Brenner proposed that Zone I began in the Barremian and extended into the ... Doyle et al. therefore transferred Brenner's species to Retimonocolpites, but this ...
Cretaceous Research (1992) 13, 337-349
Revised palynological correlations of the lower Potomac Group (USA) and the Cocobeach sequence of Gabon (BarremianAptian) James A. Doyle Department of Botany, University of California, Davis, CA 95616, U.S.A. Received 7January 1991 and accepted 30May 1991
Dating of the lower Potomac Group (Zone I), the oldest exposed Cretaceous unit of the Atlantic Coastal Plain of the USA, has relied on palynological correlations with marine sections elsewhere in Southern Laurasia. Correlation with sections in Northern Gondwana, such as the Cocobeach sequence of Gabon, has been hampered by differences due to climate. However, recently described palynofloras from intermediate areas show more overlap of elements, allowing stepwise correlations between the two provinces. A key element is the coarsely reticulate monosulcate angiosperm BrenneripoUis, which appears near the base of the Aptian in England. Brenneripollis occurs in basal Potomac samples from Delaware City well D12 and outcrop localities previously dated as Barremian, favoring an early Aptian age. Other lower Zone I angiosperms include 'ClavatipoUenites', Stellatopollis, SimilipoUis, Afropollis and very rare tricolpates. Upper Zone I, which contains more reticulate tricolpate pollen, has been dated as early Albian, but associated BrenneripoUis grains correlate better with the middle Aptian of Egypt. This age is also favored by the presence of the zonasulculate genus Schrankipollis in upper Zone I and the Aptian of Egypt. Zone C-VII of Gabon, which contains the first reticulate tricolpates and zonasuiculate species of AfropoUis, has been dated as early Aptian. However, it seems older than the presumed basal Aptian of Egypt, which contains Brenneripollis and striate tricolpates, not seen until Zone C-VIII, and there are putative late Barremian occurrences of Afropollis in Nova Scotia, England, Morocco, Libya and Israel. Together with reports of tricolpates from the Barremian and Aptian of England, these results eliminate the supposed lag in migration of tricolpate-producing plants from Gondwana into Laurasia, but Northern Gondwana does appear to have been their main center of diversification. KEY WORDS: Potomac; Cocobeach; Barremian; Aptian; Atlantic Coastal Plain; Gabon; palynology; angiosperms.
1. Introduction The Potomac Group of the Atlantic Coastal Plain of the USA and the Cocobeach sequence of Gabon have figured prominently in paleobotanical studies of the early evolution of angiosperms and have served as important palynostratigraphic reference sections for their respective areas (Brenner, 1963; Doyle, 1969; Doyle & Hickey, 1976; Doyle & Robbins, 1977; Doyle et al., 1977, 1982; Hickey & Doyle, 1977). These sections are entirely nonmarine, and the primary tools for dating are plant fossils, especially pollen and spores, which allow correlation with better-dated marine deposits elsewhere. Direct palynological correlation between the two sections is difficult because of provincial differences in presence, abundance and ranges of elements, owing to climatic and migrational effects. The Potomac flora represents the subtropical Southern Laurasia province of Brenner (1976), while the Cocobeach represents the tropical Northern Gondwana province. As a result, most palynostratigraphic comparisons have focused on correlations within one or the other province, where consistencies in palynofloral composition and sequences of taxa support the assumption that ranges are similar. In the present review, I will argue that this situation is changing as a result of recent data from intermediate areas such as Morocco, Egypt and Israel, where 0195-6671/92/040337 + 13 $03.00/0
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greater overlap of Laurasian and Gondwanan elements allows stepwise correlations between the Potomac and Cocobeach sections. The general effect of these data is to push certain key events in both regions lower down in the Cretaceous. I will concentrate on the angiosperms, which were evolving rapidly at this time, and I will consider only the lower part of the Potomac Group, since this is most controversial and most important for angiosperm evolution. My intention is not to provide a comprehensive summary of the data, but rather to call attention to evidence that revisions of assumptions are in order and to groups that may provide critical tests. 2. Potomac correlations
