SciELO - Scientific Electronic Library Online

 
vol.48 issue1Revision and update of the checklist of copepods (Crustacea: Hexanauplia) of the Colombian CaribbeanHydrocarbon contamination in mangrove sediments of the Mira river estuary, Colombian Pacific coast, affected by crude oil spills author indexsubject indexarticles search
Home Pagealphabetic serial listing  

Services on Demand

Journal

Article

Indicators

Related links

  • On index processCited by Google
  • Have no similar articlesSimilars in SciELO
  • On index processSimilars in Google

Share


Boletín de Investigaciones Marinas y Costeras - INVEMAR

Print version ISSN 0122-9761

Bol. Invest. Mar. Cost. vol.48 no.1 Santa Marta Jan./June 2019  Epub Sep 16, 2019

https://doi.org/10.25268/bimc.invemar.2019.48.1.762 

Research Articles

Sandstone reefs in the Gulf of Salamanca, continental shelf of the Colombian Caribbean

1 Universidad Nacional de Colombia - Sede Caribe, Instituto de Estudios en Ciencias del Mar-CECIMAR, c/o INVEMAR, Calle 25 2-55, Rodadero Sur, Playa Salguero, Santa Marta, Colombia. sezeas@unal.edu.co.

2 Universidad Nacional de Colombia - Sede Medellín, Departamento de Geociencias y Medio Ambiente, Facultad de Minas, Medellín, Colombia. gbernal@unal.edu.co, mweber@unal.edu.co.

3 Laboratorio de Datación por Luminiscencia, Centro Nacional de Investigación sobre la Evolución Humana, CENIEH, Burgos, España. lopezgi.phd@gmail.com, gloria.lopez@cenieh.es.

4 Grupo de Investigación en Ecología y Diversidad de Algas Marinas y Arrecifes Coralinos, Universidad del Magdalena, Santa Marta, Colombia. rgarciau@unimagdalena.edu.co.


ABSTRACT

In tropical seas there are submerged hard bottoms that harbor corals but that are not coralline in origin. This is the case for the "Banco de las Ánimas" sector in the continental shelf of the Gulf of Salamanca, Colombian Caribbean. In its upper portion (14-16 m in depth), there are low mounds of sandstone blocks and slabs, conforming reefs, colonized by coralline biota and sparse corals. To confirm their lithology an initial petrographic analysis was carried out, which showed the rocks are made up of fine-grained sands, mature in texture, cemented by dolomite. It is proposed that these reefs were formed in a beach-dune-lagoon system during an ancient sea level, similar to the recent coastal bar of Salamanca. In these high-evaporation, supratidal saline environments, they could have been formed as beach rocks or as eolianites, by aragonite cementation, modified later into dolomite. Whether the foundation of the deeper coral formations of the bank is also sandstone or in fact coralline, remains to be determined.

KEY WORDS: Beach rock; Sandstone reef; Sea level changes; Banco de las Ánimas; Southwestern Caribbean

RESUMEN

En los mares tropicales hay fondos duros sumergidos que albergan corales pero que no son de origen coralino. Este es el caso del sector del Banco de las Ánimas, en la plataforma continental del golfo de Salamanca, mar Caribe colombiano. En su parte superior (14-16 m de profundidad) hay montículos bajos de bloques y lajas de arenisca, a manera de arrecifes, colonizados por biota coralina y corales dispersos. Para confirmar su fitología, se realizó un primer análisis petrográfico, que mostró que las rocas están conformadas por arenas de grano muy fino, maduras en textura, cementadas por dolomita. Se propone que estos arrecifes fueron formados en un sistema playa-duna-laguna de un antiguo nivel del mar, similar a la actual barra costera de Salamanca. En estos ambientes supramareales salinos de alta evaporación, se pudieron formar como rocas de playa o como eolianitas, por cementación de aragonita, que fue convertida posteriormente en dolomita. Falta determinar si el basamento de las formaciones coralinas de mayor profundidad del Banco también es de arenisca, o es efectivamente coralino.

