PROBABLE VOLCANO-SEDIMENTARY COMPLEXES COMING OUT FROM INFORMAL LITHOSTRATIGRAPHIC UNITS OF SEVERAL MESOZOIC AGES OBSERVED IN CENTRAL NORTHERN SONORA , MEXICO

Stratigraphical field work in central Sonora, looking for lithodemes, has been done around the type areas of following informal stratigraphical units: a)Represo formation, of Aptian/Albian age b)Cucurpe formation, of Oxfordian age; c)Caracahui formation and d)Santa Rosa formation; both of Sinemurian age. Preliminary field observations permit us to arrive to following provable volcano/sedimentary complexes (PVSC): I. Santa Ana PVSC: In the Santa Ana region, east of the Sierra de Cucurpe and west of cerros La Pima crops out a very thick siltstone (reworked tuffs) and some other andesitic to basaltic volcanic material that interfinger and/or interstratified with calcareous sediments of the Represo formation, of Aptian/Albian age. II. La Colgada PVSC: In Sierra El Jucaral area, has been described and defined a thick volcanic and volcano-sedimentary arc magmatic sequence that began in early Jurassic and evolved until late Jurassic. One of the sedimentary intervals is the Cucurpe formation that in one way or in another it changes facies with the La Colgada Formation. The last one lithostratigraphical unit must to be changed to a lithodeme: a volcano-sedimentary complex. III. Benjamin Hill PVSC: This lithodeme crops out around Sierra de Caracahui where previous workers have misunderstood its stratigraphical relationships and so, they have asigned a wrong tertiary and quaternary age to Los Cochinitos and El Colorado volcanic sequences, the field observations have proved that both, the acidic and the intermediate volcanics are interstratified and interfingered with the Caracahui fm., which yield a Sinemurian age fauna. IV. Cerro Mayo PVSC: This complex crops out in Cerro Mayo area, located northward El Remolino and San Hipolito ranchs and El Aguila and Las Cruces ranchs to the south. The volcano-sedimentary sequence is mainly conformed by basaltic flows and carbonate sediments, which in one locality yielded a Sinemurian age. Perharps this PVSC is a faces change of the Caracahui and Santa Rosa enviroments, because they have fauna of the same age. V. San Antonio PVSC: Outcrops of this PVSC are located around the southern tip of Sierra de Santa Rosa and so it is associated, either to the Santa Rosa fm. of Sinemurian age, or with San Luis fm. of inferred Cretaceous age. the volcanic material is mainly basic (basalts) and the sediments are epiclastic and carbonated ones.