AÇOREANA, 2007, Supl. 5: 112-125

NEOGENE SHALLOW-MARINE PALEOENVIRONMENTS AND PRELIMINARY STRONTIUM ISOTOPE CHRONOSTRATIGRAPHY OF ,

Michael Xavier Kirby 1, 2, Douglas S. Jones 2, Sérgio P. Ávila 3, 4, 5

1 Geobiological Research Laboratory, 265 Cross Street, Middletown, CT 06457, USA 2 Florida Museum of Natural History, University of Florida, Gainesville, FL 32611, USA [email protected] 3 Departamento de Biologia, Universidade dos Açores, 9501-801 , Azores, 4 Centro do IMAR da Universidade dos Açores, 9901-862 Horta, Azores, PORTUGAL 5 MPB – Marine PalaeoBiogeography Working Group of the University of the Azores, Rua da Mãe de Deus, 9501-801 Ponta Delgada, Azores, PORTUGAL

ABSTRACT The fossils of Santa Maria Island in the Azores archipelago represent one of the few shallow-marine communities of Neogene age between Europe and North America. Before the evolutionary and biogeographic implications of these fossils can be understood, however, their associated depositional environments and geo- logic ages must be determined. Here we present preliminary results from sedi- mentary facies analysis and strontium isotope chronostratigraphic analysis of sed- iments and fossils from two localities on Santa Maria Island that provide a window into shallow-marine environments and communities within the mid-Atlantic Ocean during the Neogene. Pedra-que-Pica on the southeastern corner of Santa Maria contains strata of fine-grained lithic calcarenite, coquina, and fine- to medi- um-grained lithic wacke that represent a regressive sequence from transition-zone to foreshore environments. The second locality at Pedreira do Campo on the south- western-side of Santa Maria contains limestone and fine- to coarse-grained lithic arenite that represent a regressive sequence from shallow bank to shoreface-fore- shore environments. Strontium isotopic results from Pedra-que-Pica and Pedreira do Campo indicate that these localities contain fossils that range from late Miocene to late Pliocene in age. Three molluscs collected from the coquina at Pedra-que-Pica have an average 87Sr/86Sr composition of 0.709018±0.000008 that represents an average estimated age of 5.51±0.21 Ma. Three pectinid bivalves collected from the limestone at Pedreira do Campo show a wide range in 87Sr/86Sr ratios, from 0.708885 to 0.709078, which represent estimated ages from 10.03 to 2.24 Ma, respec- tively. These results help to place the shallow-marine communities of Santa Maria Island into a geologic context that will aid our understanding of how these com- munities relate to the broader evolutionary and biogeographic history of the Atlantic basin during the Neogene.

INTRODUCTION evolved into separate and distinct Mediterranean, western Atlantic- he Neogene Epoch was an impor- Caribbean, and eastern Pacific com- Ttant time when shallow-marine munities. It was during the Neogene faunas of the former Tethys Sea that the seaway between North and KIRBY ET AL: NEOGENE OF SANTA MARIA ISLAND 113

