Late Permian to Late Jurassic “microproblematica” of Saudi Arabia: Possible palaeobiological assignments and roles in the palaeoenviromental reconstructions Open Access

Palaeoenvironmental interpretation of carbonates is critical for optimal predictive support for hydrocarbon reservoir exploration, as well as developing intra-reservoir layering for reservoir modelling and improved production strategies. Within very shallow-marine and marginal-marine lithofacies, subtle palaeoenvironmental variations are not easily detectable due to the limited sedimentological variation and a lack of rich microfossil assemblages. The information presented here derives from detailed micropalaeontological analysis of subsurface shallow-marine carbonates from Saudi Arabia (including wells shown in Figure 1) containing less common microfossils, often considered as “microproblematica,” for which palaeoenvironmental interpretation is poorly defined. This study of non-foraminiferal microfossil types will assist determination of palaeoenvironmental variations in the otherwise problematic, very shallow to marginal-marine palaeoenvironmental spectrum of Late Permian to Late Jurassic age Saudi Arabian carbonates. Their distribution, together with other fossil microorganisms, is illustrated with reference to cored subsurface sections to reveal palaeoenvironmental interpretation and depositional cyclicity. Despite their poor biostratigraphic value, and their relative scarcity, integration of these undervalued microfossils with foraminifera, calcareous algae and microbialites enables their use for high-resolution palaeoenvironmental interpretation and for subdivision of reservoir-bearing carbonates. The proposed palaeoenvironmental interpretations for each of the groups is based on their vertical stacking arrangement as seen by the author during semi-quantitative micropalaeontological analysis of thousands of carbonate thin sections, including the wells used to illustrate the biofacies tiering (Figure 1) and especially in the Jurassic sequence of Saudi Arabia. Studies of this nature provide the only insight into the preferred palaeoenvironments of non-foraminiferal microfossils for which this type of information is absent or sparse (Figure 3).

Early micropalaeontological analysis of thin sections of Permian, Triassic and Jurassic carbonates, collected by Iraq Petroleum Company geologists, revealed certain microfossils that were difficult to assign to precise taxonomic groups. These were the “microproblematica” that have been identified from four wells during recent micropalaeontological analysis of cored sub-and intra-evaporitic carbonates of the Upper Permian to Triassic Khuff Formation, and the Upper Jurassic Arab and Hith formations in Saudi Arabia (Figure 3). The six problematical groups identified here include:

• (1) the possible cyanobacterial tube Aeolisaccus dunningtoniElliott, 1958 (Figure 4);

• (2) the chambered cyanobacterial and sinuous cylinder Gakhumella cf. huberi Zaninetti (Figure 5);

• (3) the form here assigned to the ribbon-like gastropod microcoprolite Prethocoprolithus centripetalusElliott, 1962 (Figure 6);

• (4) the encrusting alga Thaumatoporella parvovesiculifera (Raineri, 1922; Pia, 1927) (Figure 7);

• (5) the crustacean microcoprolite genus FavreinaBrönnimann 1955 (Figure 8); and

• (6) the agglutinated polychaete tube Terebella lapilloides Münster (in Goldfuss, 1833) (Figure 9).

For the first time, their presence and sequential distribution in thin-sections of core plugs can be related to palaeoenvironmental variations within the carbonate hydrocarbon reservoirs. They can assist recognition of very shallow-to marginal-marine high-frequency depositional cycles and definition of intra-reservoir layers that may not be evident in the sedimentological description.

The Khuff Formation (Powers, 1968; Powers et al., 1966) is of Late Permian to Early Triassic age and consists of a succession of four carbonate units termed, in the subsurface and in ascending order, as the Khuff D, C, B and A that are separated by anhydrite beds. Micropalaeontological analysis of the Late Permian succession (Hughes, 2005b, 2009c; Vachard et al., 2005) indicates shallow-marine to intertidal conditions during the deposition of the carbonate sediments. The cyanobacterial tube Aeolisaccus dunningtoni is present within the intertidal to shallow subtidal carbonates of the Khuff C.

The predominantly carbonate succession of the Jurassic Shaqra Group (Figure 2) of Saudi Arabia is composed of seven depositional sequences of the Marrat, Dhruma, Tuwaiq Mountain, Hanifa, Jubaila, Arab and Hith formations (Hughes, 2005b; Hughes and Naji, 2009; Powers, 1968; Powers et al., 1966). The depositional environments represented by these formations range from deep to shallow-marine that become hypersaline in the Upper Jurassic Arab and Hith formations (Azer and Peebles, 1998). A microbialite cylinder composed of en echelon chambers, assigned to Gakhumella cf. huberi is present in the Late Jurassic carbonates of the Arab Formation (Hughes, 2010). The shallow-marine carbonates of the Arab C, B and A members of the Arab Formation and the carbonates of the Manifa Member of the Hith Formation contain microbialite microgranules in which the agglutinated tube Terebella lapilloides is rare to common. Microcoprolites are well-preserved and include faecal ribbons of Prethocoprolithus centripetalus and the highly internally-structured faecal pellets assigned to the genus Favreina. The encrusting calcareous form Thaumatoporella parvovesiculifera is well-represented, and considered as a separate group of green algae (De Castro, 1990) and, of the forms interpreted here, represents the most open-marine environment.

These frequently neglected biocomponents often represent the only microfossils present in marginal- to very shallow-marine carbonates. This study has used their vertical stacking order to show their potential contribution to enhanced palaeoenvironmental subdivision of these environmentally stressed marginal-marine carbonate settings. The results of this investigation are summarized in Figure 3. Examples of micropalaeontological charts, displaying the semi-quantitative distribution of these forms, are provided in later sections of the contribution where their stacking and replacive relationships are evident.

Aeolisaccus is a distinctively shaped microfossil considered to be shells of a small, extinct, pteropod by Elliott (1958) with the type species Aeolisaccus dunningtoniElliott, 1958. Another form, Aeolisaccus kotori was later identified by Radoicic (1959). This form resembles the photosynthetic cyanobacterial filaments Scytonema, some species of which form dark mats in intertidal environments, where it may be the only microfossil present, as suggested by Golubic and Barghoorn (1977). It is unfortunate that so few fossilised specimens of this form exist to enable a confident identification, but available illustrations of comparative specimens all display aspects that compare well with those encountered in the Saudi Arabian samples. To avoid confusion, until more advanced work is possible, the original nomenclature of Aeolisaccus dunningtoni by Elliott (1958) is retained.

