and Fregenal-Martínez 2010). The Leza Fm has many features in common with these ancient freshwater wetlands, but the close interrelationship between the continental and marine realms suggests that its general depositional system should instead be termed “coastal wetlands.” The term “coastal wetlands” is very commonly used for modern environments in geomorphology (e.g., Perillo et al. 2009) and it has been proposed for some ancient coal-bearing coastal deposits (e.g., Greb et al. 2006; Rygel et al. 2006; Calder et al. 2006), but it is still not widespread in sedimentology for the description of ancient environments. Modern coastal wetlands include a wide spectrum of environments, such as freshwater marshes and lakes, mangroves, tidal flats, lagoons, or salt marshes (Wolanski et al. 2009), which resemble the variety of paleoenvironments recorded in the studied unit. Coastal wetlands can occur associated with any of the general coastal systems discussed above (i.e., deltas, estuaries, tidal flats, and lagoons; Perillo et al. 2009), but they can also occur as a sedimentary system per se. In fact, one of the most representative modern systems of coastal wetlands, the SW area of the Florida Everglades, does not occur as part of any of the aforementioned general settings, being formed by different environments at the marine-influenced distal area of a very low-gradient and broad drainage system without major distributary channels (VanArman 1984; Platt and Wright 1992; Marshall et al. 2009). Therefore, ancient coastal wetlands deposits may be part of broader coastal systems and thus could be classified differently (such as part of a delta plain, or as estuarine tidal flats), but when ancient deposits lack evidence to interpret them as part of those systems, as is the case of the Leza Fm, “coastal wetlands” may be the most accurate sedimentological term for their classification.
Other modern coastal wetlands can be evaluated as analogues of the studied deposits. The Camargue coastal wetlands (SE France), for example, show a varied salinity in their water bodies (from fresh to hypersaline), both in their spatial distribution (Britton and Podlejski 1981) and in their evolution through time (Muller et al. 2008). However, the Camargue wetlands differ from the Leza Fm in that they are mainly siliciclastic and are part of a broader deltaic system. Furthermore, the studied deposits present the peculiarity of being closely associated with coarse clastic alluvial-fan systems. This situation makes the Leza Fm relatively unusual, because alluvial fans may drain into continental wetlands (e.g., Quade et al. 1995; Dorado Valiño et al. 2002; Grenfell et al. 2009), but coarse clastic deposits are only occasionally observed in similar coastal settings (e.g., Fernández et al. 1988; Allen and Gastaldo 2006; Ielpi 2013). In this regard, a better analogue for the Leza Fm may be the Mesopotamian marshes (SE Iraq and SW Iran), which are drained by the Tigris and Euphrates rivers, but are also laterally associated with alluvial fans (Baltzer and Purser 1990; Aqrawi 2001; Plaziat and Younis 2005; Heyvaert and Baeteman 2007). These marshes are currently located ∼ 100 km inland, but since they have low elevation (< 10 m), they were configured as coastal wetlands during higher sea-level periods of the Holocene (Heyvaert and Baeteman 2007). As with the Leza Fm, the sedimentary record of these Mesopotamian wetlands contains clastic deposits, from fluvial and alluvial discharges, carbonate deposits, formed in shallow water bodies, and evaporites (Baltzer and Purser 1990; Plaziat and Younis 2005). Although the general tectonic and climatic context of the Mesopotamian wetlands differs from that of the Cretaceous unit studied here, their Holocene sedimentological evolution may provide an analogue for the complex interrelationship of alluvial-fan discharges, freshwater input, evaporation, and marine influence observed in the Leza Fm.
Concluding Remarks: Sedimentological Criteria For the Characterization of Ancient Coastal Wetland Systems
Comparison of the detailed sedimentological case study presented here with published information on similar modern and ancient systems (cited in the previous section) provides the opportunity to find clues that may contribute to a general sedimentological characterization of ancient coastal wetlands. Ideally, coastal wetland deposits could record the complete transition from purely continental to wholly marine areas, but this transition is not always observable due to erosion, tectonics, or outcrop conditions, hindering their recognition. In addition, coastal wetlands may occur as part of broader-scale depositional systems (e.g., deltas or estuaries), or as depositional systems themselves (as in the case of the Leza Fm). If they are part of a broader system they may be easier to recognize and could be classified differently (as part of the delta plain, or as estuarine tidal flats, for example). But if they are not associated with any established coastal system, “coastal wetlands” may be their most appropriate classification, and the following general features may provide some useful criteria for their recognition and characterization in the sedimentological record:
Since coastal wetlands are very low-gradient and commonly broad systems, with the water table at or close to the surface, they are likely to produce sedimentary units dominated by shallow-water facies. Therefore, even subtle variations of the water table can cause exposure of wide areas, entailing that desiccation and edaphic features may be very abundant. However, since the water table of coastal wetlands is at least partially controlled by sea level, total and prolonged desiccation periods may not be common, especially in tide-influenced areas.
Water bodies are the main locus of sedimentation in coastal wetlands. Salinity may be strongly variable in the same system, and even adjacent water bodies may show contrasting salinities depending on the local influence of freshwater and seawater. Therefore, coastal wetlands are prone to record a wide spectrum of interrelated and interbedded facies. Variations in the controlling factors of the system (e.g., tectonics, eustasy, climate) may produce relevant changes in the water bodies (e.g., area, depth, water sources, salinity) and, thus, sharp vertical transitions between contrasting facies can be expected.
In addition to this complex array of facies, if coastal wetlands are located on a tectonically active setting, local generation of relief may produce alluvial fans. Their base level would be the water table of the coastal wetlands, and sediment coming from the alluvial fans may be deposited in the water bodies.
The variety of shallow aquatic environments provides a setting favorable for organic development, and coastal wetlands commonly produce coal-bearing deposits. Biotic communities of freshwater and marine affinities may be preserved in the same coastal wetland unit. However, even subtle or short-lasting changes in a particular sedimentary environment may significantly affect its biotic content but not its lithological features, and therefore, the same facies may include contrasting fossils (e.g., freshwater and marine) in different localities or at different moments of its evolution. Furthermore, desiccation, anomalous salinities, and salinity variations are stressful factors that limit diversity of most metazoans, and thus, coastal wetland deposits can also be relatively poor in fossils or dominated by low-diversity communities. Nevertheless, coastal wetlands are likely to host abundant and diverse microbial communities, which may be preserved as microbial carbonates. In any case, microbial organic matter can be preserved in the sediment, producing dark bituminous facies.
This research was funded by the Spanish DIGICYT Project CGL2011-22709, by the research group “Sedimentary Basin Analysis” UCM-CM 910429 of the Complutense University of Madrid, and by a FPU scholarship from the Spanish Department of Education. We thank reviewers Paul Wright and Concha Arenas, as well as associate editor Peter Burgess and editor Leslie Melim for carefully reading the original manuscript, and for all their constructive comments, which have been of great help to improve the quality of this article. We are also thankful to Beatriz Moral, Gilberto Herrero, and Juan Carlos Salamanca for thin-section preparation, to Modesto Escudero and José Andrés Lira for technical and computer assistance, and to Laura Donadeo and María Victoria Romero for help with the bibliography.