The Potomac Group includes the oldest exposed sediments of the Atlantic Coastal Plain in Virginia, Maryland and Delaware, deposited on Paleozoic or Triassic rocks (Glaser, 1969). Much older Cretaceous sediments (Berriasian) exist to the east in the subsurface (Doyle, 1983). In Maryland three formations can be recognized: the sandy Patuxent Formation, the organic-rich Arundel Clay, and the heterogeneous Patapsco Formation; but elsewhere, where the Arundel Clay is not recognizable, the Potomac cannot be subdivided lithostratigraphically. The first and still most comprehensive monograph on Potomac palynology was that of Brenner (1963), based on two wells and numerous outcrop samples in Maryland. Brenner recognized two zones, Zone I and Zone II, with Zone II divided into two main subzones, II-A and II-B. These units were defined primarily by the appearance of new taxa, secondarily by changes in frequency. Zone II was defined by the appearance of several index spores, gymnosperms and, most importantly, several species of tricolpate pollen, the basic type for "higher" dicots (excluding Magnoliidae) or "eudicots" (Doyle & Hotton, 1991). Brenner assumed that the tricolpates were the first definite angiosperms, but later studies showed that earlier monosulcates, such as 'Clavatipollenites', with reticulate, columellar exines, are also angiosperms (Doyle, 1969; Muller, 1970; Doyle et al., 1975; Walker & Walker, 1984). Brenner proposed that Zone I began in the Barremian and extended into the Aptian, based on the presence of 'Clavatipollenites', which was known from the late Barremian of England. He compared the tricolpates of Subzone II-B with assemblages from the late Albian of Portugal (Groot & Groot, 1962), and he suggested that Subzone II-A might be early Albian. As a result of studies on angiosperm pollen from the Potomac Group, Doyle & Robbins (1977) further subdivided the sequence and extended Brenner's zonation to younger beds in north-eastern Maryland (the former "Maryland Raritan Formation"), designated Subzone II-C and Zone III. Doyle & Hickey (1976) and Hickey & Doyle (1977) used these data to correlate Potomac Group plant megafossil localities, which had been poorly dated. The most notable departure from Brenner's scheme was the extension of rare tricolpates into upper Zone I. Doyle & Robbins (1977) used the appearance of these tricolpates and the reticulate monosulcate 'ClavatipoUenites' rotundus Kemp (then called Retimonocolpites dividuus) to distinguish the upper part of Zone I from the lower part, which they thought might extend into the Barremian. Another criterion for subdivision used by Doyle & Hickey (1976) and Upchurch & Doyle (1981) was the presence in lower Zone I of small gymnospermous bisaccates apparently related to Decussosporites Brenner (aft. Decussosporites; Figure la, b), which are replaced in upper Zone I by typical D. microreticulatus. Doyle & Robbins (1977) placed the base of Zone II near the early-middle Albian boundary,
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based on reports of rare tricolpates and 'Clavatipollenites' rotundus in the early Albian of England and one of Brenner's Zone II index spores (Apiculatisporis babsae) in the middle Albian (Kemp, 1970; Laing, 1975). They dated middle Subzone II-B as late middle Albian, based on correlation with a rich angiosperm flora described by Hedlund & Norris (1968) from beds of that age in Oklahoma. More recently, Walker & Walker (1984, 1986), Ward et al. (1989) and Doyle et al. (1990) used SEM and TEM to provide additional data on the morphology, diversity and systematic atfinities of Potomac angiosperm pollen. The most important new data on the age of the Potomac and Cocobeach sections come from studies by Hughes et al. (1979) and Hughes & McDougall (1990) on angiosperm pollen in the Wealden and Lower Greensand of England, and by Schrank (1983) and Penny (1986, 1988a,b, 1989) on the Lower Cretaceous of Egypt. One of the key groups shared by these floras consists of monosulcate pollen with a coarse reticulum, supratectal spinules and no columellae (Figure lj-l). Members of this group were first described from the Potomac Group by Brenner (1963) as species of the spore genus Peromonolites (P. reticulatus and P. peroreticulatus), but SEM and TEM work by Doyle et al. (1975) showed that they are angiosperms. Doyle et al. therefore transferred Brenner's species to Retimonocolpites, but this is inappropriate because the type species of Retimonocolpites is a rather different form with a ring-furrow (see Doyle et al., 1990). Juh~isz & G6cz~in (1985) transferred Brenner's and several new species to the new genus BrenneripoUis. This transfer was questioned by Penny (1988a), in a detailed study of similar pollen from Egypt, because Juh~isz & G6cz~ described the type species, B. pellitus, as having columellae. However, their light micrographs do not convincingly demonstrate that B. pellitus has columellae, and in all other respects it resembles the Potomac species (cf. Ward et al., 1989). Therefore I will continue to use the name Brenneripollis for this complex. For stratigraphic purposes, Brenneripollis is significant because it appears at the base of the Lower Greensand in England (Hughes et al., 1979), which belongs to the first ammonite zone of the Aptian in the Boreal realm (Casey, 1961; Rawson et al., 1978). Related grains extend down to the early Barremian, but they differ in having columellae (Hughes & McDougall, 1990). The exact age of this event has become controversial because of recent magnetostratigraphic evidence that the uppermost Wealden (upper part of the Vectis Formation, or Wealden Shales on the Isle of Wight) correlates