ABSTRACT Geologic mapping on the Chortís block has defined two major Mesozoic units which are older than the well-known, lower Cretaceous Yojoa Group. The two units have been considered part of the same stratigraphic element, the Honduras Group. The Agua Fría Formation, a dominantly clastic mixed continental-marine unit, has been dated as Middle Jurassic based on recent discoveries of ammonites and plant fossils. Dates as old as Rhaetian for similar rocks have been reported in the literature, but have not been confirmed. No unfaulted contact between the Agua Fría Formation and the Yojoa Group has been found. The Yojoa Group is commonly underlain by a conglomeratic unit, the unnamed siliciclastic member of the Honduras Group, that is lithologically distinct from the Agua Fría Formation. Available palynological data indicate that the unnamed siliciclastic member is restricted to the lower Cretaceous. The Honduras Group as currently defined is an inappropriate stratigraphic term. The two units of the group are not contiguous and have significant differences in lithology, and, especially, in age. Inasmuch as its usage is causing stratigraphic confusion. I recommend abandoning the term Honduras Group. The Agua Fría Formation is left as an independent stratigraphic element. The unnamed siliciclastic member is herein named “the Tepemechín Formation” after a river south of Lake Yojoa. Although thicknesses as great as 1200m have been reported for the Agua Fría Formation, it is limited to rather narrow basins probably of strike-slip origin. By contrast, the Tepemechín Formation is widespread yet thinner and was deposited immediately prior to the deposition of the Yojoa Group limestones. Sedimentologically, the formation is similar to known rift deposits. Rifting probably occurred in the earliest Cretaceous and was followed by subsidence represented by the limestone. Thus, the Chortís block experienced strike-slip faulting in the Middle Jurassic followed by rifting throughout the block in the Early Cretaceous whereas neighboring regions experienced “pure” rifting from the Middle Jurassic to the Cretaceous as evidenced by thick deposits of continental redbeds.

We document a previously unrecognized, thin-skinned arc-continental collisional zone, termed here the Colon fold-thrust belt, which trends northeastward for 350 km near the Honduras-Nicaragua border region. The Colon belt occurs in three collinear segments: (1) a 200-km-long belt of remote but well-exposed Jurassic–Late Cretaceous rock outcrops described from original geologic mapping presented in this study; (2) a 75-km-long subsurface belt of Jurassic–Late Cretaceous rocks known from onland seismic reflection studies and exploration drilling for oil; and (3) an offshore 75-km-long subsurface belt of Late Cretaceous to Eocene rocks known from exploration studies. These three segments share a continuity of the deformation front and associated folds, as well as a similar timing of fold-thrust deformation (segment one: post-Campanian; segment two: post–Late Cretaceous; segment three: post-Cretaceous and possible to Eocene); and all segments display southeastward-dipping thrusts and related northeastward-verging folds that structurally elevate Cretaceous rocks. The structural position of the Siuna belt of oceanic island arc affinity to the south of the Colon fold-thrust belt, its association with calc-alkaline volcanic rocks of the Caribbean arc, and its Campanian (75 Ma) emplacement age, suggest that the Siuna belt was overthrust to the north and northwest onto the hanging wall of the Colon fold-thrust belt. The northwestward-transported Colon fold-thrust belt and adjacent Siuna belt document a Late Cretaceous collisional event between a south-facing continental margin of the Chortis block of northern Central America and an eastward and north-eastward-moving, Early to Late Cretaceous Caribbean arc system.

