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Pete Emmet presented this talk at the Caribbean Workshop in Austin, TX, Sept. 20, 2002.
Click here to open a Powerpoint presentation of Pete's slides.

My involvement in geological and geophysical research in Honduras began with a masters thesis at The University of Texas at Austin (Emmet, 1983a) which was supervised by W.R. Muehlberger. Students from The University of Texas had been instrumental over a period of about 15 years (1968-1983) in performing some of the earliest detailed geological and geophysical studies in Honduras under Muehlberger’s supervision (Slide 1). Most of this work was performed in a relatively small area in central Honduras, and in retrospect it appears that the geographical focus of this early work led to an under-appreciation of the variablility of stratigraphic units and the diversity of structural styles in Honduras. More recent work, some of it being performed at Institute for Geophysics (UTIG) at The University of Texas at Austin, is beginning to clarify that record.

Through shear serendipity I found myself immersed in Honduras geology and geophysics again in 1985 and 1986 while working for Aero Service, a division of Western Geophysical. In this context I was the lead interpreter for a country-wide aeromagnetic survey of Honduras and I also contributed to an assessment of the hydrocarbon potential of the country performed by the Litton Resources Group (LRG; Aero Service, Western Geophysical and Core Lab) that utilized a comprehensive exploration data base that was shipped from Honduras to Houston for that purpose (DeBuyl and others, 1986). More recently I have been involved in efforts to ‘salvage’ this database, still in storage at WesternGeco in Houston until earlier this year, in collaboration with UTIG, the Dirección General de Energía (DGE) of Honduras and the Japanese Geological Institute (JGI). This effort has resulted in the preservation of a large amount of paper exploration data (reports, well logs, seismic profiles, aeromagnetic contours and depth-to-basement maps, etc) and digital field records for approximately 150 offshore seismic profiles (Slide 2). The ongoing study of these data will serve the parallel goals of stimulating the exploration effort for hydrocarbon resources in Honduras and identifying and publishing important details of the geology and geophysics of this critical and poorly-understood area in the Caribbean.

The first focus of my work in the current UTIG / DGE / JGI collaboration has been to study the offshore area immediately adjacent to eastern Honduras over which a seismic survey was shot by Texaco in the early 1980s. The onshore geology (Slide 3) is taken from a geological compilation that I made as part of the LRG hydrocarbon assessment study, and shows a Quaternary embayment (yellow) in the coastal areas and a Tertiary basin (gray) inland. Mesozoic sedimentary rocks (green), Paleozoic metamorphic rocks (blue) and intrusive and volcanic rocks (red and brown, respectively) comprise the other units. The age of the Tertiary basin (gray) is not known with certainty but is suspected to be Eocene. The location of an exposed thrust-fold belt in the Montañas de Colon is shown by the red thrust fault.

A shallow area extends seaward along the Nicaragua Rise and is known as the Gracias a Dios platform. To the south is an area of thicker Tertiary sediments known as the Mosquitia basin. To the north of the Gracias a Dios platform is a deep Cenozoic trough known as the Tela basin (Slide 4). The water bottom is nearly flat across Gracias a Dios platform and Mosquitia basin and does not exceed 200 m except to the north in the Tela basin. A 2D seismic survey was shot by Texaco in this area in the early 1980s. The locations of two key profiles, Lines 80-68 and 80-74, and a key exploration well, Main Cape-1, are shown in Slide 4. The seismic data were reprocessed in 1984 by Geophysical Services, Inc (GSI) and were interpreted by a consultant, Don Rockwell, who prepared a proprietary report that was illustrated by a set of interpreted seismic profiles and both two-way time and depth maps for three horizons: Top Eocene, Top Cretaceous and Top Yojoa (?) Limestone (Rockwell, 1985). The structural configuration at the Top Cretaceous horizon is illustrated in Slide 5. Contours are in depth. Major faults are shown in red. The map shows a structural nose that projects to the northeast offshore beneath the Gracias a Dios Platform. A short seismic line segment (Line 80-68) shows the deep structural configuration (Slide 6) beneath the platform where a profound angular unconformity (top Cretaceous; green horizon) overlies a fold-thrust structure that involves a high-velocity unit (blue horizon) suspected to be carbonate strata of the Lower Cretaceous Yojoa Group. This thrust-fold structure has the same northerly vergence and is believed to be analogous to the thrust-fold structures described by Rogers (1995) onshore in the Montañas de Colon (Slide 7). Note in the cross section that Cretaceous volcanic rocks (map unit Kv) are associated with the Cretaceous carbonate and siliciclastic rocks in the Montañas de Colon. Mafic volcanics are not common in the Mesozoic stratigraphy of central Honduras (Finch, 1981) but appear to be increasingly common to the east (Carpenter, 1954; Simonson, 1977; Emmet, 1983a; Rogers, 1995).