PALABRAS CLAVE: Rocas de playa; Arrecifes de arenisca; Cambios del nivel del mar; Banco de Las Ánimas; Caribe suroccidental

"Banco de las Ánimas" is a reef formation with incipient coral development, located on the Colombian Caribbean continental shelf, in the Gulf of Salamanca, off Ciénaga Grande de Santa Marta (Blanco et al., 1994; Díaz et al., 2000). The formation has about 8 km x 2 km in area, at a depth of 26-30 m, on the edge of the continental shelf. These authors considered it was formed less than 1000 years ago in the ancient delta of the Magdalena River, and that the hermatypic organisms and associated flora and fauna consolidated the sandstones as the coastline recessed to its current position (Blanco et al., 1994). Recent studies show that the upper outer shelf above the bank (<20 m) consists of bioturbated soft bottoms with interspersed hard bottom patches of coral rubble and sandstone slabs, colonized by reef biota (Jerez et al. 2017; Navas et al., 2017). With the aim of characterizing the hard formations and to determine their lithology, in September 2016 and May 2017, observations and collection of rocks and loose rubble were carried out by SCUBA diving between 14 and 16 m in depth, in Zone 1 of Jerez et al. (2017) (11° 02' 4.02"-12.768" N; 74° 24' 22.787"-26.037" W). Standard petrographic analyses of sample thin sections under transmitted light were undertaken, with mineralogical and textural descriptions and visual statistical quantification of minerals percent occurrence.

The studied reefs are formed by mounds of tens to hundreds of meters in extension, elevated up to 1-2 m above the flat sandy bottom (Figure 1). Detailed in situ observations and fragmentation of rocks with hammer and chisel showed that these mounds are not built by corals but formed almost exclusively of blocks and slabs of sandstone, colonized by a few corals and by abundant reef biota. The blocks are heavily eroded in a labyrinth fashion and often show recent bivalve perforations; the surfaces exposed to the water column are encrusted with crustose coralline algae and other fouling organisms, up to 1 cm in thickness, whereas the buried parts are partly clean. Loose slabs dominate de landscape, the smaller disseminated around the mounds by waves, despite being partially attached to each other by sponges and calcareous algae. There are also cylindrical and finger-shaped forms with a sandstone core (Figure 2). From a sample of 22 rubble loose pieces, 2 to 11 cm in the largest size, 50% were sandstone, 23% were mollusk shells, and 27% were rhodoliths; none were of coral origin. Massive corals were found, live and dead, up to 1-2 m in diameter, but rather low and quite sparse; when dead, they were easily distinguished by being convex, compared to the flat slabs and the irregular and labyrinthine blocks.

Figure 1 Underwater photographs of the sandstone reefs of Gulf of Salamanca. (A) Massive dead coral (convex, co) and slab (flat, sl). (B) Irregular block (bl), strongly eroded. The images on the right correspond to fragmentation carried out in-situ with hammer and chisel to determine internal composition (white for coral skeleton and overgrowth by crustose coralline algae, gray for sandstone). Notice the strong external fouling by algae and invertebrates (f), and the internal perforation by worms and bivalves (p). For scale use the metered rod in some of the photos. 

Figure 2 Cylindrical forms and rhodoliths in rubble among Gulf of Salamanca sandstones. (A) Inferior side of an irregular cylinder; this side, partially buried in the sandy substratum, has little fouling (for scale see the caliper). (B) Finger-shaped cylinder, reminiscent of a branching coral, but when broken it showed its sandstone core. (C) Rhodolith, made up of successive layers of crustose calcareous algae. 