Elliott (1958) described Aeolisaccus as gently tapering, thin-walled tubes from the Upper Permian to Middle Jurassic carbonates of Iraq, named for their resemblance to an aeronautical windsock. He named the new species Aeolisaccus dunningtoni, after H.V. Dunnington who first encountered the forms in the subsurface of northern Iraq. Another species, Aeolisaccus kotori, was defined by Radoicic (1959) for cylindrical thick-walled forms from the Turonian. A more accurate relationship to ancient scytonematacean cyanobacterium was proposed by De Castro (1975), based on morphological and wall ultrastructure similarity with the extant cyanobacteria Scytonema. He placed the new genus Decastronema in the Phylum Cyanobacteria, Order Nostocales, Family Scytonemataceae. A change of the generic name to Decastronema, was applied by Golubic et al. (2006) for Santonian to Lower Campanian sediments, resulting in the designation Decastronema kotori n. gen. n. comb. This genus was recently used for a new species, Decastronema barattoloi, from the Eocene of the Adriatic region (Cosovic et al., 2008). As the genus Aeolisaccus was originally defined from the Jurassic and older sediments, its use is retained. Gaillot and Vachard (2007) and Gaillot et al. (2009) have recently described similar forms from the Upper Permian from the Middle East and China where they have compared them with Earlandia species and foraminiferal genera Ramovsia/Earlandia.

It is difficult to improve Elliott’s (1958), original description of Aeolisaccus dunningtoni. Aeolisaccus dunningtoni clearly displays features that are consistent with oblique sections across a cylindrical tube (Figure 4) for which apparent tapering or acuminate shape results from the oblique sectional view. The degree of apparent convergence of the sides of the tube is a function of the orientation of the thin section, and any appearance of a conical form is considered illusionary. The tubes are circular in transverse section and have a diameter of 125 μm and a maximum length of 750 μm, although no complete tubes have been encountered and they are often fractured. It should be stated here that other micropalaeontologists still consider these shapes to represent conical tubes of the foraminifera Earlandia (D. Vachard, written communication, 2012). The wall ranges in thickness from 25 μm to 35 μm, and displays a dense, dark brown character in transmitted light that could imply an organic composition. Some thin sections permit the recognition of the wall ultrastructure (Figure 4.9), in which a series of small, stacked plates are considered to represent stacked growth cone-shaped layers that are well described from Scytonema (Golubic et al., 2006, their figures 4–6). A detailed, and highly informative, description of the wall ultrastructure is provided by Golubic et al. (2006), in which the dark colour is attributed to post-depositional iron enrichment by chelation with the organic matter within the sheath. Biocrystallisation of Scytonema (Freytet and Verrecchia, 1998) provides a mechanism for cyanobacterial preservation in a process that may have similarly assisted fossilization of Aeolisaccus, even though this process has not been described in other fossil cyanobacteria.

Aeolisaccus is present in Late Permian to Late Jurassic carbonates of the Tethyan margins. In Saudi Arabia, the forms assigned to Aeolisaccus dunningtoni have been identified in samples from: (1) Upper Permian, Changhsingian Khuff C carbonates of the Khuff Formation (Hughes, 2005b, 2009c), where they are called Earlandia by Vachard et al. (2005) in the same beds; (2) Middle Jurassic, Upper Callovian carbonates of the Tuwaiq Mountain Formation; (3) Upper Jurassic, Oxfordian carbonates of the Hanifa Formation; (4) Upper Jurassic, Kimmeridgian to Tithonian Arab D and Arab C carbonates of the Arab Formation; and (5) Manifa reservoir carbonates of the Hith Formation (Hughes, 2004, 2005b, 2009b).

The forms assigned to Aeolisaccus dunningtoni foraminifera, are typically found within mudstones or wackestones that are barren of foraminifera, or within packstones and grainstones that display a very low foraminiferal diversity (Figures 9 to 12). They are considered to represent supratidal to possibly upper intertidal conditions, when in-situ, based on the distribution of extant, morphologically similar, possible cyanobacteria equivalent to Scytonema that occupies tidal flats and flourishes during times of freshwater flooding (Browne et al., 2000). When present with foraminifera or calcareous algae, they are considered to have been penecontemporaneously transported into shallow-subtidal settings (Figure 3).

A comprehensive discussion of the form assigned to Gakhumella is provided by Hughes (2010). They consist of colonies of hollow, possibly intertwined and not necessarily mutually supporting, gently and randomly curving, vermiform, chambered cylinders. They have a maximum length, observed in thin section, of 300 μm, with a width of 40 μm. It is difficult to confidently assign a biological affinity to these forms but a cyanobacterial affinity is suggested by their similarity with different representatives of this group, e.g. Gakhumella huberi, recorded from the Permian of Iran (Zaninetti, 1978) and from the Tithonian of Spain (Granier, 1986), Barremian Hormathonema sp. (Masse, 1979), Frutexites sp. (Hofmann and Grotzinger, 1985; Walter and Awramik, 1979) and the Epiphyton and Renalcis diagnostic features are described by Pratt (1984).

The forms considered to be Gakhumella have only been identified in Saudi Arabia within carbonates from the Arab A and Arab C members, and they are of Kimmeridgian age (Hughes, 2005b).

Gakhumella seems to form within microcaverns, as illustrated in Hughes (2010, Figure 3), in the type of growth described by Shapiro (2004, his Figure 3.1). It has also been found within microbialite microgranules associated with stromatolites. The lower part of the Arab A and Arab C carbonates contain foraminifera including Redmondoides lugeoni (Figure 10.5), Trocholina alpina (Figure 10.9) Nautiloculina oolithica, Quinqueloculina spp. (Figure 10.8) and small Valvulina spp. (Figure 10.7), with the microcoprolite Favreina salevensis present in the upper part. The upper part of the carbonate displays dendroidal forms of Gakhumella and is barren of microfossils, except for rare Terebella lapilloides and an apparently monospecific assemblage of thin-walled ostracod biofacies.