with the basal Aptian of the Tethyan realm, as defined by foraminifera (Kerth & Hailwood, 1988). The significance of this result will remain uncertain until the Tethyan foraminiferal zonation is correlated more explicitly with the Tethyan ammonite zonation. However, it seems safe to use the presence of non-columellar BrenneripoUis as evidence for an Aptian rather than Barremian age. As noted by Hughes et al. (1979), this provides a criterion for dating the lower Potomac Group, since rare specimens of this group occur at the base of the Potomac in the D12 reference well at Delaware City (D12-770; Doyle & Robbins, 1977; Ward et al., 1989) and at lower Zone I outcrop localities, such as the classic Dutch Gap plant megafossil locality of Fontaine (1889; Ward et al., 1989). Upchurch & Doyle (1981) suggested that other Potomac outcrop samples in which Brenneripollis had not been found might be Barremian, but this is now doubtful. The best example is the sample that provided the grains of 'Clavatipollenites' cf. hughesii studied with SEM and TEM by Doyle et al. (1975) and Walker & Walker (1984), which was collected by Cleaves (1968) from immediately above the basement during excavation of the Susquehanna Aqueduct in north-eastern Maryland. This sample
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(Aq 27) contains the bisaccate aft. Decussosporites characteristic of lower Zone I (Figure la, b). Not only 'ClavatipoUenites' (Figure lc-e) but also other angiosperms in Aq 27 are shared with the presumed late Barremian of England. One such group has finer sculpture at the proximal pole and the sulcus margins (Figure If, g); comparable English forms were figured by Hughes et al. (1979) as PERFOTECTSPOT. These grains were placed in Liliacidites by Doyle & Robbins (1977; L. sp. B) and Walker & Walker (1984, 1986), but they fit better in SimilipoUis G6czfin & Juhfisz (1984). Another group is SteUatopoUis (Figure lh), which has triangular projections on top of the reticulum; relatives from England were reported by Hughes et al. (1979) and Hughes & McDougall (1990) as SUPERRET-CROTON and SUPERRET-TRIANG. Aq 27 also contains the coarsely reticulate genus Afropollis (Figure li), which is most characteristic of Northern Gondwana but was previously reported from one other lower Potomac sample (Doyle et al., 1982). Afropollis was originally thought to appear in the early Aptian, but it has more recently been extended into the Barremian of England (Penny, 1989; Hughes & McDougall, 1990) and various localities in Northern Gondwana, as discussed below. Although BrenneripoUis was not seen in several slides examined previously, further research has revealed a single typical, non-columellar specimen of B. peroreticulatus (Figure lj-l). With the discovery of Brenneripollis, the flora in sample Aq 27 becomes more comparable with floras from other lower Zone I samples (e.g., D12-770, Dutch Gap, Trent's Reach, Potomac Run), which contain Brenneripollis, many of the same associated angiosperms, and aft. Decussosporites (cf. Doyle & Robbins, 1977; Hickey & Doyle, 1977). Besides the taxa listed above, angiosperms that occur in Aq 27 and other lower Zone I samples include 'Clavatipollenites' sp. A, which is larger and more coarsely reticulate than 'C.' hughesii, and the large, granular, possibly magnolialian monosulcate Lethomasites fossulatus (Figure lm), described by Ward et al. (1989) from D12-770 and Dutch Gap. A more surprising similarity between Aq 27 and D12-770 is the occurrence of very rare reticulate tricolpate grains (three specimens observed in Aq 27, one in D12-770), which appear to represent two related species, differing in size and coarseness of sculpture (Figure In-r). This extension of tricolpates becomes less anomalous with the report by Hughes & McDougall (1990) of isolated reticulate tricolpate grains in the late Barremian and Aptian of England. The fact that many lower Zone I angiosperms occur in the Barremian of England does not conflict with the inferred Aptian age, since they all belong to complexes that range into definite Aptian and/or Albian horizons elsewhere. Evidently Brenneripollis is highly sporadic in this interval, so it is dangerous to use its absence as evidence for a Barremian age. Penny's (1988a) work on Brenneripollis in Egypt also provides new evidence on the age of upper Zone I. This illustrates how the Egyptian flora acts as a link between Laurasia and Gabon: BrenneripoUis is rare in Gabon, but abundant and diverse in Egypt. It appears above a flora recalling the Barremian of England and the basal
hughesii Couper, Aq 27-1n, 13.3x 101.3; f, g, Similipollis sp., Aq 27-1g, 17.2x 105.4; h, SteUatopoUis sp. (with zonate aperture), Aq 27-1g, 17.8 x 103.9; i, AfropoUis sp. (probably A. aft. zonatus Doyle et a/., with zonasulculus displaced part way toward the equator), Aq 27-1n, 9.9 x 94.4; j-l, Brenneripollis peroreticulatus (Brenner) Juh~isz & G6cz~n, Aq 27-1n, 16.4 x 99.8; m, Lethomasites fossulatus Ward et al., Aq 27dk, 17.9x 104.2; n, o, Tricolpites sp., Aq 27-1b, 17.4 x 89.6; p-r, Tricolpites sp., Delaware City well D12, Delaware, D12-770-1b, 6.7 × 106.3.