From the petrographic analyses, it was found that rock samples are composed of very fine grain sand (grains of 100 μm in diameter, with micas up to 500 μm) cemented by dolomite, which gives them a crystalline appearance. The estimated clasts-cement proportion was 50-50% (Figure 3). The clasts were angular, some euhedral, with immature composition, which implies little transport. Their composition was quartz and potassium feldspar (60%), plagioclase (10%), amphiboles (10%), opaque (some were pyritoids) (8%), muscovite and biotite (5%), organic particles (4%), and other (fossils, clinozoisite, zircon, epidote) (3%). The biotites were not deformed, confirming the chemical cementation (Figure 3B). The cement composition was mainly polygonal crystals (95%), and rarely with a fibrous-radial habit around the nucleation points (5%).

Figure 3 Petrographic microscope photographs of thin sections of Gulf of Salamanca sandstones, showing in (A) the high selection, the clasts shape and the 50-50% clasts- 

Sandstone reefs have been described from different parts of the world such as Brazil (De Oliveira-Soares et al., 2016), USA (Garrison et al., 2016) and Greece (Moissette et al., 2013). The sandstones can either be beach rocks or eolianites (cemented sand dunes), in both cases formed in coastal supratidal environments. In recessing coasts, such as Salamanca, these formations are then submerged, and once within the continental shelf, they can outcrop as reefs, becoming substratum for coral and algal growth (Turner, 2005). Beach rocks and eolianites are formed in tropical and subtropical coasts, mainly microtidal, through the fast cementation of sediments (months to years, Turner, 2005), by interstitial deposition of carbonates (Vousdoukas et al., 2007; Erginal et al., 2012), in supratidal environments for beach rocks (Kelletat, 2006), and in inundation or burial ashore conditions for eolianites (Copper and Green, 2016). Their clasts can be terrigenous (quartz, chert, feldspar, dense minerals, volcanic material), biogenic (ooids, mollusk shells, skeletal debris) or even anthropogenic (ceramic, trash); they could be sands or massive sandy gravels (Vousdoukas et al., 2007). The calcium carbonate cement can precipitate by mixing of meteoric and marine waters, CO2 degassing in shallow underground water, sea water evaporation, or from microbial activity (Turner, 2005; Erginal et al., 2012). The cements can be calcitic or aragonitic, depending on temperature, salinity, pH and Mg abundance, aragonite being predominant in warm environments, which can later dolomitize (replacement of calcium by magnesium, and change of structure (Vousdoukas et al., 2007).

Several features of Salamanca sandstones, such as the high degree of selection and the fine clast size, are indicative of eolianites. The cylindrical shapes are reminiscent of crab supratidal burrows filled by wind-transported sand. The formational environment of Gulf of Salamanca sandstone reefs would be similar to that existing nowadays in the coastal bar of Salamanca, i.e., microtidal warm beach-dune-lagoon system of predominantly terrigenouos sediments, and with supratidal salt flats, in which the evaporation processes can produce cementation of the wind-deposited sand, with later dolomitization.

To understand the process of formation and evolution of these sandstone reefs, it would be necessary to carry out studies in the current bar of Salamanca, as well as in the Gulf reefs, to determine how beach rocks/ eolianites can be formed therein, the associated lithofacies to their formation, and their age. Additionally, the "Banco de las Ánimas" proper (26-30 m in depth) should be explored in detail to determine if its basement is coralline or sandstone. This knowledge would allow a more precise reconstruction of the relative sea-level changes in the Magdalena river delta.

ACKNOWLEDGMENTS

Field trips were made under the Project "Bajo de las Ánimas: a little known coral formation", funded to R.G.-U. by Universidad del Magdalena (Fonciencias VIN2016111). The Marine and Coastal Research Institute - INVEMAR is thanked for granting access to the information of the project "Viability of a project of coralline restoration in the Banco de las Ánimas, Magdalena Department", cooperation agreement Nr. 005 - 2016 PNSA-INVEMAR. The Faculty of Mines of Universidad Nacional de Colombia, Medellín Campus, is also thanked for the thin sections and the laboratory analyses. Contribution 478 of "Instituto de Estudios en Ciencias del Mar" - CECIMAR, Universidad Nacional de Colombia, Caribbean Campus.

