The systematic strAtigraphy of the Pacific littoral of Costa Rica has been
reorganized in three supergroups: l. Papagayo: mafic and ultramafic oceanic
basements and volcanically or tectonically associated sediments. 2. Garza:
oceanic sedimentary cover including pelagics (Sabana Grande Group) and slope
deposits (Sámara Group). 3. Mal País: neritic sedimentary cover, subdivided
into 5 epochs.
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A major, upper Santonian tectonic event divides the geologic history into
two phases: l. The Nicoya Complex with its Bathonian/Callovian - Santonian
(BAUHGARTNER 1984) oceanic sedimentary cover and the Santa Elena Peridotite
form together a nappe edifiee with a southern vergenee (BOUR􀂯'-et al. in
press). 2. Since the Campanian, this structure is part of the stable isthmic
crust, the substrate of the studied upper Senonian-Cenozoic sedimentary seque􀂰
ces, affected by open folding and normal faulting only. No accretionary struetures
relat, that latest Senoniao, have been observed. Oeean floor continued to form duriog
the latest Senonian-early Paleoeene in a primitive island are environment (Quepas,
Osa).
Campanian-middle Eocene pelagic (and background) sedimentation followed
the paleooceanographic trends observed in the Pacifico amplified by local
(equatorial. eastern ocean margin) high fertility. A shallow ccn ls refleeted
by Santonian-Iower Campanian and upper Paleocen-middle Eoeene radiolarian-rieh
siliceous deposits. In the late Campanian-Maestrichtian a drastic drop of the
ccn overruled subsidence and resulted in deposition of foraminifer-rich pelagic
limes tones. Thick Maestrichtian-Paleocene turbidite and mass flow sequences
were deposited in NW-SE trending elongate hasins and document the erosion of an
emerged andesitic volcanic are, the product of active subduction. surr?unded by
narrow carbonate and siliciciastic shelves. set aboye steep. unstable slopes
A tectonic event at che middle/late Eocene boundary caused mayor gravítative
slidíng and slumping of the upper part of che Dceanic sequen ce , subsequent uplift
and a regional, unconformable cnset of shallow carbonate deposition, while
turbiditc sedimentation persisted in sorne areas (Santa Elena, Quepas).
An interred thickened oceanic crust in isastatie equilibrium should form a sea
floor ae subphotic depths. However. tectonic uplift has repeatdly brought che
substrate to shallow depth and allowed che deposition of unconformable, chin,
shortlived, neritic sequences. formed duriog five succesive epochs:
Epoch l. Campanian-Maestrichtian: The structural1y highest parts of the upper
Santonian orogen were uplifted to subaeyial and shallow photic depths where rudistid
limes tones developed.
Epoch 2. Paleocene-early Eocene: Volcanic activity related to subduction gave
rise to an island arc surrounded by shallow platforms NE of the study area, only
knawn from reworked carbonate clasts in slope deposits (Samara, Quepas).
Epoch 3. Middle-late Eocene: Tectonic uplift brought most of the area to
shallow photic depth which led to the formation of the widespread larger foraminifer
- red algal limestones.
Epoch 4. Oligocene-early Miocene: Mast of the central area was emerged and
only near the trench (Nasara-Mal País) thin shallow clastic sequences developed,
while turbidite basins persisted in the NW and SE (Dept. Rivas, Nicaragua, Fila
Costeña).
Epoch 5. Late Miocene-Pleistocene: Littoral deposits are concentrated around
the south of Nicoya, in areas of recent uplift (FISCHER 1980).