South America closed (Emiliani et al., Pedreira do Campo) in order to infer 1972; Keigwin, 1978), thereby divid- their depositional environments ing a once continuous marine (paleoenvironments). We also ana- province into distinct Pacific and lyze the strontium isotopic composi- Atlantic communities (Woodring, tion of fossil shells in order to esti- 1966; Vermeij, 1978; Vermeij and mate the geologic ages at these Petuch, 1986). Also during the localities as a first step toward a Sr Neogene, the remnants of the Tethys chronostratigraphy of the fossils and Sea continued to contract as the sediments of Santa Maria Island. This African tectonic plate collided with method of age dating is appropriate the Eurasian plate. Although many for the fossils of Santa Maria because Neogene localities in the western and past studies elsewhere have shown eastern Atlantic Ocean have been that the Neogene was a time of rapid- described, there are few opportuni- ly increasing 87Sr/86Sr in the global ties to examine shallow-marine local- ocean and, therefore, particularly ities between these two ends of an amenable to dating and correlating ocean basin. marine sediments using strontium The Azores archipelago, located isotopes (e.g. Hodell et al., 1991; almost in the middle of the Atlantic Miller et al., 1991; Jones et al., 1993; Ocean, provides an opportunity to Hodell and Woodruff, 1994; examine shallow-marine communi- Mallinson et al., 1994; Oslick et al., ties of the Neogene. The Azores arch- 1994; Miller and Sugarman, 1995; ipelago consists of nine islands, but Martin et al., 1999; McArthur et al., despite six centuries of occupation by 2001). These results place the fossil people, only one island has been communities at Pedra-que-Pica and found to contain fossils (Mayer, 1864; Pedreira do Campo into a geologic Cotter, 1888-1892; Ferreira, 1955; framework in order to better under- Krejci-Graf et al., 1958; Zbyszewsky stand the biogeography and evolu- and Ferreira, 1962b; Ávila et al., 2002; tion of these shallow-marine commu- Ávila, 2005). This is the island of nities during the Neogene. Santa Maria, which is the southeast- ern-most island in the archipelago. The fossils and associated sediments MATERIALS AND METHODS of Santa Maria offer a window into shallow-marine communities and We conducted fieldwork in May environments within the mid- 2005 at two localities on Santa Maria Atlantic Ocean during the Neogene Island, Pedra-que-Pica and Pedreira (Zbyszewski and Ferreira, 1962a; do Campo. Two stratigraphic sec- Serralheiro and Madeira, 1993; Ávila tions were measured upsection using et al., 2002; Cachão et al., 2003). In this the method of eye height and study, we analyze the sedimentary Brunton compass described by facies of outcrops at two localities on Compton (1985). Pedra-que-Pica is Santa Maria (Pedra-que-Pica and located on the southeastern corner of 114 AÇOREANA 2007, Supl. 5: 112-125

Santa Maria at Baixa do Sul was formerly used to mine basalt for (N36°55.806’, W25°01.482’), about construction material. Pedreira do 0.76 km west of the lighthouse on Campo is now part of a natural mon- Ponta do Castelo (Fig. 1). The lower ument and the area is protected for its portion of the outcrop is exposed in a geological, paleontological, biological wave-cut platform about 1709 m2 in and cultural significance (Cachão et area in the intertidal zone. The upper al., 2003). Cachão et al. (2003) have portion of the outcrop is exposed in a recently described the international sea cliff that is several hundred importance of the fossils and geology meters in height. at Pedreira do Campo, particularly Pedreira do Campo is located on toward understanding the geologic the southwestern corner of Santa history of the North Atlantic and the Maria (N36°56.818’, W25°08.119’), colonization of the Azores Islands by about 1.0 km east of marine biota. and about 1.4 km southwest of Pico We analyzed three fossil speci- do Facho (Fig. 1). The outcrop is mens of molluscs (oyster, spondylid, exposed in an abandoned quarry that and pectinid) from Pedra-que-Pica

FIGURE 1. Map of Santa Maria Island, Azores, showing the location of Pedra-que-Pica and Pedreira do Campo. KIRBY ET AL: NEOGENE OF SANTA MARIA ISLAND 115 and three specimens of pectinid meter equipped with seven Faraday bivalves from Pedreira do Campo in collectors and one Daly detector in order to determine the ratio of the Department of Geological 87Sr/86Sr of the low-magnesium cal- Sciences at the University of Florida. cite composing the shell (Table 1). Sr was loaded onto oxidized tungsten These data allow us to estimate the single filaments and run in triple col- geologic age for each fossil specimen. lector dynamic mode. Data were For isotopic analyses, we first ground acquired at a beam intensity of about off a portion of the surface layer of 1.5 V for 88Sr, with corrections for each shell specimen to reduce possi- instrumental discrimination made ble contamination. Areas showing assuming 86Sr/88Sr=0.1194. Errors in chalkiness or other signs of diagenet- measured 87Sr/86Sr are better than ic alteration were avoided. Powdered ±0.00002 (2 sigma), based on long- low-magnesium calcite samples were term reproducibility of NBS 987 drilled from the interior of each shell (87Sr/86Sr=0.71024). Age estimates using a hand-held Dremel tool with a were determined using the Miocene carbide burr. Approximately 0.01 to portion of Look-Up Table Version 0.03 g of powder was recovered from 4:08/03 associated with the strontium each fossil sample. The powdered isotopic age model of McArthur et al. calcite samples were dissolved in 100 (2001).