Abstract The Chicontepec Formation in eastern central Mexico has long been explored for its oil accumulations and contains several large unconformities, which in the past seem to have been miscorrelated across the Tampico-Misantla Basin. The formation is of current interest because it may help us to understand the mechanisms of delivery of the Wilcox sands in the deep Gulf of Mexico. In the northern part of the Chicontepec play, outcrop data indicate that a major unconformity occurs at about 55 Ma ( i.e. , the Paleocene/Eocene boundary). Outcrops overlying this unconformity show a “classic” section about 150 meters thick; mass transport complexes on top of the unconformity are in turn overlain by sheet sandstones, channels, and channel levee complexes. In the central part of the play, there is a major unconformity in the Eocene, between 47 and 53 Ma (probably late early Eocene). In the southern part of the play (in the Agua Fria Field area) the first major erosional unconformity is between 43.6 and 50.4 Ma. In the extreme southern part of the play, in the Presidente Alemán area, the oldest unconformity is at approximately 40 Ma. The unconformities all have been generated in submarine environments. Without the benefit of seismic and high-resolution biostratigraphic data, earlier workers miscorrelated these unconformities and assumed that they were one large unconformity, forming the Chicontepec “paleocanyon.” All the unconformities are regional events and occur throughout the basin. The 43.6 to 50.4 Ma erosional event (unconformity A) is a correlatable seismic event across a large proportion of the Tampico-Misantla Basin. This event has been cored in at least two wells. In the closest well to the canyon, a 110 meter thick mass transport deposit immediately overlies the unconformity. The mass transport deposit consists of a basal 30-meter pebbly mudstone debrite, overlain by an 80 meter thick slump. Recent interpretation in the Agua Fria, Tajin and Coapechaca ( ATC ) fields, west of Poza Rica, Veracruz, has allowed more detailed mapping of the erosional unconformities and a better estimate of the volume of material removed at unconformity A. In this area, the feature is up to 6 km wide and, in some places more than 600 meters of consolidated Paleocene and Cretaceous strata have been removed. Production from the ATC fields west of Poza Rica, is from the infill of the “paleocanyon” (canyon-fill trap) and the pre-erosion strata (canyon truncation trap), and the trapping mechanism is both stratigraphic and diagenetic in nature.

In the Galisteo drainage basin south of Santa Fe, a fold and several faults related to the Rio Grande rift deform late Eocene–Oligocene dikes, laccoliths, and the Espinaso Formation. The largest rift-related feature, a northerly plunging syncline, comprises the south end of the Santa Fe embayment of the Española Basin and the northern end of the Estancia Basin. The east limb of the syncline is cut by northerly trending, graben-forming, normal faults of the Agua Fria fault system in the Santa Fe embayment. East of the Tijeras-Cañoncito fault system, the east limb of the Estancia Basin is disrupted by down-to-the-west, normal faults of the Glorieta Mesa boundary fault and the Hub Mesa fault system. The fold is offset by down-to-the-northwest movement, and a small component of right slip, on the Tijeras-Cañoncito fault system, which separates the two basins. The above-mentioned rift-related fold and north-trending faults are superimposed across the southeastern margin of the San Luis uplift and the younger Galisteo Basin. Geologic maps and drill data reveal four, and possibly five, phases of Laramide deformation associated with recurrent movement along the Tijeras-Cañoncito fault system: (1) a possible Late Cretaceous, cryptic phase of strike slip associated with elevation of the highest portions of the Santa Fe Range uplift to the north-northeast; (2) the early Paleocene San Luis uplift that formed a southwest plunging, V-shaped anticlinal nose whose southeast limb is the Lamy monocline, which extends 25 km southwest from Precambrian basement at the margin of the Santa Fe Range at Cañoncito to the Cretaceous Menefee Formation; (3) following erosional beveling, the collapse of the southern shoulder of the San Luis uplift, forming a portion of the north-northeast–trending, latest Paleocene–Eocene Diamond Tail subbasin, the axial portion of which lies along the trend of the Tijeras-Cañoncito fault zone; (4) minor Eocene uplift which interrupted deposition in the basin; and (5) Eocene subsidence across the broader Galisteo subbasin and deposition of the Galisteo Formation and latest Eocene–Oligocene Espinaso Formation. Late Eocene–Oligocene intrusions in Los Cerrillos and the Ortiz Mountains deformed the Cretaceous and Tertiary host rocks. Across the Tijeras-Cañoncito fault system, the northwest-trending erosional edge of the Campanian Point Lookout Sandstone displays 400 m of pre–Diamond Tail Formation, right-lateral separation, and the Diamond Tail Formation shows no lateral offset between the overlapping San Lazarus and Los Angeles faults. Although the axis of the Galisteo Basin parallels the fault system, and the basin has been proposed to have formed in a releasing bend of a strike-slip fault along the Tijeras-Cañoncito fault system, any major Laramide strike-slip movement pre-dates the deposition of the Diamond Tail Formation and the formation of the Lamy monocline. The faulted core of the pre–Diamond Tail Lamy monocline, initially up ~800 m on the northwest, was reactivated during rift development and downdropped on the northwest by ~250 m of dip slip. An earlier period of movement (either early Laramide or older strike slip or down-to-the southeast Pennsylvanian movement) is suggested by contrasting thicknesses of Paleozoic formations across the fault zone.