REFERENCES

Blanco, J.A., J.M. Díaz, G. Ramírez y M.L Cortés. 1994. El Banco de las Ánimas: una amplia formación arrecifal desarrollada sobre un antiguo delta del río Magdalena. Bol. Ecotropica, 27: 10-18. [ Links ]

Cooper, J.A.G. and A.N. Green. 2016. Geomorphology and preservation potential of coastal and submerged aeolianite: Examples from KwaZulu - Natal, South Africa. Geomorphology, 271: 1-12. [ Links ]

De Oliveira-Soares, M., S. Rossi, F.A. Santos-Martins and P. Bastos de Macedo. 2016. The forgotten reefs: benthic assemblage coverage on a sandstone reef (Tropical South-western Atlantic). J. Mar. Biol. Ass. UK, 97(8): 1585-1592. [ Links ]

Díaz, J.M., L.M. Barrios, M.H. Cendales, J. Garzón-Ferreira, J. Geister, M. López-Victoria, G.H. Ospina, F. Parra-Velandia, J. Pinzón, B. Vargas-Ángel, F.A. Zapata y S. Zea. 2000. Áreas coralinas de Colombia. INVEMAR, Serie Publicaciones Especiales 5, Santa Marta. 176 p. [ Links ]

Erginal, A.E., N.G. Kiyak, M.Z. Ozturk, M. Avcioglu, M. Boczu and E. Yigitbas. 2012. Cementation characteristics and age of beachrocks in a fresh - water environment, Lake Iznik, NW Turkey. Sed. Geol., 243-244: 148-154. [ Links ]

Garrison, E.G., J.C. Hale, C.S. Cameron and E. Smith. 2016. The archeology, sedimentology and paleontology of Gray's Reef National Marine Sanctuary and nearby hard bottom reefs along the mid continental shelf of the Georgia Bight. J. Archaeol. Sci. Rep., 5: 240-262. [ Links ]

Jerez S., D., Morales-Giraldo, S. Millán, P. Quintero y C. Ricaurte. 2017. Anexo 1. Caracterización de los fondos marinos del Banco de las Ánimas, departamento del Magdalena. Informe Técnico Final Convenio de Cooperación No. 005-2016 PNSA-Invemar, Santa Marta. 43 p. [ Links ]

Kelletat, D. 2006. Beachrock as a sea level indicator? Remarks from a geomorphological point of view. J. Coast. Res., 22(6): 1558-1564. [ Links ]

Moissette, P., E. Koskeridou, J.J. Cornée and J.P. André. 2013. Fossil assemblages associated with submerged beachrock beds as indicators of environmental changes in terrigenous sediments: Examples from the Gelasian (Early Pleistocene) of Rhodes, Greece. Palaeogeogr., Palaeoclim., Palaeoecol., 369: 14-27. [ Links ]

Navas, R., A. Acosta, L. Sánchez, J. González, M. Ontiveros, J.A. Rodríguez-Rodríguez y P. Obando. 2017. Anexo 4. Caracterización del ecosistema asociado al sector de las Ánimas, departamento del Magdalena. Informe Técnico Final, Convenio de Cooperación No. 005-2016 PNSA-Invemar. 61 p. [ Links ]

Turner, R.J. 2005. Beachrock. 183-186. In: Schwartz, M.L. (Ed.). Encyclopedia of coastal science. Kluwer, Dordrecht, Netherlands. 1213 p. [ Links ]

Vousdoukas, M.I., A.F. Velegrakis and T.A. Plomaritis. 2007. Beachrock occurrence, characteristics, formation mechanisms and impacts. Earth - Sci. Rev., 85: 23-46. [ Links ]

This is an open Access article under the CC BY-NC-SA

Received: March 13, 2018; Accepted: September 04, 2018

Creative Commons License Este es un artículo publicado en acceso abierto bajo una licencia Creative Commons