First Circum-Pacific Record of Dimorphinites (Ammonoidea, Upper 
Bajocian) in the Precordillera of Northern Chile 

S.R. Femandez-Lopez1*, G.B. Chong Diaz2 

1. Departamento de Paleontologia, Facultad de Ciencias Geologicas and Instituto de Geologia Economica, 28040-Madrid, Spain 
(E-mail: sixto@geo.ucm.es) 

2. Departamento de Ciencias Geologicas, Universidad Catolica del Norte, Antofagasta, Chile (E-mail: gchong@ucn.cl) 

New ammonite fossil-assemblages containing 
morphoceratids have been discovered in the Quebrada 
San Pedro area, northern Chile, Region de Antofagasta, 
Comuna Sierra Gorda, approximately 40 km ESE of 
Sierra Gorda town and 75 km south of Calama city. The 
outcrop is located 1 km north of the composite section 
described as Quebrada de San Pedro and 10 km south of 
the classical localities of Caracoles Mining District 
previously studied by diverse authors (Gottsche, 1878; 
Steinmann, 1881; Moricke, 1894; Westermann, 1967, 
1981; Hi 11 ebrandt, 1970, 1973, 1977, 2001; Wester­
mann and Riccardi, 1979, 1980; Jensen and Quinzio, 
1979, 1981; Groschke and Hillebrandt, 1985; Hille­
brandt et aI., 1986; Groschke et aI., 1988; Riccardi et aI., 
1990a, b; Riccardi and Westermann, 1991a,b; Fernan­
dez-Lopez et aI., 1994; Groschke and Hi 11 ebrandt, 
1994). 

In the Caracoles-Quebrada de San Pedro area, 
overlying unconformably Triassic volcanic and meta­
morphic rocks, there is a succession of Jurassic, 
fossiliferous deposits belonging to the Caracoles Group 

(Harrington, 1961; Garcia, 1967; Montano, 1976; 
Marinovic and Garcia, 1999; Ardill et aI., 1998). The 
lowest marine deposits are brown, calcareous sand­
stones and fine-grained conglomerates belonging to the 
Coronado Formation. This lithostratigraphic unit is 
followed by yellow and brown shales with inter­
calations of limestones and calcareous sandstones, of 
the Torcazas Formation, developed in shallow marine 
settings. In overlying levels, there are yellow and green 
shales with intercalations of limestones, belonging to 
the Mina Chica Formation, developed in offshore 
settings. The lithostratigraphic position of the new 
ammonite fossil-assemblages is the top of the Torcazas 
Formation, below the Mina Chica Formation. In this 
locality, the last beds of the Torcazas Formation 
correspond to a centimetric interval of grey wackestone 
to packstone limestones containing resedimented 
specimens of Upper Bajocian ammonites, without 
traces of reelaboration. These ammonites have been 
collected by one of the authors (F.L.) during several 
field studies, between 2006 and 2010. 

Age Lithostratigraphy Column Lithology Setting 

Caracoles Formation 18 m 
Grey and blue limestones with Shallow 

~ intercalations of green shales marine 

u 
Mina Chica Formation 

-=4' 75 m Yellow and green shales with Offshore ·Cii intercalations of limestones 
VI --ca ... 
~ --, 

~ 
..2! Yellow and brown shales with Shallow "C 

Torcazas Formation 100 m intercalations of limestones "C marine 
i and calcareous sandstones 

------=t 

---t 
.:::.::.:::.::- :A Brown calcareous sandstones Shallow 

Corona do Formation 
............ 40 m :0 "' .......... 0.. ••• ".":. and fine-grained conglomerates marine 

.. ~: .. Gg: :~7-~": 
<Ii 

. . . -= . - Violet and red silicified volcanic rocks, ca Agua Dulce Formation . . 
Volcanic . t: ...:.. . ~ including lava flows, tuffs and breccias 

~ . . . 
FIg. 1 LIthostratIgraphy and hthology of study area 

Among the ammonoids of these fossil- assem­
blages, Sphaeroceratidae are dominant, being 
Megasphaeroceras [M+m] the most common am­
monite genus. Stephanoceratidae, Perisphinctidae and 
Oppeliidae are scarce, but Cadomites [M]-Polyplectites 
[m], Vermisphinctes [m ]-Prorsisphinctes [M] and Oxy­
cerites [M]-Paroecotraustes [m] are well represented. 
Morphoceratidae, Lissoceratidae and Strigoceratidae 
are very scarce, respectively represented by Dimor-

phinites [M]- Vigoriceras [m], Lissoceras [M]-Micro­
lissoceras [ m] and Strigoceras [M]. Besides Am­
monitina, Phylloceratina and Lytoceratina are 
extremely scarce, but they occur. Among the mor­
phoceratids of this stratigraphical interval, macro­
conchs of Dimorphinites dimorphus (D'Orbigny) and 
microconchs of Vigoriceras defrancii (D'Orbigny) 
have been identified. Since Dimorphinites dimorphus 
(D'Orbigny) [M+m] has only been known from the 



Parkinsoni Zone, Upper Bajocian, of Europe, North 
Africa and south part of the Transcaucasus (cf. Dietze 
and Bernt, 2009), this standard zone is now extended to 
the SE Pacific. Some associated taxa, such as 
Vermisphinctes [m]-Prorsisphinctes [M], Strigoceras 
[M] and Oxycerites [M]-Paroecotraustes [m] also 
indicate the uppermost Bajocian age and Tethyan 
affinities. 

Key words: Jurassic; Marine stratigraphy; Palaeo­
biogeography; South America 

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