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Potomac, with two species of Stellatopollis and several reticulate monosulcates. Most Potomac Brenneripollis grains, usually identified as B. peroreticulatus, are comparable to two of Penny's 'biorecords' (essentially equivalent to species), RETIMONOHEDGEHOG and RETIMONO-SPOTSPINES, which differ mostly in size. Penny showed that grains in the smaller RETIMONO-HEDGEHOG size range are replaced abruptly at 7310ft (2228m) by larger grains. As Penny noted, B. peroreticulatus from its type locality in upper Zone I (United Clay Mine, in the Arundel Clay) falls in the size range of the older RETIMONO-HEDGEHOG group, whereas a grain from the middle Albian of Oklahoma figured by Walker & Walker (1984) falls in the range of RETIMONO-SPOTSPINES. Penny's section is not dated by marine fossils, but his palynological correlations and extrapolation from definite Albian horizons higher in the well both imply that the turnover of species occurs near the boundary between the middle and the late Aptian. If these species have similar ranges in the two areas, these data suggest that upper Zone I lies well below the top of the Aptian--perhaps, considering that lower Zone I is probably also Aptian, in the middle Aptian. This implies that the boundary between Zone I and Zone II is probably much older than the early-middle Albian boundary, where it was placed by Doyle & Robbins (1977). More evidence for lowering the boundary between Zone I and Zone II is provided by an elliptical, zonasulculate pollen group described from the Aptian of Egypt by Schrank (1983) as Retimonocolpites mawhoubensis. Schrank noted that this species is similar to grains described by Brenner (1963) from upper Zone I of the Potomac Group (William Allen Clay Mine, in the Arundel Clay) as Schizosporis microreticulatus (Figure 2a, b). Doyle et al. (1990) showed that these forms resemble each other at the SEM level and transferred them to the new genus Schrankipollis. Most significantly, Penny (1988b) also reported Schrankipollis in his section, from about 1300ft (396m) above the appearance of Brenneripollis to the same level where RETIMONO-HEDGEHOG (compared with upper Zone I B. peroreticulatus) is replaced by RETIMONO-SPOTSPINES, This consistency of ranges increases confidence in correlation between the Potomac and the Egyptian section and supports dating of upper Zone I as middle Aptian. Several tricolpate types are now known from upper Zone I, as defined on absence of Brenner's (1963) non-angiospermous index species for Zone I! and corroborated by lithostratigraphic relations. The most common, which has foveo-reticulate sculpture and slightly thicker exine at the poles, was figured by Wolfe et al. (1975), Doyle & Robbins (1977), and Hickey & Doyle (1977) as aft. Tricolpites crassimurus (Figure 2c, d). It occurs at United Clay Mine, in Delaware City well DI2 (D12-695), at the Baltimore megafossil locality of Fontaine (1889), and at the Drewrys Bluff megafossil locality of Doyle & Hickey (1976) and Upchurch (1984). Tricolpites sp. A of Doyle & Robbins (1977), which is smaller and has a more open reticulum, is known only from well D12 (D12-715, D12-710). Since then, one or two other species have been found that conform to the genus Rousea, distinguished by sculpture that grades from fine at the poles to coarse at the equator. These occur at William Allen Clay Mine and at Fontaine's Fredericksburg megafossil locality (Figure 2e-g). The tricolpate figured from the Aptian of England by Hughes & McDougall (1990) is also of the Rousea type. Further indirect evidence for dating the boundary between Zone I and Zone II comes from a palynoflora reported by Beach (1981; pers. comm.) from the Glen Rose Formation of Texas, which is faunaUy dated as early Albian. This flora includes several tricolpate types, including large and striate forms not known below