Abstract 0.0 Bell Ranch northeast of Mayer, Arizona. Proceed on dirt road down the Agua Fria River. 0.9 Road leaves valley bottom and climbs to the south through metasedimentary rocks. 1.3 Road from the Binghampton and Copper Queen mines joins this road on the left. 2.7 Crossing small cattleguard. 3.1 View in the distance at 9:00 to 10:00 shows drill pad used by Billiton to explore the Copper Mountain area. Drill pad is now used as a quarry for mining of hematite-stained metarhyolite. 3.7 Road makes sharp turns around small hills. Visible in the distance at 12:00 are high peaks in the southern Bradshaw Mountains near Horsethief Basin. 4.0 Road crests small ridge. High peak in the far distance at 12:00 to 12:15 is Brady Butte, elev. 6,400 ft (fig. 53). The peak and the ridge extending to the south are composed of the 1750±10 Ma Brady Butte Granodiorite, the oldest dated rock in Arizona (Anderson and others, 1971). The pluton intrudes metavolcanic rocks similar to those seen between Prescott and Mayer, and has been deformed and recrystallized during regional metamorphism (Blacet, 1966; DeWitt, in press; Blacet, 1985). Unconformably overlying the pluton is the Texas Gulch Formation (fig. 43), a sequence of metamorphosed volcaniclastic and sedimentary material that has been multiple deformed (O'Hara and others, 1978; Karlstrom and O'Hara, 1984; Karlstrom and Conway, 1986) at the northern margin of the pluton. Also visible from this location at about 12:00, directly over the town of Mayer and the smelter stack, are the

This study describes the geology of a well-exposed but previously unmapped section of Paleozoic–early Cenomanian metamorphic, sedimentary, and igneous rocks in the Frey Pedro study area of the Agalta Range of east-central Honduras. The objective of the study is to use these new structural, stratigraphic, biostratigraphic, and geochemical data to better constrain the geologic and tectonic history of this part of the Chortis block during the period of time from Aptian to early Cenomanian. The study revealed that the topographic Agalta Range exposes a thick stratigraphic section (3.5 km) deposited in an Albian-Aptian intra-arc rift and on the rift shoulders. This rift feature, named here the Agua Blanca rift, presently trends northwest and is parallel to three other belts of deformed Cretaceous rocks in Honduras (the Comayagua, Minas de Oro, and Montaña de la Flor belts) that also may correspond to Cretaceous intra-arc rifts produced during the same phase of intra-arc extension. These other three deformed belts are west of the Agalta Range and also form topographically elevated mountain ranges. Albian-Aptian calc-alkaline volcanic rocks and pyroclastic flows of the Manto Formation record arc affinity, while the volcaniclastic wedges of the Tayaco Formation record syn-rift deposition. These rift and arc-related units occupy the stratigraphic position between two major, extensive, shallow-water carbonate units, the lower and upper Atima Formations, also of middle Cretaceous age. Thickening of volcaniclastic rocks of the Tayaco Formation strata in the Agua Blanca rift accompanied erosion of the adjacent rift shoulders and eruption of Manto calc-alkaline volcanic rocks both within and adjacent to the Agua Blanca rift. The Agua Blanca intra-arc rift was inverted by a regional shortening event presumably in the Late Cretaceous. Rocks within the rift were intensely shortened, while rocks on the rift shoulders were shortened less because they are underlain by more competent metamorphic basement rocks. In order to better understand the Aptian–early Cenomanian tectonic setting for intra-arc rifting and subsequent rift inversion on the Chortis block, we reconstructed the Chortis block relative to the terranes studied by previous workers in southern Mexico. Five geologic and tectonic features were selected for realigning the two now widely separated areas: (1) areas of Precambrian basement outcrops, (2) areas of similar Mesozoic stratigraphy, (3) aligned trends of Mesozoic volcanic arc rocks exhibiting a similar arc geochemistry, (4) aligned trends of Late Cretaceous folds and thrusts, and (5) alignment of magnetic boundary.