Gakhumella is rarely seen, but should also be expected in the Arab B as this member of the Arab Formation was mostly deposited within very shallow-marine, hypersaline conditions. The upwards succession is interpreted to represent a single depositional sequence that commenced with initial flooding conducive for ooid and foraminiferal facies to be deposited, followed by gradual shallowing and probable hypersaline conditions dominated by laminated microbialite. The uppermost part of this latter lithofacies contains the chambered cylinders, and a very shallow-marine, restricted, possibly hypersaline or poorly oxygenated depositional environment, related to gradual shallowing of the sequence, is interpreted. The absence of foraminifera suggests highly adverse environmental conditions for which cyanobacterial colonisation may indicate salinity range of between 70 and 140 ppt (Warren, 2006, Figure 9.9). Restricted marine, moderately high-energy intertidal palaeoenvironments also support Gakhumella-resembling forms such as the Palaeozoic Fasculus dactylus (Königshof and Glaub, 2004).

The upper part of the Arab A and Arab C carbonates in Saudi Arabia, is interpreted as having been deposited in adverse, probably hypersaline, shallow-marine conditions that favoured the development of dendroidal, digitate microbialites, containing cavities that supported the growth of uniserial, crescentic-chambered, tubular and locally bifurcating microstructures of probable cyanobacterial origin. These microtubules are localised in the upper part of the inter-evaporitic Arab A carbonates that commenced with normal-marine salinity, elevated energy, shallow-marine lagoonal conditions with ooids that progressively shallowed and became hypersaline or oxygen deficient in the uppermost part. The cylinder-forming cyanobacteria seemed to have flourished colonially within restrictive marine conditions that excluded all shell-bearing forms, except for a monospecific assemblage of thin-walled unornamented ostracods resembling species of the hypersaline tolerant genus Cytherella (Mette, 1997).

Prethocoprolithus centripetalus was first described as a coprolite (Elliott, 1962) and faecal ribbon (Elliott, 1980) with a distinctive “enrolled m” cross-section. The forms are typically well preserved, and can be easily distinguished except in longitudinal sections where the parallel sides can resemble agglutinated tubes of the sabellid worm Terebella lapilloides Münster with their diagnostic annular cross-sections.

Elliott (1962) established the genus Prethocoprolithus from the Upper Jurassic Najma Formation of Iraq, equivalent to the Arab Formation in Saudi Arabia (Sharland et al., 2001), to accommodate elongate cylindrical coprolites with a central tubular cavity. A bivalve affinity is based on the knowledge that bivalve intestines have thickened, ciliated longitudinal ridges that impart a varied longitudinal ornament on the compacted mud and faecal ribbon (Moore, 1931; Cox, 1960; Arakawa, 1970). Bivalve faecal ribbons similar to P. centripetalus had been previously published, but not named (Edge, 1934; Moore, 1939; Manning and Kumpf, 1959), although similarity to pellets of the modern patellid gastropod was noted by Elliott (1962). Allen (1953) provided evidence that a single species can produce faecal pellets of differing shape. Although Elliott (1962, 1980) clearly defined the tubes as being of coprolites, Flügel (2004, his plate 92.10) mistakenly infers that Prethocoprolithus is a worm tube associated with Terebella lapilloides Münster. The similarity of certain thin sections has led some authors to consider the two forms to be synonymous (Wahlman, 1988; Delvolvé et al., 1994).

Typical preservation of the Saudi Arabian specimens includes the paired gutter-shaped tubes, together with numerous oblique sections, as well as broken fragments of these partial tubes. The paired tubes (Figure 5) are of variable shape in detail, although the overall diagnostic shape is common to all unbroken specimens. The form consists, in transverse section, of two U-shaped, enrolled gutter-like elements that are joined by a bar. This view is commonly encountered in thin sections. Another shape is that which results from a longitudinal section through the coprolite that, depending on the orientation of the section, ranges from two parallel walls, to one in which the walls are connected by a curved wall. Both of these “end members” represent orthogonal longitudinal and oblique longitudinal sections through a partial, but not complete cylindrical tube, unlike that mistakenly interpreted by Elliott (1962). The “rounded ends” described by Elliott (1962) and interpreted correctly by Elliott (1980) are interpreted here to represent the appearance of oblique longitudinal sections through the gutter-shaped parts of the coprolite.

The faecal ribbon width ranges from 2.00–2.56 mm, with the component U-shaped elements ranging from 0.80–1.20 mm. Ribbon thickness ranges from 0.16–0.4 mm, based only on the transverse sections, as apparent thickness can vary with longitudinal section obliquity. In longitudinal thin-sections, these forms range from 1.70–2.16 mm in length in variably compressed fragments, and this compression is also present in associated peloids. Sections that display a clearly developed annular appearance in thin section are assigned to the sabellid worm tube Terebella lapilloides (Figure 9).

Forms that resemble a single U-shape in cross-section have been encountered in Cenomanian carbonates, where they are associated with rudists that resemble Eoradiolites spp. These forms have not been seen before, and are interpreted to represent transverse sections of a faecal ribbon consisting of a single trough. Their intimate association with rudist fragments suggests that they may represent rudist faecal ribbons.

Prethocoprolithus centripetalus has, until recently, only been identified in thin sections of carbonates from the Arab C, B and A members of the Arab Formation and from the Hith Formation and are, therefore, of Kimmeridgian to Tithonian age (Hughes, 2005b). Rare specimens have been encountered in the Berriasian carbonates of subsurface Saudi Arabia, where they are associated with a highstand shallowing event within an unusually diverse biofacies that includes Pseudocyclammina lituus, miliolids, Terebella lapilloides, Valvulina sp. and echinoid fragments.

These forms are present within fine-grained micropeloidal grainstones and are frequently associated with laminated microgranular carbonates considered to represent stromatolites within shallow subtidal settings. Figures 11 to 15 display the vertical succession of Prethocoprolithus centripetalus-bearing sediments and their relationship with other microbioclasts. Typically found with few associated microfossils, and suggestive of very shallow, possibly hypersaline conditions, Prethocoprolithus centripetalus is occasionally found with, possibly allochthonous, dasyclad Salpingoporella cf. mellitae, Thaumatoporella parvovesiculifera and the foraminifera Trocholina alpina (Figure 10.9) and Redmondoides lugeoni (Figure 10.5). This biofacies typically alternates with the stromatolites of the Late Jurassic formations. The presence of rare Prethocoprolithus centripetalus with Pseudocyclammina lituus, miliolids, Terebella lapilloides, Valvulina sp. and echinoid fragments in the Berriasian carbonates of the Sulaiy Formation is attributed, if in-situ, to stressed shallow-marine conditions at the base of a shallow-marine depositional cycle.