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Subzone II-B in the Potomac. If Subzone II-B extends down into the early Albian, Subzone II-A may be either late Aptian or early Albian. These data have important implications concerning origin and migration of tricolpate-producing eudicots. Brenner (1976) documented a poleward spread of tricolpate pollen, appearing in the late Barremian or early Aptian of Israel and the Aptian of Brazil in Northern Gondwana, but not until the middle Albian in Southern Laurasia and the late Albian or Cenomanian in Northern Laurasia. Later, Hickey & Doyle (1977) performed a similar analysis, adding monosulcate angiosperms. Based on the recognition of tricolpates in the early Albian of England (Kemp, 1970; Laing, 1975) and upper Zone I, considered coeval at that time, Hickey & Doyle extended the range of tricolpates in Laurasia into the early Albian. However, with the present evidence that upper Zone I is middle Aptian and the discovery of tricolpates in the probable early Aptian of lower Zone I and the Barremian of England (Hughes & McDougall, 1990), much more extension of this group is indicated. The previous failure to recognize tricolpates below the Albian of England is not surprising in light of the rarity of tricolpates in the lower Potomac and the marine character and impoverished land-derived palynoflora of the English Aptian. From a practical point of view, these observations imply that future palynostratigraphic subdivisions of the Potomac Group and correlative sections cannot be based on the presence or absence of broad morphological groups such as tricolpates, but must instead concentrate on the specific composition of the angiosperm flora.
3. Cocobeach correlations
The Cocobeach sequence of Gabon and Congo consists of rift deposits associated with opening of the South Atlantic Ocean, similar to deposits in coastal basins of Brazil. The Cocobeach was zoned palynologically by Jardin6 et al. (1974) and Doyle et aL (1977, 1982). The lower part consists of thick, organic-rich shales and turbidites, interpreted as deep lake deposits. In Gabon, these lacustrine sediments are followed by lighter and sandier beds marking infiUing of the lakes by deltas (Zone C-V) and then by fluvial beds (Zone C-VI through C-VIII). These rocks are overlain unconformably by salt deposits and associated basal clastics (Zone C-IX), marking the first influx of marine water into the rift, and then by marine carbonates (Madi~la Series). The sequence in Congo differs in that it lacks Zones C-VII and C-VIII, so the present discussion will focus on Gabon. The oldest marine fossils for dating the Gabon section are late Aptian and Albian ammonites in the Madi~la carbonates (Reyment & Tait, 1972), but lower beds have been correlated with marine sections elsewhere in Northern Gondwana using palynology (Doyle et al., 1977, 1982). Thus the gymnosperm Dicheiropollis etruscus of Zones C-II through C-IV is known elsewhere up to the early Barremian. Monosulcate angiosperms appear sporadically in Zone C-V and regularly in Zone C-VI. Zone C-VI angiosperms are similar to those in the basal Potomac and the Barremian of England, including several monosulcates with even reticulate sculpture (generally coarser than typical 'ClavatipoUenites'), two StelIatopotlis species, and Similipollis (Liliacidites sp. 1 of Doyle et al., 1977). There is also one major endemic group, not reported from either Laurasia, Egypt or Morocco, which resembles 'ClavatipoUenites' but has a continuous tectum, originally described from Brazil as Inaperturopollenites crisopolensis
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(Regali et al., 1974) but renamed TucanopoUis by Regali (1989). Zone C-VII is characterized by the last occurrence of TucanopoUis, especially forms with a round aperture, and the appearance of AfropoUis, an abundant element throughout Northern Gondwana, which was known to appear near the base of the Aptian in Algeria, Morocco and the southern Alps. Zone C-VII species of AfropoUis are zonasulculate, with a ring-furrow around the distal pole (A. opercu/atus) or at the equator (A. zonatus). Zone C-VII also includes the first rare tricolpates, with reticulate sculpture (Figure 2h-k); judging from their depth below the late Aptian marine beds, these seemed much older than the first tricolpates in Laurasia, then thought to be early Albian. Still below the marine beds, in Zone C-IX or Zone C-VIII (which is more poorly known), zonasulculate species of Afropollis are replaced by inaperturate forms (A. aft. jardinus). Zone C-IX also includes new monosulcates, including Brenneripollis and 'Clavatipollenites' rotundus, noted in upper Zone I of the Potomac, and diverse new tricolpates, including forms with striate sculpture (not seen in the Potomac until Zone II) and gemmate sculpture (not known outside Northern Gondwana). These data led Doyle et al. (1977, 1982) to propose that Zone C-VI is Barremian, Zone C-VII early Aptian, and Zone C-IX late Aptian. This would be consistent