PALAEOENVIRONMENT RECONSTRUCTION BASED ON ...provides a monospecific ostracod assemblage represented...
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1. INTRODUCTION The late Messinian Lago-Mare event, which spans from 5.50 to 5.33 Ma (Krijgsman et al., 1999, with refs.) is characterised by peculiar ostracod and molluscs assemblages, dominated by Paratethyan species that entered the Mediterranean realm due to the humid global climatic phase that affected this area (Griffin, 2002) while the western closure of the Mediterranean- Atlantic seaway was still active. Thus, after the Messinian salinity crisis the hypersaline Mediterranean waters were diluted to brackish conditions and the Paratethyan ostracods were enabled to spread into the whole Mediterranean area as documented by several papers (Gliozzi & Grossi, 2004 with refs.). Several studies (Gliozzi & Grossi, 2004; Grossi et al., 2008 with refs.) showed that under the term “lago-mare” or “caspibrackish” several salinity and depth fluctuations occurred. These variations were inferred mainly from geochemical and mineralogical studies and, on a less extant, on the ecological requirement of the ostracod species, since the autoecological data of the majority of the taxa characterising the lago-mare biofacies are unknown and only very few species collected in the late Messinian lago-mare assemblages are still living in the Paratethyan relict areas (Black Sea, Caspian Sea, Aral Sea) [i.e. Amnicythere multituberculata, Amnicythere palimpsesta and Amnicythere propinqua (= Amnicythere cymbula)]. Anyway, in recent years multivariate statistical analyses have been applied successfully to marine and non-marine ostracod assemblages to infer the palaeoenvironmental evolution (Gliozzi & Grossi, 2004 with refs.). The data set analysed by Grossi et al. (2008) lead to recognize seven different types of ostracod assemblages, each one characteristic of a different palaeoenvironment (fig. 6). 2. GEOLOGICAL SETTING AND STRATIGRAPHY The Malaga Basin is a marginal depression of the Alboran region (westernmost Mediterranean), about 100 km far from the Strait of Gibraltar (fig. 1). The Late Miocene outcrops are restricted to a narrow band running through Álora, Pizarra and SE of Cártama parallel to current Guadalhorce river course. Lithostratigraphical and palaeontological analyses have been carried out on the topmost Messinian LM unit, a new stratigraphic unit just below the marine deposits of the Lower Pliocene (Guerra-Merchán et al., 2000) (fig. 2), pertaining to the Loxocorniculina djafarovi Zone (last 60-80 ka of the Messinian) (Grossi et al., 2007). The LM unit is made of two fining-upward sequences separated by a discontinuity due to synsedimentary tectonic activity. Each sequence is composed of an alluvial conglomeratic lower member and a lutitic upper member. The LM unit has been sampled at two localities, respectively on the right (Outcrop 1) (fig. 3) and left (Outcrop 2) side of the Guadalmedina river. REFERENCES – Esu, 2007. Geobios 40, 291-302. Gliozzi & Grossi, 2004. Revista Espagnola de Micropaleontologia 36(1), 157-169. Gliozzi & Grossi, 2008. Palaeogeogr., Palaeoclimatol., Palaeoecol., 264, 288-295. Griffin, 2002. Palaeogeogr., Palaeoclimatol., Palaeoecol.182, 65-91. Grossi et al., 2007. E.O.M. VI, abstract book. Grossi & Gennari, 2008. Atti del Museo Civico di Storia Naturale supp. 53, 67-88. Grossi et al., 2008. Boll. Soc. Paleont. Ital. 47 (2), 131-146. Guerra-Merchán et al., 2000. Geotemas 2, 108-110. Krijgsman et al., 1999. Nature 400, 652-655. Laskar et al., 2004. Astronomy & Astrophysics 428, 261-285. Rosenfeld & Vesper, 1977. Aspects of ecology and zoogeography of recent and fossil ostracoda. The Hague, 55-66. PALAEOENVIRONMENT