The incidence and abundance of these coprolites may provide insights into variations in sedimentation and its inverse relationship with organic biomass availability. Sediment affects the grazing efficiency of gastropods because of the increased inorganic content of the detritus (Cohen et al., 1993). Rivers and Michel (written communication, 2012) state that “… to maintain the same caloric intake, a greater volume of food must be digested; this necessitates grazing of greater areas”.

The characteristic “double U” cross-sectional shape provides a clue to the life habitat of the host mollusc. The physiological advantages of an increase in surface area of the internal alimentary canal, suggested by the double trough shape, may include a higher rate and efficiency of nutrient and water absorption. Possibly adverse environmental conditions, related to hypersalinity or elevated temperatures, may have favoured such a digestive tract modification. Torsion of the visceral mass in gastropods causes the anus and mouth to be closely located, for which ingestion of potentially fatal expelled faecal matter would be avoided by a rigid faecal ribbon. Gastropods can efficiently extract water, as well as nutritive elements, from faecal matter during its passage through the alimentary canal (Brough and White, 1990; Myers and Burch, 2001) and as this process would be more efficient by an increase in the surface area produced by the “double U” represented by the faecal ribbon of P. centripetalus. Preservation of the sigmoid cross-section and brittle fragmentation of the forms suggests that the faecal ribbon was rigid enough to be preserved after its expulsion from the host.

P. centripetalus displays a variety in ribbon thickness that is considered to be probably related to the food availability, although numerous variations are noted within the same thin section, and has no particular stratigraphic value. It is apparent, therefore, that P. centripetalus was deposited by mollusks that occupied very shallow-marine conditions with adversely elevated salinity conditions, accompanied by possible low nutrient availability. It should be noted that a “single U” form has recently been observed, by the author, in thin sections where rudist and chondorodontid oyster fragments are present. There is no suggestion that these “C” shaped forms represent broken fragments of Prethocoprolithus centripetalus. Further recent studies by the author of the carbonate beds within the Hith Formation, and known as the Hith “stringers”, have revealed at least four feet of packstones in which the grain type consists entirely of Prethocoprolithus centripetalus with peloids.

The calcareous form Thaumatoporella parvovesiculifera (Raineri) was first described from Cenomanian carbonates as a dasyclad alga by Raineri (1922), and named Gyroporella parvovesiculifera. It consists of a calcareous meshwork that is loosely comparable with a honeycomb, consisting of a single layer of cells (Figure 7). The biological assignment has been problematical (De Castro, 1990), and his publication provides excellent photographs of various sections (De Castro, 2002, Plate 1). Elliott (1957) considered the form, named Polygonella incrustata, to be a solenoporacean, Johnson (1966) a codiacean, and Flügel (1983), as possibly sponge. Flügel (2004) later reconsidered possible assignments of T. parvovesiculifera to the microproblematica, red algae, green algae and sponges and concluded that the form should be considered as a new group of green algae following De Castro (1990), of which T. parvovesiculifera is the type species. De Castro (1988, 1990) assigned the form to the new Family Thaumatoporellaceae, and new order Thaumatoporellales, and this assignment was followed by Cherchi and Schroeder (2005), despite informally considering the forms to represent calcified cyanobacteria. T. parvovesiculifera is currently considered to belong to the Phylum Chlorophyta, or calcareous green algae.

Thaumatoporella parvovesiculifera is a common biocomponent of shallow-marine carbonates of Middle Triassic to Late Cretaceous age (Flügel, 2004), although a Ladinian (Middle Triassic) to Palaeocene age is stated by Senowbari-Daryan (1984). Schlagintweit (2011) has proposed a Late Permian earliest range, based on the possible relationship with the presumed praecursor Pseudovermiporella sodalica, but this relationship is not correct (D. Vachard, written communication, 2012) as P. sodalica is a miliolid foraminifera and invalid as a relative of T. parvovesiculifera. D. Vachard (written communication, 2012) has suggested that the problematic dasyclad Koninckopora pruvosti could represent a better ancestral candidate, of Palaeozoic age.

Thaumatoporella parvovesiculifera is a common microfossil of shallow-marine carbonates and is especially well-represented within the packstones of the Arab D (Hughes, 2004, 2005a, 2009b, 2009c). It displays a highly irregular morphology, owing to its attached and often encrusting growth. Cylindrical forms are common within the Kimmeridgian grainstones of the Arab Formation of Saudi Arabia, and they are considered to have probably encrusted plants, of which no trace is preserved. It is found with the dasyclad algae Clypeina jurassica, and a diverse foraminiferal assemblage typical of normal salinity conditions. Its association with Clypeina indicates a water depth within the photic zone, while the packstone fabric suggests a less than fair-weather wave-base depth. Its presence within the non-Clypeina foraminiferal grainstone biofacies also suggests tolerance of elevated water turbulence within fair-weather wave base.

Palaeoenvironments favoured by Thaumatoporella have been summarized by Leinfelder (1992), and include back-reef and reef-front/crest. Palaeobathymetric ranges for Thaumatoporella have been proposed by Banner and Simmons (1994) from the tentative calcareous algal affinities. They provided a depth range of 10–20 m for sheets without any encrusting substrate within the chlorophyte and rhodophyte zone, to 20–45 m within the exclusively rhodophyte zone. The present study extends the palaeoenvironmental tolerance of Thaumatoporella parvovesiculifera from normal marine, into low intertidal, possibly hypersaline conditions. Figure 7.1 displays the close relationship between T. parvovesiculifera and microgranular microbialite and Gakhumella species. The open-marine, stromatoporoid bank depositional environments in the Tuwaiq Mountain, Hanifa and Arab formations (Hughes, 2004; Hughes et al., 2009) display encrustations by Thaumatoporella parvovesiculifera (Figures 7.11, 7.12 and 7.14).

