Magnetostratigraphy and magnetic properties of the Jurassic to Lower Cretaceous Giron Group (northern Andes, Colombia)

We report paleomagnetic results from the Jurassic to Lower Cretaceous continental sedimentary succession exposed in the eastern limb of the Los Yariguies anticlinorium, Eastern Cordillera, Colombia. About 820 m of a stratigraphic section of the upper part of the Giron Group (Angostura del Rio Lebrija and Los Santos Formations) was sampled to construct a magnetic polarity stratigraphy. A total of 199 independent samples that yield interpretable and acceptable data have a characteristic remanent magnetization component (ChRM) isolated between 400 degrees C and 680 degrees C in progressive thermal demagnetization. Demagnetization behavior and rock magnetic properties are interpreted to indicate that hematite is the principal magnetization carrier with a possible contribution by magnetite in some parts of the section. After tilt correction, 123 samples are of normal polarity (declination [D] = 44.9 degrees, inclination [I] = +9.7 degrees, R= 110.64, k= 9.87, and alpha(95) = 4.3 degrees), and the other 76 accepted samples are of reverse polarity (D=216.4 degrees, l= -6.1 degrees, R= 68.29, k= 9.72, and alpha(95) = 5.5 degrees). The statistical reversal test conducted on virtual geomagnetic poles is positive (class B). Based on paleontologic age estimates for the Cumbre and Rosablanca Formations, we assume a Berriasian age for the Los Santos Formation. The magnetostratigraphic data from the Giron Group strata are interpreted to suggest an age for the sampled part of the section between early Kimmeridgian and early Valanginian (ca. 157-139 Ma). The age of the Angostura del Rio Lebrija Formation is estimated as between early Kimmerid-gian and early Tithonian (ca. 157-146.5 Ma). The age of the Los Santos Formation is estimated between early Tithonian and early Valanginian (146.5-139.3 Ma). With our proposed, but nonunique, correlation with the Geomagnetic Polarity Time Scale, the Jurassic-Cretaceous boundary is interpreted to be located within the Los Santos Formation. The Giron Group is characterized by two periods of high (>8 cm/k.y.) and two periods of low (< 2 cm/k.y.) sedimentation rates. An inferred clockwise rotation of similar to 44 degrees, based on paleomagnetic declination data from the Giron Group, is similar to rotation estimates reported in some previous studies in the general area, and this facet of deformation could be related to local and regional response to displacement along regional-scale strike-slip faults.