RECONSTRUCTION BASED ON MOLLUSC AND OSTRACOD ASSEMBLAGES OF THE MESSINIAN LAGO-MARE EVENT IN THE MALAGA BASIN (S SPAIN) 13° Congress R.C.M.N.S. – Regional Committee on Mediterranean Neogene Stratigraphy, Naples, 2-6 September 2009 Francesco GROSSI 1 , Elsa GLIOZZI 1 , Antonio GUERRA-MERCHÁN 2 , Francisco SERRANO 2 , Daniela ESU 3 , Serge GOFAS 4 In particular, three alternating assemblages are recognizable: “Tyrrhenocythere assemblage”, “Cyprideis-Loxoconchidae-Tyrrhenocythere assemblage” and “Cyprideis-Loxoconchidae assemblage” (Grossi et al., 2008). These assemblages point to a relatively shallow (few tens of meters) palaeoenvironment, whose water-mass underwent salinity oscillations from fresh-water to low mesohaline. Just one sample (MLG6) provides a monospecific ostracod assemblage represented by Cyprideis anlavauxensis, pointing to a shallowing pulse. Planktonic and benthonic foraminifers are sporadic and poorly preserved, of Cretaceous and Cenozoic ages. Their concomitance with abundant and well preserved brackish mollusc and ostracod faunas suggests their reworking. The upper sequence (samples MLG32-MLG48) is marked by the lack of molluscs and by a change in the ostracod communities, that become oligotypic (3-4 species). Assemblages are dominated by Cyprideis agrigentina, alternatively accompanied by T. pontica and Loxoconchidae (L. djafarovi, Loxoconcha mülleri and L. kochi) or only by Loxoconchidae. Near the top of the upper sequence a short interval (samples MLG44-MLG47) displays more diversified ostracod faunas and L. djafarovi become again dominant. From a palaeoenvironmental point of view, the upper sequence seems to be still characterised by a shallow waterbody with small salinity oscillations between oligo- and mesohaline range. Marine foraminifers are very rare and present only in some samples, but it is necessary a more detailed search for determine their autochthonous or reworked nature. Outcrop 2 In the pelitic portion of the upper sequence (samples MLG49-MLG68), molluscs are absent and ostracods are rather abundant and well preserved, but two barren intervals are recognizable: the first at the base of the sequence (samples MLG49-MLG51) the second near the upper part (samples MLG61-MLG64). Assemblages are mainly monotipic, made of abundant Cyprideis agrigentina. Foraminifers are present only in one sample (MLG55) and they are very rare and poorly preserved. Cyprideis is an euryhaline genus, ranging from freshwater to hyperhaline waterbody. Thus, in order to depict palaeosalinity, it was performed a detailed analysis on the Cyprideis agrigentina specimens, following Rosenfeld & Vesper (1977) which demonstrated the correlation between the normal sieve-pore canals shape and the water salinity in the genus Cyprideis. The SEM analyses on the sieve-pore canals shapes of C. agrigentina were performed on MLG52 specimens. On the whole, 50 sieve-pore canals were counted: 10 have an irregular or elongated shape (20%) and 40 were rounded (80%) (fig. 4k). According to Rosenfeld & Vesper (1977), the strong dominance of rounded sieve-pore canals seems to confirm a brackish environment. Near the top, a short interval (MLG65-MLG68) records the presence of oligotypic freshwater ostracod assemblages characterised by Ilyocypris, Potamocypris and Candoninae. 