Stratigraphic implications of palaeomagnetic data from Honduras

Geophys. J . Int. (1992) 108, 855-864 Stratigraphic implications of palaeomagnetic data from Honduras W. A. Gose’ and R. C. Finch2 ’ Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712-7909, USA ’Department of Earth Sciences, Tennessee Technological University, Cookeville, TN 38505, USA Accepted 1991 September 19. Received 1991 August 1; in original form 1991 January 7 SUMMARY Key words: Honduras, palaeomagnetism, stratigraphy. 1 INTRODUCTION Geologic field mapping in Honduras faces four main difficulties: poor accessibility, limited outcrop areas, lack of marker beds, and scarcity of diagnostic fossils. Because of logistical problems, only the central part of the republic has been mapped in some detail, and our interpretations pertain only to this area. In this subtropical environment, fresh outcrops are largely restricted to river beds and usually are of limited stratigraphic extent. The paucity of good outcrops combined with the lack of marker bed makes it often impossible to correlate between isolated outcrops. The direction of the natural remanent magnetization (NRM) of rock samples can, under certain conditions, be used for stratigraphic correlations. Most magnetostratigraphic studies, such as the correlation of sedimentary cores from the ocean basins (e.g Kennett 1980) or the exact definition of the Cretaceous/Tertiary boundary (e.g. Alvarez et al. 1982), are based on the occurrence of reversals of the geomagnetic field and their correlation with the sea-floor magnetic anomaly time-scale. Variations in the direction of the NRM other than reversals form the basis of archaeomagnetism and have been used for the stratigraphic correlation of recent lake sediments (e.g. Creer & Tucholka 1982) as well as older rocks (e.g. GrommC, McKee & Blake 1972). Another means of using palaeomagnetism for correlation is to compare the magnetic pole position of an unknown rock unit with the pole positions obtained from an established stratigraphic column. This will not only yield a relative age, but also an absolute age with an accuracy which depends on the age resolution of the known apparent polar wander path as well as the associated errors in the palaeomagnetic data (e.g. Porath 1967; Kyle, Ulrich & Gose 1987). Palaeomagnetic results from the Mesozoic section of Honduras delineate an apparent polar wander path which is well suited for this approach. Critical to our work is the fact that some of the rock units had been dated either by fossils or radiometrically. These dated units form the ‘calibration points’ for the apparent polar wander path, from which the ages and stratigraphic positions of unknown rock units can be inferred by comparison. In this manner we were able to resolve several stratigraphic problems. 2 GENERAL STRATIGRAPHY OF CENTRAL H O N D U R A S The stratigraphic section in central Honduras consists of three major parts: a poorly known metamorphic basement of Palaeozoic or older age, a Mesozoic sequence of clastic and carbonate sedimentary rocks, and a Tertiary volcanic cover. The Mesozoic stratigraphy of central Honduras (Fig. 855 Palaeomagnetic results from mainly Cretaceous rocks in Honduras delineate a distinctive apparent polar wander path (APWP) which is well suited for magnetostratigraphy. T h e A P W P is based o n palaeomagnetic d a t a from palaeontologically dated units in the established stratigraphic column a n d three radiometrically dated intrusions. T h e ages and stratigraphic positions of sedimentary units whose stratigraphic assignments have been debated, can be inferred by comparing their palaeomagnetic pole postion with t h e known A P W P . T h e results demonstrate the usefulness of magnetostratigraphy for resolving stratigraphic problems in the absence of fossils o r marker beds, a n d for areas of discontinuous exposures. For regional interpretations, the most important conclusion is that strata in Honduras that have been assigned t o the Todos Santos Formation were not deposited on the same tectonic plate as the Todos Santos Formation of Guatemala and southern Mexico. T h e r e is n o evidence that t h e Honduran ‘Todos Santos’ strata are depositionally related t o t h e true Todos Santos Formation. T h e name ‘Todos Santos Formation’ should be abandoned in Honduras and for t h e entire Chortis Block. W . A . Cose and R. C. Finch 856 NAASTRICHTIAN VALLE DE ANGELES GROUP CANPANIAN SANTONIAN TURONIAN v) 2 s . - - - -3 CENOMANIAN J4ITIOUC f N U < ALBIAN APTIAN , ~ Y O J O A GRDlJP HALlTERlVlAN VALANGINIAN BERRIASIAN --7-7- EL P L A N I I u v) v) I P Figure 1. Stratigraphic column for central Honduras after Finch (1981) which was used during the palaeomagnetic sampling and the stratigraphic section proposed by Donelly et al. (1990). 1) has been reviewed and revised by Finch (1981). The Jurassic-Early Cretaceous clastic strata have been the subject of further revision by Ritchie & Finch (1985), Finch (1985) and Donelly et al. (1990). The oldest sedimentary unit in Honduras is the Honduras Group (Finch 1985; Ritchie & Finch 1985; Donelly et al. 1990). This group includes the El Plan Formation (Carpenter 1954), the ‘Agua Fria Formation’ (Ritchie & Finch 1985), and unnamed dark-coloured clastic strata that directly overlie metamorphic basement and underlie Cretaceous limestone in various localities throughout Honduras (e.g., the ‘basal siliciclastic unit’ of Simonson 1977). The El Plan Formation consists of dark grey shale and siltstone members interbedded with gray to buff sandstone members; minor conglomerate lenses and calcareous beds are present in both members (Carpenter 1954). Mills el al. (1967) and Mills & Hugh (1974) extended the use of the name El Plan throughout Honduras; however, the name should probably be restricted to its type area in the San Juancito mining district. The name ‘Agua Fria Formation’ (Ritchie & Finch 1985) is being used informally by geologists currently mapping in Honduras due to problems with the definition of the E l Plan Formation. ‘Agua Fria’ strata include thick sections of grey to green shale and siltstone, sandstone, abundant massive quartz pebble conglomerate units and minor coal-bearing strata. Jurassic ferns and cycads have been identified at five localities, and Bajocian to early Bathonian ammonites have I been identified at another five localities (Ritchie & Finch 1985; David. G. Taylor, personal communication, May, 1989). The Honduras Group also includes clastic strata referred to as the ‘Todos Santos Formation’. This name was introduced by Sapper (1894) to represent a red bed sequence exposed near the village of Todos Santos in the Altos Cuchumatanes in northwestern Guatemala (site G1 in Fig. 6). It overlies the Permian Ch6chal Formation and underlies the Ixcoy or Coban Formations, two carbonates considered to be time-equivalent with the Atima Formation of Honduras (McBirney 1963; Burkart 1965; Anderson et al. 1973). The uppermo (...truncated)