µl of 3.5 N HNO3 and then loaded onto cation exchange columns packed with strontium-selective RESULTS crown ether resin (Eichrom Technologies, Inc.) to separate Sr Sedimentary Facies Analysis at Pedra- from other ions (Pin and Bassin, que-Pica 1992). Sr isotope analyses were per- Pedra-que-Pica contains an expo- formed on a Micromass Sector 54 sure of 7.5 to 40.0 m of sediments that Thermal Ionization Mass Spectro- lie between two basalt flows. We

TABLE 1. Strontium isotope data and age estimates from Santa Maria Island, Azores.

Sample Locality 87Sr/86Sr error (%) Age (Ma)* Age range (Ma)*

PC1 A Pedreira do Campo 0.709064 0.0010 2.78 3.88-2.36 PC1 B Pedreira do Campo 0.709078 0.0010 2.24 2.58-1.89 PC1 C Pedreira do Campo 0.708885 0.0010 10.03 10.34-9.75 PP1 I Pedra-que-Pica 0.709027 0.0010 5.28 5.56-4.96 PP1 II Pedra-que-Pica 0.709012 0.0008 5.67 5.82-5.49 PP1 III Pedra-que-Pica 0.709016 0.0012 5.59 5.82-5.25

87Sr/86Sr relative to NIST987 = 0.710248 * Ages from look-up tables in McArthur et al. (2001) 116 AÇOREANA 2007, Supl. 5: 112-125 interpret the sediments and lower discordant. Rare basalt clasts up to 30 basalt flow as part of the Touril cm in size are present in the lithic Complex as defined by Serralheiro et wacke. A boulder conglomerate con- al. (1987) (Fig. 2). We interpret the taining basalt clasts overlies the lithic upper basalt flow as part of the over- wacke at this location, as illustrated lying Facho-Pico Alto Complex as in Figure 2. The contact between defined by Serralheiro et al. (1987). these two units is erosive and the The bottom of the Touril Complex boulder conglomerate clearly infills a was not seen at this locality as a result fluvial channel. About 50 m to the of being below sea level. The top of east beyond the channel, however, the Touril Complex was seen in con- the lithic wacke is 32.5 m thick and is tact with the overlying basalt of the overlain by a basalt flow. Here, the Facho-Pico Alto Complex. The top of lithic wacke grades upsection into an the Facho-Pico Alto Complex was not immature sandstone with less dis- seen. Sediments of the Touril tinct bedding and more basalt clasts. Complex at this locality are divided There is relief along the contact into four distinct facies. The base of between the lithic wacke and the the section is marked by brecciated overlying basalt flow of the Facho- basalt pillows that are overlain by a Pico Alto Complex. fine-grained, lithic calcarenite show- We infer from the sedimentologic ing abundant bioturbation. Two sizes and fossil evidence that the deposits of Thalassinoides sp. are present: One at Pedra-que-Pica represent a regres- size that is about 1 cm in diameter sive sequence of transition zone to and another that is about 3 mm in foreshore environments. The under- diameter. Both types of burrows are lying pillow basalt indicates subma- vertical to subvertical and show rine volcanism. The cracked and branching. This facies is overlain by weathered nature of the top of the pil- 1.5 to 3 m of coquina that is rich in low basalt indicates an interval of large, disarticulated valves of exposure before deposition of the spondylids, pectinids, and pycn- overlying lithic calcarenite. This odontids, as well as in barnacles, lower sandstone formed in transi- echinoids, bryozoans, calcareous tion-zone or lower shoreface environ- algae, and coral. The coquina is struc- ments, based on the abundant biotur- tureless, except for some cross-lami- bation and grain size. The overlying nation (1 cm) near the top of the coquina represents one or more coquina. Overlying the coquina is a storm-lag deposits, where storms thick unit of fine- to medium-grained, have winnowed out most of the sand, lithic wacke that is 4.0 to 32.5 m thick. thereby leaving behind the larger The lithic wacke is well-stratified at shells. Most of the bivalves that we the base and contains planar bedding observed were concave down, sug- about 1 cm thick. There are internal gesting winnowing. The overlying erosive surfaces within the lithic well-stratified sandstone most likely wacke where the planar bedding is formed in a foreshore to upper KIRBY ET AL: NEOGENE OF SANTA MARIA ISLAND 117