1 Dip. Scienze Geologiche, Università degli Studi Roma Tre, Largo. S. Leonardo Murialdo, 1, 00146-Roma, Italy; [email protected] 2 Dept. Ecología y Geología, Facultad de Ciencias, Universidad de Málaga, 29071-Malaga, Spain 3 Dip. Scienze della Terra, Università “La Sapienza”, P. le A. Moro, 5, 00185-Roma, Italy 4 Dept. Biología animal, Facultad de Ciencias, Universidad de Málaga, 29071-Malaga, Spain Fig. 4 – Selected ostracod species from the Malaga Basin: a. Loxoconcha eichwaldi; b. Zalanyiella venusta; c. Loxocorniculina djafarovi; d. Amnicythere sp.2; e. Loxoconcha rhombovalis; f. Camptocypria sp.1; g. Cypria sp.; h. Loxoconcha mülleri; i. Amnicythere propinqua; j. Cyprideis agrigentina; k. C. agrigentina sieve-pore canals (a-j: bar scale is 100 micron; k: bar scale is 5 micron) Fig. 6 – Taxonomic composition and palaeoenvironmental meaning of Lago-Mare ostracod assemblages (Grossi et al., 2008) Fig. 5 – Selected mollusc species from the Malaga Basin: A. Theodoxus mutinensis (7.0 mm); B. Theodoxus sp. (10.2 mm); C. Melanopsis narzolina (height 22.0 mm); D. Saccoia sp. 1 (5.3 mm). E. Saccoia cf. S. congermana (5.0 mm, level 4); F. Melanoides curvicosta (height 18 mm); G. a group of Dreissena, in life position within a large (38 mm) left valve of Pontalmyra sp. 1; H. Dreissena ex gr. D. rostriformis (15 mm, level 4; I. Pontalmyra partschi (12.2 mm); J. Pontalmyra sp. 2 (21 mm); K. Pachydacna (Parapachydacna) sp. (12 mm); L. Prosodacnomya sp. (12.5 mm, level 6); M. Lymnocardiinae gen. sp., right minute species (1.7 mm); N. Lymnocardiinae gen. sp., left valve of a juvenile (1.7 mm). Fig. 1 – Location of the studied area. A: The region of Malaga within the Betic Cordillera. B: Geological scheme of the Malaga Basin. C: Synthetic lithological column. 1: Internal Zone; 2: Flysch units of the Campo de Gibraltar Complex and Oligocene-Early Miocene transgressive deposits; 3: Tortonian; 4: Early Pliocene; 5: Pleistocene; 6: Holocene fluvial deposits. 4. CONCLUSION On the whole, the molluscs and ostracod assemblages record a very shallow brackish waterbody with salinity fluctuations between freshwater and mesohaline range that, towards the top, become decisely freshwater. A general reduction in salinity near the Messinian/Zanclean boundary is recorded also in central and eastern Mediterranean Lago-Mare successions (Grossi et al., 2008). Fig. 3 - The Lago-Mare deposits in the studied area. A: Geological cross-section across the outcrop. B: Panorama showing the stratigraphic relationships between the geological units recognized in the area. C: Normal faults sealed by the Pliocene Pl 1 unit. D: Panorama showing the passage between the upper sequence of the LM unit and the Pl 1 unit. C 1-2 : Conglomeratic members of the lower- upper sequences; C 3 : Conglomeratic accumulations between the upper sequence and the Pl 1 unit; L 1-2 : Lutitic members of the lower-upper sequences. Fig. 2 - Geological scheme (A) and synthetic lithological column of the LM unit (B). 1: Substratum; 2: LM unit (latest Messinian); 3: Pl 1 unit (earliest Zanclean marine deposits); 4: Pl 2 unit (Early Zanclean marine deposits); 5: Quaternary alluvial deposits. MLG1 MLG31 MLG32 MLG48 3. PALAEONTOLOGICAL DATA AND PALAEOENVIRONMENTAL INTERPRETATION Outcrop 1 The pelitic portion of the basal sequence was sampled in detail (samples MLG1-MLG31). The palaeontological analyses have shown the presence of very well preserved mollusc assemblages only in the lower portion (samples MLG1-MLG19); they include bivalves of Paratethyan origin (Lymnocardiinae and Dreissenidae) and Mediterranean gastropods (Neritidae, Hydrobiidae, Thiaridae, Melanopsidae), typical of the Lago-Mare biofacies (Esu, 2007), (fig. 5) pointing to oligo- low mesohaline shallow water environment subject to fluviatile freshwater input. On the contrary, ostracods are abundant in all the samples (fig. 4); assemblages are made of species of Paratethyan tradition, dominated by Loxoconchidae (Loxocorniculina djafarovi and Loxoconcha eichwaldi) and Tyrrhenocythere pontica with accompanying Candoninae, Leptocytheridae and Cyprideis spp. (C. anlavauxensis and subordinated C. agrigentina).