Rodlike faecal pellets that display an internal arrangement of calcified canals are assignable to the microcoprolite genus FavreinaBrönnimann, 1955. Canaliculate pellets were first described from Triassic sediments by Joukowsky and Favre (1913), with additional data provided by Favre and Richard (1927). They were identified as faecal pellets of the crustacean genus Anomura by Moore (1932), who suggested that the arrangement of the calcitic canals possibly represented different species. The peculiar internal fabric of crustacean coprolites was also observed by Edge (1934). The different types have been related to various crustaceans and assigned the genus CoprolithusParejas (1935, 1948). Potential producers of similarly structured pellets also include the Glypheoidea crustaceans (Forster and Hillebrandt, 1984; Forster, 1985). Favreina specimens have been related to the Callianassa shrimp burrow Thalassinoides from Miocene sediments by Kennedy et al. (1969), and from the Cretaceous Chalk by Kennedy (1967) and Bromley (1967). Their association with Anomura is confirmed by Kuss and Senowbari-Daryan (1992), who describe three genera Diapasolutum, Favreina and Palaxius from Albian to Turonian shallow subtidal carbonates from Egypt and Jordan. Their association with the burrows is considered to be genetic, and not a product of transport from a host different than the burrow former, especially in view of the likely resistance of these pellets to transport. Taxonomic assignment of callianassid shrimps to either the Infraorder Thalassinidea (Martin and Haney, 2005) or Anomura (Becker and Chamberlain, 2006) may suggest that some forms of these microcoprolites may not necessarily be the product of anomuran shrimps (Senowbari-Daryan et al., 2007). The presence of the internal structures distinguishes crustacean microcoprolites from gastropods and worm pellets (Kornicker, 1962; Powell, 1974).

Favreina is an ichnogenus, named after Favre by Brönnimann (1955, 1972, 1976), which describes rod-like pellets that are characterised by numerous longitudinal calcite-filled canals. The additional genera Palaxius, Helicerina, Parafavreina and Thoronetia were defined by Brönnimann and Norton (1961) and Senowbari-Daryan et al. (2009). These internally structured pellets are coprolites and found in sediments as old as Late Carboniferous (Masse and Vachard, 1996). They form microfossils that are subcircular in cross-section, and broadly rectangular in longitudinal section (Figure 8) Transverse sections reveal the characteristic internal fabric with a variety of sieve-like patterns considered to be species specific (Elliott, 1962). The origin and patterns of the canals are considered to represent canals created by impingement of alimentary fleshy processes and cylinders, known as pyloric fingerlets (Kennedy et al., 1969; Powell, 1974; Schweigert et al., 1997), that are preserved by calcification. Although not seen in the present study, calcitic envelopes can also surround the microcoprolite, and are attributed to diagenetic replacement of an organic mucilaginous coat (Frangoulis et al., 2005). Cross-sectional patterns of these longitudinal canals allow the discrimination of different taxa, based on their number, shape and arrangement (Brönnimann, 1972; Schweigert et al., 1997).

Generic subdivision using the shape of the canals in transverse section, was considered unnecessary owing to the presence of transitional canal shapes (Elliott, 1962) from samples of Middle Triassic, Upper Jurassic, Eocene and Miocene from Iraq. Palik (1965) considered that the use of two form genera was a practical approach. The genus Favreina was defined by Brönnimann (1955) as subrectangular and rounded, dark fragments of apparently homogeneous texture, ranging from about 0.50 to 1.50 mm in length and about 0.20 to 0.40 mm in width. Longitudinal sections are subrectangular, with long, thin, straight, parallel canals arranged in a regular intermittent pattern. Transverse sections are subcircular to oval outline canals represented by minute subcircular to eye-shaped pores, either arranged in two or more flattened oblong rings, or distributed more or less irregularly. The presence of distinctive canals within certain pellets is sufficient to enable recognition of Favreina, for which Elliott (1962) considers the detailed canal variations to be of specific rather than generic value. He provides a useful table (Elliott, 1962, his Table 1) that clearly displays the characteristic variations of the canal distribution of the species identified from Iraq.

In the Upper Jurassic carbonates of Saudi Arabia (Figure 8), the Favreina pellets are often ovoid due to sediment compaction but the canals are relatively well-preserved. According to Brönnimann (1955), the flattened or centrally depressed side of the coprolite has been termed the “ventral” side, and the rounded side considered “dorsal” (Figure 8.2, 8.4 and 8.6). Based on the orientation of the canals in transverse sections, Favreina salevensis (Parejas, 1948) is confirmed (Figure 8.13 to 8.15), and the resemblance of other forms made to F. cf. prussica (Parejas, 1948) (Figures 8.11 and 8.12) and described briefly by Elliott (1962) and to F. cf. dinarica Brönnimann 1976 (Figures 8.1 to 8.6) as defined by Brönnimann (1976). Longitudinal sections cannot be used for species identification without more detailed biometric analysis, although the low density of canals (Figures 8.7 to 8.10) suggests a closer affinity with Favreina cf. salevensis/dinarica in preference to F. aff. prussica.

Elliott (1962) provided the following description of Favreina salevensis: “pellet of up to about 1.0 mm long and 0.52 mm wide, cylindrical in shape with rounded elliptical cross-section of which one side is flattened. Canals small, circular in cross-section; 40–50 in number, set in a pattern of an outer arc with central portion subparallel to the flattened side of the pellet, and at right angle to this two inner parallel rows which recurve back inside the outer arc to unite parallel to it.” The specimens from the Upper Jurassic of Saudi Arabia (Figures 8.13 to 8.15) display a lower density of canals in cross-section (approximately 35), arranged in two relatively clearly defined loops with one additional row of canals that parallels the outline of almost half of the circumference of the pellet. Favreina cf. dinarica was defined for forms that display an extended peripheral ring of canals in transverse section (Brönnimann, 1976, his Figure 4D). Such forms have been identified in the Saudi Arabian material (Figures 8.1 to 8.6). Favreina aff. prussica differs from the other Jurassic species F. salevensis by a greater pellet diameter (between 0.65 and 1.26 mm), a much greater canal density of between 66 and 136 and no clearly defined pattern. The Saudi Arabian specimens have approximately 85 canals and are compared with F. aff. prussica.

The genus Favreina has a published range from the Devonian to Tertiary (Flügel, 2004), but D. Vachard (written communication, 2012) has indicated that the Devonian occurrence is actually Triassic. It is especially common in the Upper Jurassic carbonates of Saudi Arabia. Favreina salevensis and F. aff. prussica are present within carbonates from the Arab Formation, C, B and A members. F. salevensis has also been identified within the ostracod-bearing stromatolites of the Tithonian upper Hith Formation in the Saudi Arabian subsurface (Hughes and Naji, 2009), as well as within peloidal grainstones of the Toarcian Marrat Formation. Favreinid stratigraphy has been applied to the Lower Cretaceous of Argentina (Kietzmann and Palma, 2010) and a similar approach should be possible for the Saudi Arabian Mesozoic.