FIGURE 2. Stratigraphic section at Pedra-que-Pica, Santa Maria Island, Azores (N36°55.806’, W25°01.482’). 118 AÇOREANA 2007, Supl. 5: 112-125 shoreface environment, based on the conglomerate infilling the fluvial planar bedding, erosive surfaces, and channel represents Quaternary allu- general lack of bioturbation, which vium. are all indicative of modern foreshore to upper shoreface environments Sedimentary Facies Analysis at Pedreira (Reineck and Singh, 1975). The over- do Campo lying basalt of the Facho-Pico Alto Pedreira do Campo contains an Complex represents a return to active exposure of 5 m of sediments that are volcanism in this area. The boulder within the Touril Complex of

FIGURE 3. Stratigraphic section at Pedreira do Campo, Santa Maria Island, Azores (N36°56.818’, W25°08.119’). KIRBY ET AL: NEOGENE OF SANTA MARIA ISLAND 119

Serralheiro et al. (1987) (Fig. 3). The The top of the Facho Complex was bottom of the Touril Complex was not seen. not seen, but the top was seen. We infer from the sedimentolog- These sediments can be divided ic and fossil evidence that the into two facies. The base of the sec- deposits at Pedreira do Campo rep- tion is marked by bioclastic lime- resent a regressive sequence of stone (calcirudite/skeletal grain- open-ocean environments grading stone) that is rich in large benthic upsection into a shallower fore- foraminifera, bryozoans, gas- shore or shoreface environment tropods, bivalves, and rhodoliths of with subaqueous volcanism. The calcareous algae. The base of the limestone most likely formed on a limestone is not exposed. Overlying shallow bank or shoal within the the limestone is a well-stratified photic zone, based on the photosyn- lithic arenite that is 4.5 m thick, thetic organisms (rhodophytes and which is overlain by about 20 m of benthic foraminifera) and sedimen- pillow-basalt flows of the Facho tology. The absence of fine-grained Complex of Serralheiro et al. (1987). terrigenous material suggests that