Transcript of PALAEOENVIRONMENT RECONSTRUCTION BASED ON ...provides a monospecific ostracod assemblage represented...
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1. INTRODUCTION
The late Messinian Lago-Mare event, which spans from 5.50 to 5.33 Ma (Krijgsman et al., 1999, with refs.) is characterised by peculiar ostracod and molluscs assemblages, dominated by Paratethyan species that entered the Mediterranean realm due to the humid global climatic phase that affected this area (Griffin, 2002) while the western closure of the Mediterranean-Atlantic seaway was still active. Thus, after the Messinian salinity crisis the hypersaline Mediterranean waters were diluted to brackish conditions and the Paratethyan ostracods were enabled to spread into the whole Mediterranean area as documented by several papers (Gliozzi & Grossi, 2004 with refs.). Several studies (Gliozzi & Grossi, 2004; Grossi et al., 2008 with refs.) showed that under the term “lago-mare” or “caspibrackish” several salinity and depth fluctuations occurred. These variations were inferred mainly from geochemical and mineralogical studies and, on a less extant, on the ecological requirement of the ostracod species, since the autoecological data of the majority of the taxa characterising the lago-mare biofacies are unknown and only very few species collected in the late Messinian lago-mare assemblages are still living in the Paratethyan relict areas (Black Sea, Caspian Sea, Aral Sea) [i.e. Amnicythere multituberculata, Amnicythere palimpsesta and Amnicythere propinqua (= Amnicythere cymbula)]. Anyway, in recent years multivariate statistical analyses have been applied successfully to marine and non-marine ostracod assemblages to infer the palaeoenvironmental evolution (Gliozzi & Grossi, 2004 with refs.). The data set analysed by Grossi et al. (2008) lead to recognize seven different types of ostracod assemblages, each one characteristic of a different palaeoenvironment (fig. 6).
2. GEOLOGICAL SETTING AND STRATIGRAPHY
The Malaga Basin is a marginal depression of the Alboran region (westernmost Mediterranean), about 100 km far from the Strait of Gibraltar (fig. 1). The Late Miocene outcrops are restricted to a narrow band running through Álora, Pizarra and SE of Cártama parallel to current Guadalhorce river course. Lithostratigraphical and palaeontological analyses have been carried out on the topmost Messinian LM unit, a new stratigraphic unit just below the marine deposits of the Lower Pliocene (Guerra-Merchán et al., 2000) (fig. 2), pertaining to the Loxocorniculina djafarovi Zone (last 60-80 ka of the Messinian) (Grossi et al., 2007). The LM unit is made of two fining-upward sequences separated by a discontinuity due to synsedimentary tectonic activity. Each sequence is composed of an alluvial conglomeratic lower member and a lutitic upper member. The LM unit has been sampled at two localities, respectively on the right (Outcrop 1) (fig. 3) and left (Outcrop 2) side of the Guadalmedina river.
REFERENCES – Esu, 2007. Geobios 40, 291-302. Gliozzi & Grossi, 2004. Revista Espagnola de Micropaleontologia 36(1), 157-169. Gliozzi & Grossi, 2008. Palaeogeogr., Palaeoclimatol., Palaeoecol., 264, 288-295. Griffin, 2002. Palaeogeogr., Palaeoclimatol., Palaeoecol.182, 65-91. Grossi et al., 2007. E.O.M. VI, abstract book. Grossi & Gennari, 2008. Atti del Museo Civico di Storia Naturale supp. 53, 67-88. Grossi et al., 2008. Boll. Soc. Paleont. Ital. 47 (2), 131-146. Guerra-Merchán et al., 2000. Geotemas 2, 108-110. Krijgsman et al., 1999. Nature 400, 652-655. Laskar et al., 2004. Astronomy & Astrophysics 428, 261-285. Rosenfeld & Vesper, 1977. Aspects of ecology and zoogeography of recent and fossil ostracoda. The Hague, 55-66.
