17MB PDF - Association for Mexican Cave Studies

112GEOLOGYPaleozoic noncaverniferous metamorphic rocks crop out throughoutthe southwestern half of Guerrero and cover nearly 50% of thestates of More1os and Guerrero. Farther north these metamorphicsare the basement rocks for later deposits. Cenozoic extrusivesand Tertiary pyroclastics and nonmarine conglomerate, limestone,and gypsum cover 35% more of the area. The thick, cave-formingCretaceous limestones crop out in a rectangular area from Chi1pancingoto south of Cuernavaca, totaling about 15% of the area ofthe two states (see map on page 114).These Cretaceous limestones have been divided into severalformations; only the two cavernous units will be describedo Bothare lower Cretaceous. The More1os Formation comprises the majorpart of the limestone and dolomite strata of the western part ofMore1os and the northern and central parts of Guerrero. It isprobably the best cave-forming limestone in the area of interest,but not enough field work has been done to verify this. Thestrata are generally uniformly bedded with medium to thick beds.The limestone texture is variable, but contains no intercalatedclay beds. White, cream, and gray are the most common colors ofthe limestone, and the dolomites are buff to brown or black. Thethickness of the More1os Formation is highly variab1e~ ranging froma little less than 900 meters in central More1os and ~OO metersnorth of Acuit1apan, to 400-500 meters around Igua1a. Severalhundred meters of the More1os Formation are also present aroundChi1pancingo.The Cuaut1a Formation, which is separated from the underlyingMore1os Formation, is commonly exposed between Cuernavaca andCuaut1a where it reaches a thickness of 750 meters. The lithologyis variable and is frequently difficult to distinguish from theMore1os Formation. It appears to represent a large marine bankoriented principally east-west. Upper Cretaceous rocks are notknown to be cavernous.Many periods of tectonic activity are recorded in the rocksof Guerrero. The first movements left their imprint on the Paleozoicmetamorphics. Following the deposition of the Cretaceoussediments was a period of broad uplift and block-faulting. Nextcame a period of important folding and faUlting attributed to theLaramide Revolution. North-south anticlinal mountain ranges andsynclinal valleys dominate the tectonic topography of northeasternGuerrero. Most of the anticlines are of Cretaceous rocks, and thevalleys are filled with post-Laramide Tertiary volcanics and nonmarinesediments.Few data have been gathered on spe1eogenesis in Guerrero andMore1os. Bretz (1955, p. 372, 373) describes his idea of cavernmakingin the immediate area of Cacahuami1pa and lists a fou~partphysiographic history of the region which includes:1) A tectonic topography produced by Laramide folding.2) An erosional topography developed on mountain structures.The range of limestone hills is notched, perforated, andetched from the tectonic topography. The early, largephreatic caves were developed at this time.3) Introduction of volcanic fills in the valleys.4) Cessation of volcanism and inauguration of the presentcycle of erosion, with stream valleys and barrancas eroded