FIGURE 4. Scatter plot showing estimated geologic ages of Pedra-que-Pica and Pedreira do Campo as derived from measurements of the 87Sr/86Sr in fossil mollusc shells from these localities. 120 AÇOREANA 2007, Supl. 5: 112-125 this bank or shoal was far from any the north Atlantic. Both localities subaerial sources of terrigenous studied in this report contain sediment. The sandstone probably deposits that formed in shallow- formed in a foreshore or shoreface marine environments, with Pedra- environment, based on the planar que-Pica representing a regressive bedding. The tuffaceous nature of sequence from transition-zone to the sandstone indicates that ash foreshore environments, and falls were a common occurrence Pedreira do Campo representing a from one or more nearby volcanoes. regressive sequence from shallow 87Sr/86Sr Chronostratigraphy bank to shoreface-foreshore environ- Strontium isotopic results from ments. Although both localities con- Pedra-que-Pica and Pedreira do tain regressive sequences, they differ Campo indicate that these localities in their sedimentology. Pedra-que- contain fossils that range from late Pica contains abundant terrigenous Miocene to late Pliocene in age (10.0 sediment that is clearly derived from to 2.2 Ma) (Fig. 4, Table 1). The three volcanic and bioclastic sources. fossil shells collected from the Pedreira do Campo, on the other coquina at Pedra-que-Pica have an hand, contains limestone with very average 87Sr/86Sr composition of little terrigenous input, but with 0.709018±0.000008 that represents an abundant photosynthetic organisms average estimated age of 5.51±0.21 (large benthic foraminifera and Ma, which is Messinian age in the late rhodophytes). We infer from this pat- Miocene. The three fossil shells col- tern that Pedra-que-Pica was very lected from the limestone at Pedreira near emergent land, whereas do Campo show a wide range in Pedreira do Campo was not as close 87Sr/86Sr ratios, from 0.708885 to to emergent land, but was instead on 0.709078, which represent estimated a shallow bank or shoal either isolat- ages from 10.03 to 2.24 Ma, respec- ed or on the windward edge of Santa tively (Fig. 4). Maria Island. If the former was the case, then the sediments at Pedreira do Campo may record the complete DISCUSSION AND CONCLUSIONS submergence of Santa Maria Island between eruptive phases, with the The fossils of Santa Maria Island last eruptive phase re-establishing an in the Azores archipelago represent emergent island in the Pliocene (rep- one of the few shallow-marine com- resented by the overlying Facho-Alto munities of late Neogene age Pico Complex). between Europe and North America The estimated geologic ages of in the north Atlantic Ocean. The rari- these two localities are different as ty of similar localities makes our well (Fig. 4). Pedra-que-Pica is latest results important in the consideration Miocene in age, whereas Pedreira do of the evolutionary and biogeograph- Campo is either as old as late ic history of shallow-marine faunas in Miocene or as young as late Pliocene. KIRBY ET AL: NEOGENE OF SANTA MARIA ISLAND 121