An association between the Thalassinoides with the burrow-creating crustacean Glyphaea udressieri and the microcoprolite Favreina was observed by Sellwood (1971), and interpreted to represent a shallow subtidal environment. Kennedy et al. (1969) proposed that the Favreina crustacean lived at, or close to, low water mark and tidal flats, their burrows probably reached waterlogged sediments.

The palaeoenvironmental implications of Favreina-bearing carbonates have received minimal attention, but are here concluded to represent carbonate sediments deposited within intertidal to shallow subtidal environments (Figure 3). In the Saudi Arabian carbonates, Favreina specimens are present with a typically depleted biofacies that contains ostracods and rare dasyclad fragments (Figures 11 to 15). Rarely are they found with Redmondoides lugeoni (Figure 10.5), Valvulina spp. and Nautiloculina oolithica. Comparison with the preferred ecology of extant crustacean (Griffis and Suchanek, 1991) provides an intertidal to shallow subtidal setting for carbonates in which these forms are recovered. Variations in the geometric patterns have been proposed (Brönnimann and Norton, 1961; Brönnimann and Masse, 1968; Brönnimann et al., 1972) for generic subdivision into the genera Palaxius, Helicerina, Parafavreina and Thoronetia. These forms have not been identified in the Saudi Arabian material, possibly due to their Triassic range of which similar age carbonates of the Jilh Formation in Saudi Arabia have been little studied by the author.

As part of Clypeina-Campbelliella striata and Epimastopora cekici and Clypeina cf. solkani biofacies, Maticec et al. (1997) interpret a shallow lagoon environment shallowing upwards into “upper subtidal”/intertidal conditions for uppermost Tithonian to Early Creatceous carbonates from Croatia. Lower Cretaceous carbonates from southeastern Slovenia (Dozet and Sribar, 1998) contain Favreina salevensis with Salpingoporella annulata, Thaumatoporella parvovesiculifera, Pseudocyclammina lituus and charophytes and have been interpreted as intertidal. Favreina-bearing carbonates from the Upper Jurassic Smackover Formation of the United States Gulf Coast have been variably assigned to shelf, shelf margin and patch-reef, lagoon and ramp environments based on their coexisting biocomponents by King and Hargrove (1991). Present-day shrimp burrows and mounds produced by modern Callianassa and Upogebia “ghost shrimps” contain faecal pellets with complex internal structures that resemble favreinid pellets (Seilacher, 2007).

The propensity for faecal pellet movement prior to lithification has been investigated by Wanless et al. (1981), based on studies of faecal pellets in modern carbonates of south Florida and the Bahamas. Hardened pellets, such as Favreina, have a narrow range of settling behaviour and tend to accumulate within sediments with a narrow grain size range. They are most readily preserved in ancient rocks as ovoid, uncompacted grains, whereas soft faecal pellets compact easily and amalgamate into a pseudo wackestone, forming a “grumulose” pattern in structures termed Spongiostromides, Spongiostromata or loferites (Cayeux, 1935; Bathurst, 1971). D. Vachard (written communication, 2012) states that these structures are composed of microbialites and not micropellets. Pellets tend to fall with their greatest cross-sectional area perpendicular to the settling path (Briggs et al., 1962; Wanless et al., 1981). The orientation of Favreina pellets, wherever preserved following transport due to their weak calcification (D. Vachard, written communication, 2012), could possibly indicate palaeocurrent directions.

Simple, unbranched, rectilinear finely agglutinated tubes clearly visible in carbonate thin sections have been assigned to Terebella lapilloides Münster (in Goldfuss, 1833) but have been variously described as a problematic organism (Schorr and Koch, 1985), agglutinated boring (Flügel and Steiger, 1981), allogromid foraminifera (Jansa et al., 1972) and agglutinating worm (Klieber, 1985). The Order Terebellida (Bellan, 2001) is based on the extant form Terebella, and are classified as agglutinating marine polychaete worm tubes. It should not be confused with the curvilinear agglutinated, but internally laminated, tube described as Thartharella (Elliott, 1980), a microcoprolite from the Upper Jurassic originally referred by Elliott (1962) to Prethocoprolithus centripetalus and P. cucumeriformis. Terebella tubes (Figure 9) consist of siliciclastic sand and silt grains, carbonate and organic matter (Flügel, 2004). They form rarely observed communal aggregations within the Arab A carbonates.

Early Carboniferous to Recent (Flügel, 2004). In the Early Carboniferous, they are rare in the late Visean and early Serpukhovian, but become relatively common in the late Serpukhovian (Vachard, written communication, 2012).

Palaeoenvironmental affinities of Terebella are poorly documented. Low-diversity associations of Terebella with the microproblematic canaliculate form Tubiphytes of possible sponge affinity (Riding and Guo, 1992) are also described by Flügel (2004) from “low-ramp and middle- to outer-ramp positions,” and by Vachard et al. (written communication, 2012) as foraminiferal-associated, and considered to represent deep, low-energy environments with low sedimentation rates. Ramp mud mounds containing abundant Terebella fragments have been described by Pratt (1995). Terebella lapilloides is not common in Jurassic carbonates of Saudi Arabia, but has been found to be locally present in carbonates of the Upper Jurassic Manifa Formation. Figure 15 displays the relationship between Terebella lapilloides, Prethocoprolithus centripetalus, Favreina spp., thin ostracods and stromatolites. A close association between the abundance of Terebella and microbialites has been noted in Upper Jurassic carbonates by Dupraz and Strasser (1999), and an interesting observation regarding a preference of Terebella for dysoxic conditions has been documented by Leinfelder et al. (1996). Recent observations on sabellid-like tube communities in Dammam Bay, on the east coast of Saudi Arabia, suggest a tolerance to hypersaline intertidal conditions.

In the Carboniferous, D. Vachard (written communication, 2012) considers that Terebella is associated with microbialite build-ups and exists in relatively deep middle ramp or outer reef environments, at depths of 50–100 m. It is possible, therefore, that Terebella is opportunistic within certain adverse depositional environments, of variable depths.