with the inferred basal Aptian appearance of AfropoUis in Egypt (Penny, 1989). However, more recent data indicate that Afropollis, and therefore Zone C-VII, may extend into the late Barremian (a conclusion also reached by Regali & Viana, 1989). One occurrence of Afropollis already cited by Doyle et al. (1982), in the Nova Scotia shelf, seemed more likely late Barremian than Aptian, based on the associated dinoflagellates. Subsequently, G~beli et al. (1984) reported that Afropollis operculatus extends into the marine late Barremian in Morocco, associated with SteUatopoUis and 'Clavatipollenites.' Afropollis has also been reported from marine beds dated by dinoflagellates as late Barremian in north-eastern Libya (Thusu et al., 1988) and Israel (De Haan & Leereveld, 1990). Finally, as noted above, Penny (1989) and Hughes & McDougall (1990) reported an inaperturate form of Afropollis in the late Barremian of England. Other inconsistencies between Gabon and Egypt cast additional doubt on an Aptian age for Zone C-VII. First, the oldest AfropoUis grains in Egypt are inaperturate, not zonasulculate as in Zone C-VII. Second, in Egypt the appearance of Afropollis coincides with the first consistent appearance of tricolpates, as in Zone C-VII, but the Egyptian tricolpates are not reticulate but rather striate (Penny, 1988b), a sculpture pattern not seen in Gabon until Zone C-VIII. Penny ascribed this conflict to sampling problems, since he did see one isolated reticulate tricolpate grain in the pre-Afropollis interval. It is also possible that this interval includes other tricolpates that Penny did not recognize as such. Specifically, 'Liliacidites' aegyptiacus, which Penny (1986) interpreted as monosulcate, is very suggestive of the oldest Zone C-VII tricolpates (figured as aft. Tricolpites crassimurus by Doyle et al., 1977). Similarities include its heterobrochate sculpture pattern (with intermixed large and small lumina) and its twisted appearance, similar to the Zone C-VII grain in Figure 2h-k, in which the colpi are unevenly spaced and obliquely orientated (considered a primitive transitional state in origin of the tricolpate condition by Doyle & Hotton, 1991). When studying strew mounts with SEM, as Penny did, it may be impossible to tell whether a grain with one furrow is monosulcate or tricolpate with two colpi on the opposite side. Zone C-VII also differs from the lower Aptian of Egypt in lacking BrenneripoUis, which is not definitely known until Zone C-IX. These indications that tricolpates appear in the late Barremian of Egypt
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and Gabon increase the plausibility of a Barremian age for the reticulate tricolpates reported by Brenner (1976) from an interval in Israel dated as late Barremian to early Aptian. 4. Potomac-Cocobeach correlations
Figure 3 presents two hypotheses to resolve these conflicts concerning correlation of the lower Potomac and Cocobeach sequences. On the left are the proposed correlations of the dated Lower Cretaceous of England with the Potomac on the one hand and Penny's Egyptian section on the other. The appearance of non-columeUar BrenneripoUis is equated with the base of the Aptian, but it may be slightly younger in view of the magnetostratigraphic evidence for an Aptian age of the uppermost Wealden (Kerth & Hailwood, 1988). T h e overlap of Schrankipollis and RETIMONO-HEDGEHOG is used to date upper Zone I as middle Aptian. AfropoUis is shown as extending into the late Barremian of Morocco, Libya and Israel. On the right are the two interpretations of the Gabon sequence. According to the first hypothesis, Afropollis appears at the same time in Gabon and Egypt, dated as early Aptian on association with Brenne@ollis in Egypt. This implies that the absence of BrenneripoUis and striate tricolpates in Zone C-VII is a climatic or facies effect. The underlying Zone C-VI is Barremian, which is consistent with its similarities to the Barremian of England. This implies that there was a lag in appearance of tricolpates in Gabon, with reticulate forms appearing in the early Aptian rather than the Barremian, and striate forms appearing in the late rather than early Aptian. The presence of inaperturate Afropollis species in the early Aptian of Egypt, but of zonasulculates only Zone C-VII, is a function of a less complete record of AfropoUis in Gabon. The concept that ranges of Afropollis species are incomplete (at least upward) in Gabon was