Field Trip

Field trip leaders: José Maria Landim Dominguez (UFBA) & Ana Clara Coni e Melo (UFBA)

The GeoHab 2015 field trip (May 8th, 2015) will consist of a one day cruise at the Todos os Santos bay onboard of  23 meter schooner (Schooner resort). The objective of the field trip is to provide some relaxing moments to the delegates combined with a discussion about geology, geomorphology and evolution of the bay.

Todos os Santos bay (TSB) is the second largest bay in Brazil. The bay is a feature of erosional character, with sub-bays, several islands, hard bottoms and abrasion terraces. The reduced supply of fluvial sediments into the TSB is not yet sufficient to fill the bay. These characteristics, associated with almost oceanic  salinity and temperature conditions of its waters, are responsible for the relatively preserved scenic beauty of the bay, which is a major tourist destination in the country.

Todos os Santos Bay is set on the Recôncavo sedimentary basin, part of an aulacogen or aborted rift formed during the separation of South America and Africa. The beginning of the Recôncavo basin formation occurred approximately 145 Ma (lower Cretaceous). The basin filling period ended at the end of the Aptian (115 Ma). The Recôncavo rift exhibits the geometry of a half-graben, with the greatest subsidence recorded along the Salvador fault, which is its eastern boundary and is also where the basin depocenter is located, with sediment accumulation up to 8 km in thickness. Major lithology types outcropping along the TSB shoreline are: (i) crystalline basement rocks along the Salvador and Maragogipe faults, which delimit the basin to the east and to the west; (ii) Jurassic sandstones from the pre-rift phase, restricted to the western shore of the bay; and (iii) shales, siltstones, and sandstones of the Santo Amaro and Ilhas groups from the syn-rift evolution phase of the Recôncavo basin .

Figura 2aaa

Geology of Todos os Santos bay field trip itinerary. Bathymetry and the distribution of mangrove swamps and known coral reefs are also shown. Red broken line with yellow arrows show fieldtrip itinerary.

Geology of Todos os Santos bay field trip itinerary. Bathymetry and the distribution of mangrove swamps and known coral reefs are also shown. Red broken line with yellow areas show fieldtrip itinerary.

Interestingly, the Recôncavo basin is the only region in Brazil where sediments from the syn-rift phase of the continental margin evolution are exposed, mainly on the cliffs surrounding the TSB. In the other regions of the Brazilian continental margin, these sediments are buried by piles of marine sediments, several kilometers thick, deposited during the various evolution stages of the South Atlantic.

Figura 2b

Geological cross-section of the Recôncavo sedimentary basin

Todos os Santos Bay is the result of a combination of events dating from the separation of South America and Africa to the cooling of the planet during the Neogene. The lowering of the sea level since the middle Miocene, which was accentuated during the late Quaternary, was vital to the development of the TSB. This drop in sea level  favored the differential erosion between the Jurassic-Cretaceous sedimentary rocks of the Recôncavo basin and the crystalline rocks of the neighboring Precambrian basement, topographically lowering the landscape in the sedimentary areas. During the short periods of high sea level in the Quaternary, these lower regions were flooded, and a bay was present, such as exists currently. In this sense, the BTS is an ephemeral feature, with an existence restricted to the short-duration Quaternary interglacial periods.

ITINERARY

07: 30 – Meeting at the Pestana Hotel lobby and bus ride to the Marina

08 :00 – Welcome Reception at Marina and board the Schooner Resort

08 : 30 – Departure

08:30 – 12 : 00 – Sails along the Salvador fault scarp to the Porto da Barra beach. Sail back to the  Itapagipe peninsula, passing along the way the São Marcelo fortress, Salvador Port, Monte Serrat point and Tainheiros Cove.  From Tainheiros Cove we will sail westwards to Barra do Paraguassu locality, passing along the way the Maré and Frade islands.

12:00 – 14 : 30 – Stop at the berthing pier in Barra do Paraguassu for a refreshing bath. Upon return to the Schooner Resort a buffet lunch will be served onboard.

14:30 – 16 : 30 : Return back to Salvador.

17 : 00 – Arrival at Marina and return by bus to Pestana Hotel.

FEE: US$100,00. Price includes:

  • Water, soft drinks, canned beer (5 cans per passenger) and roskas (typical local drink, prepared with tropical fruits  and vodka)
  • Lunch on board (main course and desserts), snacks (4 varieties) and fruits .
  • Diving platform aft with bounded area and floaters
  • Freshwater shower
  • Auxiliary schooner boat to beach transfer
  • Field Guide.

Passengers should bring their own beach towels.



GEOHAB