PALAEOENVIRONMENT RECONSTRUCTION BASED ON MOLLUSC AND OSTRACOD
ASSEMBLAGES OF THE MESSINIAN LAGO-MARE EVENT IN THE MALAGA BASIN (S SPAIN)
13° Congress R.C.M.N.S. – Regional Committee on Mediterranean Neogene Stratigraphy, Naples, 2-6 September 2009
Francesco GROSSI1, Elsa GLIOZZI1, Antonio GUERRA-MERCHÁN2, Francisco SERRANO2 , Daniela ESU3, Serge GOFAS4
In particular, three alternating assemblages are recognizable: “Tyrrhenocythere assemblage”, “Cyprideis-Loxoconchidae-Tyrrhenocythereassemblage” and “Cyprideis-Loxoconchidae assemblage” (Grossi et al., 2008). These assemblages point to a relatively shallow (few tens of meters) palaeoenvironment, whose water-mass underwent salinity oscillations from fresh-water to low mesohaline. Just one sample (MLG6) provides a monospecific ostracod assemblage represented by Cyprideis anlavauxensis, pointing to a shallowing pulse. Planktonic and benthonic foraminifers are sporadic and poorly preserved, of Cretaceous and Cenozoic ages. Their concomitance with abundant and well preserved brackish mollusc and ostracod faunas suggests their reworking.The upper sequence (samples MLG32-MLG48) is marked by the lack of molluscs and by a change in the ostracod communities, that become oligotypic (3-4 species). Assemblages are dominated by Cyprideis agrigentina, alternatively accompanied by T. pontica and Loxoconchidae (L. djafarovi, Loxoconcha mülleri and L. kochi) or only by Loxoconchidae. Near the top of the upper sequence a short interval (samples MLG44-MLG47) displays more diversified ostracod faunas and L. djafarovi become again dominant. From a palaeoenvironmental point of view, the upper sequence seems to be still characterised by a shallow waterbody with small salinity oscillations between oligo- and mesohaline range. Marine foraminifers are very rare and present only in some samples, but it is necessary a more detailed search for determine their autochthonous or reworked nature.
Outcrop 2In the pelitic portion of the upper sequence (samples MLG49-MLG68), molluscs are absent and ostracods are rather abundant and wellpreserved, but two barren intervals are recognizable: the first at the base of the sequence (samples MLG49-MLG51) the second near theupper part (samples MLG61-MLG64). Assemblages are mainly monotipic, made of abundant Cyprideis agrigentina. Foraminifers are presentonly in one sample (MLG55) and they are very rare and poorly preserved.Cyprideis is an euryhaline genus, ranging from freshwater to hyperhaline waterbody. Thus, in order to depict palaeosalinity, it wasperformed a detailed analysis on the Cyprideis agrigentina specimens, following Rosenfeld & Vesper (1977) which demonstrated thecorrelation between the normal sieve-pore canals shape and the water salinity in the genus Cyprideis.The SEM analyses on the sieve-pore canals shapes of C. agrigentina were performed on MLG52 specimens. On the whole, 50 sieve-porecanals were counted: 10 have an irregular or elongated shape (20%) and 40 were rounded (80%) (fig. 4k). According to Rosenfeld & Vesper(1977), the strong dominance of rounded sieve-pore canals seems to confirm a brackish environment.Near the top, a short interval (MLG65-MLG68) records the presence of oligotypic freshwater ostracod assemblages characterised byIlyocypris, Potamocypris and Candoninae.