proposed by Doyle et al. (1982) because of the overlap of ranges of zonasulculate and inaperturate Afropollis types in the late Aptian of Senegal, but not in Gabon. The worst problem with this hypothesis is that it conflicts with reports of Afropollis in the late Barremian of Morocco, Libya, Israel and England. This suggests the second hypothesis--that Zone C-VII correlates with the interval in Egypt predating Afropollis and Brenneripollis, which is most likely late Barremian. This correlation is supported by the evidence for reticulate tricolpates below the appearance of BrenneripoUis in Egypt and in England, as in Zone C-VII. The absence of BrenneripoUis, inaperturate Afropollis, and striate tricolpates in Zone C-VII is due simply to the fact that Zone C-VII is older than the beds where these forms appear in Egypt, not to climate or facies. Zones C-VIII and C-IX could extend back as far as early Aptian, but there is an unconformity of unknown length in this interval. The greatest anomaly is the absence of Afropollis below the presumed Aptian in Egypt. This is hard to ascribe to climate, since Egypt and Israel are adjacent geographically, but it might be a facies effect. More likely climatic effects are the absence of Schrankipollis in Gabon and the absence of TucanopoUis in Egypt and Morocco, although Thusu et al. (1988) did report Tucanopollis in Libya. I find this second hypothesis more plausible, since it is more consistent with the range extensions of Afropollis and trends among the tricolpates. If the second hypothesis is correct, Zone C-VI equivalents have not been recognized in Egypt. This may reflect the fact that the base of the angiospermbearing sequence coincides with a change from marine to nonmarine environments, which Penny (1986) suggests is due to regression and a possible unconformity. Zone
Revised palynologicalcorrelations
POTOMAC
ENGLAND
EGYPT
MORocco LIBYA ISRAEL
347
GABON HYPOTHESIS I
HYPOTHESISn
C-IX C-VIII
C-IX
ALBIAN SUBZONE II-A
0
APTIAN
o
O
C-VIII
ZONEI
0@6
,m !
C-VII
O @Q C-VI BARREMIAN
C-VII C-VI
Figure 3. Proposedcorrelation of the lower Potomacand Cocobeachsectionswith sectionsin England (Kemp, 1970; Laing, 1975; Hugheset al., 1979; Hughes & McDougall, 1990)and Egypt (Schrank, 1983; Penny, 1986, 1988a,b, 1989),showingtwo alternative hypotheseson correlation of Cocobeach Zone C-VII. Pollen types indicated (from top, under "EGYPT"): Schrankipollis, typical noncolumellar Brenneripollis (with range of biorecord RETIMONO-HEDGEHOGof Penny, 1989a), Afropollis, striate tricolpate, reticulate tricolpate. C-VI equivalents may be either missing or unfavorable in facies. This is not to say that the Gabon section is generally more complete; it is possible that significant time is missing between Zone C-VI and C-VII, and especially between Zone C-VII and Zone C-IX. The latter interval appears to be better represented in Egypt. Until these problems are resolved, it seems most prudent to refer to Zone C-VII pollen types as late Barremian or early Aptian in age. These include not only the asymmetrical tricolpates figured by Doyle & Hotton (1991) and in Figure 2h-k, but also the Afropollis operculatus and A. zonatus grains and winteralian tetrads (Walkeripollis gabonensis) described by Doyle et al. (1990). From a botanical point of view, these results imply that tricolpate-producing eudicots appeared much earlier in Laurasia than previously thought (Barremian, early Aptian), eliminating the supposed lag in migration of eudicots from Gondwana to Laurasia. However, it still appears that pre-Albian tricolpates were much more common and diverse in Northern Gondwana than in Laurasia, as most conspicuously exemplified by the striates. The same pattern holds for Afropollis. In addition, the increasing overlaps of supposed Laurasian and Gondwanan elements blur the previously sharper distinctions between the two provinces: the differences are more quantitative than qualitative. However, this is reason for optimism from a stratigraphic point of view: it means that there is more potential for intercontinental palynological correlations of nonmarine Cretaceous sediments than has been believed.
Acknowledgements I thank Pieter de Haan and James H. J. Penny for useful discussions, Serge Jardin6 and the Socitt6 Nationale Elf-Aquitaine (Production) for Cocobeach samples, and
348
J.A. Doyle
Jerome v. Ward for use of unpublished data on the tricolpates. This work was supported in part by NSF grant BSR-8415772.