It can be concluded, therefore, that Terebella lapilloides can occupy a range of depositional environments. Its association with Prethocoprolithus centripetalus, rare Favreina spp. and rare Aeolisaccus spp., finely granular microbialites and thin ostracods suggests shallow subtidal to intertidal, hypersaline conditions in which stromatolites flourished. A minor transgressive event interrupted the succession in Figure 15, and permitted short-lived deeper, normal salinity conditions to become established with the foraminifera Redmondoides lugeoni (Figure 10.5). The Terebella-Prethocoprolithus biofacies is overlain by a thick succession of evaporites, indicating the presence of late highstand conditions prior to establishment of an extremely hypersaline environment.

In some of the studied wells, the Arab C carbonate contains a moderately diverse microfossil assemblage that includes various microproblematic microfossils considered in this paper, together with foraminifera and other microfossils that assists in calibrating their palaeoenvironment.

The semi-quantitative distribution of microfossils has been recorded from cored Arab C, B and A carbonates (Figures 11 to 15). A gamma log is displayed for most of the cored sections together with detailed palaeoenvironmental interpretations. The length of the horizontal bar for each species is proportional to semi-quantitative abundance. For the Dunham textural column, the length of the bar is proportional to increased grain dominance of the sediments, from mudstone to grainstone respectively.

The biofacies related to the six groups considered in this paper are displayed with their co-occurring microbiocomponents (Figures 11 to 15). The section interpreted as being representative of a supratidal regime is characterised by the presence of Aeolisaccus dunningtoni. Where this species is found with other listed species, its origin is interpreted as being from penecontemporaneous transport. The intertidal regime is characterised by the presence of Favreina species, in the absence of foraminifera. Shallow subtidal conditions are typified by the presence of Prethocoprolithus centripetalus, Terebella lapilloides, Gakhumella sp., stromatolites, thick- and thin-valved, unornamented ostracods and Thaumatoporella parvovesiculifera. Favreina specimens may also be present in this regime. The shallow subtidal regime is characterised by the presence of stromatolites, small biserial agglutinated foraminifera, Trocholina alpina (Figure 10.9), Redmondoides lugeoni (Figure 10.5), Pfenderina salernitana (Figure 10.6), concentric ooids and species of the dasyclad Salpingoporella. Specimens of Favreina and Prethocoprolithus centripetalus may be present in this relatively deeper regime.

For the Arab C carbonate (Figure 11) 75 ft of core has been analysed in thin section at approximately one foot spacing. For the species considered here, the basal part of the core is characterised by the consistent presence of Favreina spp., over a thickness of 7 ft. This is followed by rare and isolated specimens of Prethocoprolithus centripetalus. Aeolisaccus specimens are located in the uppermost 6 ft of the core examined. Other microfossils in the studied section include the foraminifera Redmondoides lugeoni (Figure 10.5), Pfenderina salernitana, Trocholina alpina alpina (Figure 10.9), immature or dwarfed Valvulina sp., Reophax sp. (Figure 10.1) and Ammobaculites sp. (Figure 10.2), the dasyclad Salpingoporella cf. mellitae and the encrusting alga Thaumatoporella parvovesiculifera. The core is considered to represent a depositional sequence related to a normal salinity marine transgression that provided carbonate accumulation in between hypersaline conditions responsible for the evaporite sediments of the underlying Arab D and overlying Arab C. The presence of Favreina species, in the presence of foraminifera and dasyclads at the base of the sequence is considered to indicate shallow subtidal conditions into which Favreina specimens were possibly reworked, or in-situ, during the early transgression. The increase in microfossil diversity in the middle section of the core is considered to represent a maximum flooding zone, including rare Prethocoprolithus centripetalus and rare Favreina species. The presence of only Aeolisaccus in the upper part of the core, is interpreted to represent supratidal possibly hypersaline intertidal conditions associated with the late highstand of the sequence. The composite sequence includes up to six high-frequency sequences revealed by careful study of the microfossil distribution.

For the Arab B carbonate (Figure 12), a considerably lower diversity microfossil assemblage is evident, despite the incompleteness of core recovery. There is a lower and upper Favreina distribution, separated by the localised presence of Prethocoprolithus centripetalus, considered to represent shallow subtidal with reworked intertidal microfossils associated with the transgressive and highstand conditions of the carbonate sequence. The localised Prethocoprolithus centripetalus is possibly associated with the thick maximum flooding zone. As with the Arab C carbonate described above, this section is also stratigraphically bound by evaporites, with the Arab C and B evaporites at the base and top respectively.

For the Arab A, three wells have been selected of which the first (well 2, Figure 13) clearly displays a trend similar to that of the Arab B carbonate, described previously, in which Favreina species typify the lower and upper biofacies, separated by a middle unit containing Prethocoprolithus centripetalus. This vertical stacking is considered to represent transgressive, maximum flooding and highstand phases of the Arab B carbonate depositional sequence. Despite the paucity of microfossils, there is an interesting association between Favreina species and the foraminifera Redmondoides lugeoni (Figure 10.5), a form that is known to be tolerant of elevated energy conditions in shallow-marine settings. Its scarcity within the slightly deeper Prethocoprolithus centripetalus biofacies is difficult to explain but may be attributed to reworking of this shallow-marine form during the slightly lower energy conditions. For the second well displaying Arab A carbonate micropalaeontology (well 3, Figure 14), the trend described above is repeated, with lower and upper Favreina biofacies and a middle Prethocoprolithus biofacies. Another Favreina biofacies is present within the middle part of the core. In the almost complete absence of foraminifera due to adverse depositional conditions, the Favreina and Prethocoprolithus biofacies provide evidence for at least two high-frequency depositional cycles that would otherwise be difficult to identify.

The Arab A micropalaeontological succession (well 4, Figure 15) displays the predominance of Prethocoprolithus centripetalus that represents a marine transgression within a very shallow-marine setting. Favreina species and thin ostracods are locally subordinate. Evidence for maximum flooding is suggested by the presence of ooids and foraminifers Redmondoides lugeoni (Figure 10.5) and Valvulina sp. The presence of Terebella lapilloides, thin ostracods and finely granular microbialite represent the highstand system. In the upper part of the studied succession, finely granular microbialite, Terebella lapilloides, thin ostracods, dasyclad fragments and Quinqueloculina spp. together indicate hypersaline shallow subtidal conditions. The association of Thaumatoporella parvovesiculifera with a moderately diversified assemblage of foraminifera and associated microfossils is illustrated in Figure 15. This assemblage is typical of a shallow-marine, lagoon setting within normal salinity and clear water. It represents the most normal marine conditions of the various forms here considered, and represents the relatively deepest of the palaeoenvironmental spectrum spanning the shallow subtidal to intertidal palaeoenvironments. The Hith Formation is represented by a succession of interbedded evaporites and carbonates, most of which are represented by ooid and peloid grainstones.