The Sr ratios determined from the thermal activity associated with near- three specimens collected from the by submarine volcanism altered the coquina at Pedra-que-Pica form a ambient Sr ratio of the water for short tight cluster, giving an estimated age periods of time during formation of of 5.51±0.21 Ma. This estimated age is the limestone, such that some shells very interesting as it coincides with may have a lower Sr ratio than other the Messinian salinity crisis, when shells that reflect global Sr ratios the Mediterranean Sea is believed to more accurately. Sr ratios from mid- have desiccated several times during oceanic, ridge flows and pillow- the Messinian age (Adams et al., 1977; basalt fluids are comparatively lower Hsü et al., 1977; Krijgsman et al., than ratios from rivers bringing Sr to 1999). the ocean from eroded crustal (sialic) The Sr ratios from the three speci- rocks (Faure and Mensing, 2005). If mens collected from the limestone at seawater with lower Sr ratios sur- Pedreira do Campo, however, do not rounding the Azores from periods of form a tight cluster but are scattered, high submarine volcanism or active with two specimens clustering at vent flows was incorporated into about 2.5 Ma and the other specimen shell calcite by the molluscs, it could indicating a much lower Sr ratio and have lowered their ratios and made older age date of 10 Ma. There are them appear artificially older. three possible alternative explana- Further work is clearly needed in tions for the pattern observed in the order to test each alternative hypoth- specimens from Pedreira do Campo. esis in order to better understand the First, there may have been significant preliminary results from Pedreira do time-averaging at Pedreira do Campo. Campo. An older shell from a late Our results for Pedra-que-Pica are Miocene deposit may have been in general agreement with previous reworked into a younger death studies, but there are differences. assemblage during the late Pliocene Results are mostly congruent with at Pedreira do Campo. Second, it is previous work that inferred a late possible that some of the shells ana- Miocene to early Pliocene age for the lyzed may have undergone diagenet- sediments of the Touril Complex, ic alteration that may have altered the based on biostratigraphy (e.g. Krejci- ratio of 87Sr/86Sr. Future work will Graf et al., 1958; Zbyszewski and need to further exclude the possibili- Ferreira, 1962b). Our results are also ty of diagensis. Third, local volcanism congruent with the K/Ar radiometric may have affected the Sr ratio of the ages determined by Abdel-Monem et seawater locally where the molluscs al. (1975). They dated the basalts lived on the open bank. The three underlying the “coquina zone” (pre- shells from Pedreira do Campo are sumably Touril Complex of from limestone that is overlain by a Serralheiro and Madeira, 1993) as thick section of tuffaceous sandstone being 6-8 Ma or older, and the basalts and pillow basalt. Perhaps hydro- overlying the “coquina zone” as 122 AÇOREANA 2007, Supl. 5: 112-125 being 4 Ma or younger. These ages Pedreira do Campo in order to better for the underlying and overlying resolve its age; (2) Sr age dating of basalt flows bracket our average Sr samples from new exposures, such as age date of 5.51±0.21 Ma for the sedi- those at Ponta da Malbusca and Baía ments at Pedra-que-Pica. However, da Cré; (3) correlating the different our estimated age for the sediments exposures of fossiliferous sediments at Pedra-que-Pica are older than the around the island through Sr age dat- K/Ar age dates determined by ing and geologic field work; and (4) Feraud et al. (1980, 1981, 1984) for ultimately creating a Sr chronostrati- what they described as the “pillow graphic framework for Santa Maria complex interbedded with fossilifer- Island that will help to resolve the ous calcarenites,” which Serralheiro discrepancies between the previously and Madeira (1993) inferred as con- published age dates for the Touril taining the Touril Complex. Feraud et Complex. al. (1981, 1984) determined that these rocks are 3.8 to 3.3 Ma, which is 1.7 to 2.2 million years younger than our ACKNOWLEDGEMENTS average estimated age for the sedi- ments at Pedra-que-Pica. Finally, our We thank Mr. Pombo (Vila do results are in agreement with the geo- Porto) for information about the fos- logic review of Serralheiro and sils of Santa Maria. We also thank F. Madeira (1993), who inferred that the Cecca (University Paris VI) for dis- fossiliferous sediments in the Touril cussions and help in the field and Complex were Messinian to early Clube Naval de Santa Maria for pro- Pliocene in age, based on the previ- viding sea transportation to reach the ous studies and their own geologic locality at Pedra-que-Pica. MXK field work. Results from Pedreira do thanks A.M. de Frias Martins and Campo are not in agreement with Sérgio P. Ávila for inviting him to previous studies, which further indi- participate in the 1st Atlantic Islands cates that the initial results from Neogene International Congress, Pedreira do Campo need to be treat- June 2006. We are grateful to the ed with caution. University of the Azores for provid- These results and conclusions ing funding for this work, as well as help to place the shallow-marine to the University of Florida for fund- communities of Santa Maria Island ing the Sr analyses. We also acknowl- into a geologic context that will aid edge financial support from the our understanding of how these com- organizers of the 3rd Workshop munities relate to the broader evolu- “Palaeontology in Atlantic Islands” and tionary and biogeographic history of from FCT (Portuguese Science the Atlantic basin, as well as that of Foundation), SRAM (Secretaria the world, during the late Neogene. Regional do Ambiente e do Mar, Future work must include: (1) Sr age Governo Regional dos Açores), dating of additional samples from DRCT (Direcção Regional da Ciência KIRBY ET AL: NEOGENE OF SANTA MARIA ISLAND 123 e Tecnologia, Governo Regional dos Campo (Santa Maria, Açores): Açores) and Câmara Municipal de monumento natural. Ciencias da Vila do Porto. Terra, 5: I20-I23. S.P. Ávila was supported by grant COMPTON, R.R., 1985. Geology in the SFRH/BPD/22913/2005 (FCT - field, 398pp. John Wiley and Sons, Fundação para a Ciência e New York. 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