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Juh~isz, M. & G6cz~in,F. 1985. Comparative study of Albian monosulcate angiosperm pollen grains. Acta Biologica Szeged 31, 147-172. Kemp, E. M. 1970. Aptian and Albian miospores from southern England. Palaeontographica,Abteilung B 131, 73-143. Kerth, M. & Hailwood, E. A. 1988. Magnetostratigraphy of the Lower Cretaceous Vectis Formation (Wealden Group) on the Isle of Wight, Southern England. Journal of the GeologicalSociety, London 145, 351-360. Laing, J. F. 1975. Mid-Cretaceous angiosperm pollen from southern England and northern France. Pa/aeontolo~ 18, 775-808. Muller, J. 1970. Palynological evidence on early differentiation of angiosperms. Biological Reviews of the Cambridge Philosophical Society 45, 417-450. Penny, J. H. J. 1986. An Early Cretaceous angiosperm pollen assemblage from Egypt. Special Papers in Palaeontology 35, 121-134. Penny, J. H. J. 1988a. Early Cretaceous acolumellate semitectate pollen from Egypt. Palaeontology 31, 373-418. Penny, J. H. J. 1988b. Early Cretaceous striate tricolpate pollen from the Borehole Mersa Matruh 1, North West Desert, Egypt. Journal of Micropalaeomology 7, 201-215. Penny, J. H. J. 1989. New Early Cretaceous forms of the angiosperm pollen genus AfiopoUis from England and Egypt. Review of Palaeobotany and Palynology 58, 289-299. Rawson, P. F., Curry, D., Dilley, F. C., Hancock, J. M., Kennedy, W. J., Neale, J. W., Wood, C. J. & Worssam, B. C. 1978. A correlation of Cretaceous rocks in the British Isles. GeologicalSociety of London Special Report 9, 70 pp. Regali, M. S. P. 1989. Tucanopo///s, urn #nero novo das angiospermas primitivas. Boletim de Geocibwias da Petrobrds $, 395-402. Regali, M. S., Uesugui, N. & Santos, A. S. 1974. Palinologia dos sedimemos meso-cenoz6icos do Brasil. Boletim Tdcnicoda Petrobrds 17, 177-191,263-301. Regali, M. S. P. & Viana, C. F. 1989. Late Jurassic-Early Cretaceous in Brazilian sedimentary bas/ns: correlation with the international standard scale, 95 pp. (Petrobrfis, Rio de Janeiro). Reyment, R. A. & Tait, E. A. 1972. Biostratigraphical dating of the early history of the South Atlantic Ocean. Philosophical Transactions of the Royal Society of London, B. BiologicalSciences 264, 55-95. Schrank, E. 1983. Scanning electron and light microscopic investigations of angiosperm pollen from the Lower Cretaceous of Egypt. Pollen et Spores 25, 213-242. Thusu, B., Van der Eem, J. G. L. A., EI-Mehdawi, A. & Bu-Argoub, F. 1988. Jurassic-Early Cretaceous palynostratigraphy in northeast Libya. In Subsurface palynology of northeast Libya (eds EI-Arnauti, A. et al.), pp. 171-213 (Garyourfis University Publications, Benghazi). Upehurch, G. R. 1984. Cuticular anatomy of angiosperm leaves from the Lower Cretaceous Potomac Group. I. Zone I leaves. AmericanJournal of Botany 71, 192-202. Upchurch, G. R. & Doyle, J. A. 1981. Paleoecology of the conifers Frenelopsis and Pseudofrenelopsis (Cheirolepidiaceae) from the Cretaceous Potomac Group of Maryland and Virginia. In Geobotany II (ed. Romans, R. C.), pp. 167-202 (Plenum Publishing, New York). Walker, J. W. & Walker, A. G. 1984. Ultrastructure of Lower Cretaceous angiosperm pollen and the origin and early evolution of flowering plants. Annals of the Missouri Botanical Garden 71, 464-521. Walker, J. W. & Walker, A. G. 1986. Ultrastructure of Early Cretaceous angiosperm pollen and its evolutionary implications. In Pollen and spores:form and function (eds Biackmore, S. & Ferguson, I. K.), pp. 203-217 (Linnean Society Symposium Series 12, Academic Press, London). Ward, J. V., Doyle, J. A. & Hotton, C. L. 1989. Probable granular magnoliid angiosperm pollen from the Early Cretaceous. Pollen et Spores 33, 101-120. Wolfe, J. A., Doyle, J. A. & Page, V. M. 1975. The bases of angiosperm phylogeny: paleobotany. Annals of the Missouri Botanical Garden 62, 801-824.