These forms are of interest because of the new realisation of their potential to provide a refined palaeoenvironmental subdivision of the marginal-marine environment (Figure 3). By examining the vertical stacking of Aeolisaccus, Gakhumella, Prethocoprolithus, Favreina, Thaumatoporella and Terebella, microcoprolite biofacies and their microassociation with microbialite microgranules, ooid grainstones, ostracod, calcareous alga and foraminiferal bearing sediments, their value as indicators of supratidal, through the intertidal and shallow subtidal palaeoenvironments respectively has been revealed. This observation is of value to determine the high frequency depositional cycles within carbonate successions where, because of the scarcity of foraminifera and minimal sedimentological characters, palaeoenvironmental interpretation is difficult. Definition of previously cryptic depositional cycles within reservoir carbonates can have important implications for understanding the stratigraphic context of reservoir porosity, heterogeneity and this will assist strategies for optimising reservoir development. Their localised distribution within the Upper Permian Khuff carbonate reservoir, Middle Jurassic Hadriyah and Middle and Upper Jurassic carbonate reservoirs also provides potential for accurate rig-site stratigraphic control using biosteering techniques (Hughes, 2009a) currently employed by Saudi Aramco in the Khuff C carbonate rerservoirs.

Five of the six microfossil genera Aeolisaccus, Gakhumella, Prethocoprolithus, Thaumatoporella, Favreina and Terebella represent numerous forms initially considered as “microproblematica” by Elliott (1958, 1962, 1980). They have been analysed with other microfossils from thin sections of Late Permian and Late Jurassic carbonates from the subsurface of Saudi Arabia. Most of these carbonates are of intertidal to shallow-marine origin and are closely associated with evaporites. These unusual microfossils provide palaeoenvironmental indicators within shallow-marine carbonate successions (Figures 11 to 15), where other microfossils are poorly represented, and can assist carbonate reservoir characterisation.

With reference to Figure 3, the following five biofacies and palaeoenvironment associations are proposed:

• Aeolisaccus dunningtoni biofacies: supratidal mud flats;

• Aeolisaccus dunningtoni - Cytherella ostracod - stromatolite biofacies: supratidal mud flats to shallow intertidal;

• Gakhumella spp.- ostracod - stromatolite - Prethocoprolithus - Thaumatoporella biofacies: deep intertidal, hypersaline;

• Prethocoprolithus - Thaumatoporella - Favreina - Terebella - miliolid biofacies: shallow inner neritic, hypersaline;

• Thaumatoporella - Terebella - foraminifera - dasyclad biofacies: inner neritic, euryhaline.

Aeolisaccus dunningtoni is well-preserved and is considered to represent fossilised cyanobacterial sheaths or gently tapering tubes within carbonates of Late Permian to Late Jurassic age, with very little morphological variation. If interpreted as being in-situ, they are considered to represent supratidal conditions and indicate possible freshwater influence. Their presence within marine biofacies can be attributed to reworking within a basal marine transgressive lag or runoff. Their presence, with thin ostracods and stromatolites, indicates shallow intertidal hypersaline conditions. The chambered cyanobacteria attributed to Gakhumella spp., the ribbon-like microcoprolite Prethocoprolithus centripetalus and Thaumatoporella parvovesiculifera, the encrusting multichambered lamellar form with dasyclad affinity, constitute a biofacies considered to have accumulated within a deep intertidal environment. Supplementation of this biofacies with the complex cylindrical microcoprolite Favreina spp., the agglutinated worm tube Terebella lapilloides and rare miliolid foraminifera probably indicate shallow-marine conditions with slightly elevated salinity. The unstable and relatively hostile nature of the palaeoenvironment explains the low diversity but locally common abundance of the various forms described, mostly in the absence of foraminifera and calcareous algae. They would be best classified as environmentally opportunistic “r” strategists in the scheme of Macarthur and Wilson (1967).

These biofacies can be used to determine the palaeoenvironment and polarity of high-frequency depositional sequences in shallow-marine carbonates. The vertical stacking of various biofacies will enable detection of minor palaeoenvironmental variations within carbonate reservoirs, and permit regional correlation of depositional cycles. In addition, variations in the biofacies character will assist palaeoenvironmental and reservoir quality mapping of carbonates.

The author thanks the Saudi Arabian Ministry of Petroleum and Mineral Resources and the Saudi Arabian Oil Company (Saudi Aramco) for granting permission to publish this paper. Valuable written and oral taxonomic discussions with Ioan Bucur of the Rumanian Geological Survey, Christopher Toland of Oolithica and Matthew Williams of Geoscience Wales are acknowledged. Earlier versions of the manuscript were improved by comments from my colleagues Merrell Miller, David Cantrell, Nigel Hooker and Nassir Naji, together with Gregory Douglas and other members of the publications approval team of Saudi Aramco. The manuscript has benefitted considerably following critical comments by D. Vachard, Lille University, France. The author thanks GeoArabia’s Assistant Editor, Kathy Breining, for proofreading the manuscript and GeoArabia’s Graphic Designer Arnold Egdane for designing the paper.

Geraint Wyn Hughes is Senior Geological Consultant in the Biostratigraphy Group of Saudi Aramco’s Geological Technical Services Division. He gained BSc, MSc, PhD and DSc degrees from Prifysgol Cymru (University of Wales) Aberystwyth, UK, and in 2000 he received the Saudi Aramco Exploration Professional Contribution award, in 2004 the best paper award, and in 2006 the GEO 2006 best poster award. His biostratigraphic experience, prior to joining Saudi Aramco in 1991, includes 10 years with the Solomon Islands Geological Survey, and 10 years as Unit Head of the North Africa-Middle East-India region for Robertson Research International. Wyn’s professional activities are focused on integrating micropaleontology with sedimentology to support exploration activities and assist reservoir characterization. He maintains links with academic research as an Adjunct Professor of the King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia. He is an Associate Editor for the AAPG, Saudi Journal of Earth Sciences, GeoArabia reviewer, and a member of the British Micropaleontological Society, the Grzybowski Agglutinated Foraminiferal Society, SEPM Society for Sedimentary Geology and the Cushman Foundation for Foraminiferal Research.

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