1 Dip. Scienze Geologiche, Università degli Studi Roma Tre, Largo. S. Leonardo Murialdo, 1, 00146-Roma, Italy; [email protected] Dept. Ecología y Geología, Facultad de Ciencias, Universidad de Málaga, 29071-Malaga, Spain 3 Dip. Scienze della Terra, Università “La Sapienza”, P. le A. Moro, 5, 00185-Roma, Italy4 Dept. Biología animal, Facultad de Ciencias, Universidad de Málaga, 29071-Malaga, Spain
Fig. 4 – Selected ostracod species from the Malaga Basin: a. Loxoconcha eichwaldi; b. Zalanyiella venusta; c. Loxocorniculina djafarovi; d. Amnicythere sp.2;e. Loxoconcha rhombovalis; f. Camptocypria sp.1;g. Cypria sp.; h. Loxoconcha mülleri; i. Amnicythere propinqua; j. Cyprideis agrigentina; k. C. agrigentina sieve-pore canals (a-j: bar scale is 100 micron; k: bar scale is 5 micron)
Fig. 6 – Taxonomic composition and palaeoenvironmentalmeaning of Lago-Mare ostracod assemblages (Grossi et al., 2008)
Fig. 5 – Selected mollusc species from the Malaga Basin: A. Theodoxus mutinensis (7.0 mm); B. Theodoxus sp. (10.2 mm); C. Melanopsis narzolina(height 22.0 mm); D. Saccoia sp. 1 (5.3 mm). E. Saccoia cf. S. congermana (5.0 mm, level 4); F. Melanoides curvicosta (height 18 mm); G. a group of Dreissena, in life position within a large (38 mm) left valve of Pontalmyra sp. 1; H. Dreissena ex gr. D.rostriformis (15 mm, level 4; I. Pontalmyra partschi(12.2 mm); J. Pontalmyra sp. 2 (21 mm); K. Pachydacna (Parapachydacna) sp. (12 mm); L. Prosodacnomya sp. (12.5 mm, level 6); M. Lymnocardiinae gen. sp., right minute species (1.7 mm); N. Lymnocardiinae gen. sp., left valve of a juvenile (1.7 mm).
Fig. 1 – Location of the studied area. A: The region of Malaga within the Betic Cordillera. B: Geological scheme of the Malaga Basin. C: Synthetic lithological column. 1: Internal Zone; 2: Flysch units of the Campo de Gibraltar Complex and Oligocene-Early Miocene transgressive deposits; 3: Tortonian; 4: Early Pliocene; 5: Pleistocene; 6: Holocene fluvial deposits.
4. CONCLUSION
On the whole, the molluscs and ostracod assemblages record a very shallow brackish waterbody with salinity fluctuations betweenfreshwater and mesohaline range that, towards the top, become decisely freshwater. A general reduction in salinity near theMessinian/Zanclean boundary is recorded also in central and eastern Mediterranean Lago-Mare successions (Grossi et al., 2008).
Fig. 3 - The Lago-Mare deposits in the studied area. A: Geological cross-section across the outcrop. B: Panorama showing the stratigraphic relationships between the geological units recognized in the area. C: Normal faults sealed by the Pliocene Pl1 unit. D: Panorama showing the passage between the upper sequence of the LM unit and the Pl1 unit. C1-2: Conglomeratic members of the lower-upper sequences; C3: Conglomeratic accumulations between the upper sequence and the Pl1 unit; L1-2: Lutitic members of the lower-upper sequences.
Fig. 2 - Geological scheme (A) and synthetic lithological column of the LM unit (B). 1: Substratum; 2: LM unit (latest Messinian); 3: Pl1 unit (earliest Zanclean marine deposits); 4: Pl2 unit (Early Zanclean marine deposits); 5: Quaternary alluvial deposits.
MLG1
MLG31
MLG32
MLG48
3. PALAEONTOLOGICAL DATA AND PALAEOENVIRONMENTAL INTERPRETATION
Outcrop 1The pelitic portion of the basal sequence was sampled in detail (samples MLG1-MLG31). The palaeontological analyses have shown the presence of very well preserved mollusc assemblages only in the lower portion (samples MLG1-MLG19); they include bivalves of Paratethyan origin (Lymnocardiinae and Dreissenidae) and Mediterranean gastropods (Neritidae, Hydrobiidae, Thiaridae, Melanopsidae), typical of the Lago-Mare biofacies (Esu, 2007), (fig. 5) pointing to oligo- low mesohaline shallow water environment subject to fluviatile freshwater input. On the contrary, ostracods are abundant in all the samples (fig. 4); assemblages are made of species of Paratethyan tradition, dominated by Loxoconchidae (Loxocorniculina djafarovi and Loxoconcha eichwaldi) and Tyrrhenocythere pontica with accompanying Candoninae, Leptocytheridae and Cyprideis spp. (C. anlavauxensis and subordinated C. agrigentina).