The Mosquitia basin is characterized by closely-spaced normal faults with displacement mainly down to the southeast as seen on the map (Slide 8) and on Texaco Line 80-74 (Slides 9, 10, 11). The seismic line shows that these faults cut a thick section of Eocene and younger strata, but most of these faults appear to sole-out at the top Cretaceous (green) horizon (Slides 9-11). The underlying Yojoa (?) carbonate (blue horizon) is seen only in the northern corner of Slide 9. Note that the top Eocene (pink) horizon at some locations is elevated above a regional marker (pink dashed line, Slides 11, 12, 13) suggesting that there was a period of regional extension in the Eocene (transtension?) followed by an episode of compression (transpression?) during the Neogene. Detailed mapping, including the preparation of time-thickness and isopach maps, would clarify the structural evolution of the Mosquitia basin and help to determine the timing and sense of shear, if any, that influenced the formation of these structures. Onshore studies along the Guayape fault zone in eastern Honduras have suggested a complicated shear history (Ritchie and Finch, 1984 and 1989; Gordon, 1987, 1990 and 1991; Finch and Ritchie, 1991).

The Main Cape-1 well was drilled on a regional arch (Slide 11) and yielded a significant test of light oil. The source rocks for the oil are believed to be within the high amplitude (dark) reflections in the lower part of the Eocene package (Slides 10 and 11) which are believed to correlate to the Mosquitia and/or Coco Marina Formations of Middle to Upper Eocene age shown in the stratigraphic column in Slide 12. Geochemical modeling performed recently by JGI has demonstrated that Eocene strata in the vicinity of the Main Cape-1 well are thermally mature for oil, and also demonstrate that the thermal maturation and source rock quality of Cretaceous strata penetrated by wells offshore Honduras are favorable for significant oil generation. The Main Cape structure cannot be shown to be a closed structure based only on the Texaco lines. Many older seismic lines of lesser quality exist in the surrounding areas but could be integrated into a more detailed study. These lines appear to be of acceptable quality for defining the major structural and stratigraphic elements of this offshore province.

The aeromagnetic survey of Honduras (Aero Service, 1985) included coverage of a large part of the offshore area in eastern Honduras (Slide 13). Note that this map is co-registered with the basemap (Slide 8) that shows the coastline and the location of key wells and seismic profiles. The original (black) contours on the aeromagnetic map (Slide 13) are at a 5 nT interval. Colored contours are at a 20 nT interval. The range of total magnetic intensity values shown in the colored contours is 220 nT, and the anomalies within this range of values include essentially all those that arise from structural relief (faulting) of more-or-less uniformly magnetic rocks (crystalline basement) against essentially non-magnetic sedimentary strata. The higher-amplitude anomalies (note that the full range of values on the map is in excess of 750 nT) arise from highly magnetic mafic igneous intrusions. Many of these igneous bodies were intruded within the basement complex in ancient times. In places where the basement subcrop is an erosional unconformity the lithologic contrast between these ancient intrusions and less magnetic metamorphic basement gives rise to magnetic anomalies whose depth can be calculated and which corresponds to the top of basement. In some cases in this data set igneous bodies can be documented to have intruded as diapirs within the sedimentary section in more recent times. These observations are consistent with the late Mesozoic and Cenozoic isotopic ages of many intrusions exposed onshore reported by Horne et al 1976a and 1976b, by McDowell in Emmet, 1983a, Appendix 4, and by Manton and Manton, 1984). The most prominent magnetic anomaly in eastern Honduras, the low-high anomaly pair with an amplitude in excess of 700 nT, is shown on the extreme left-hand part of the map (Slide 13). A depth-to-basement map interpreted from the aeromagnetic data (Slide 14) shows that this major anomaly arises from a combination of causes, including discrete intrusive bodies of obvious high mafic mineral content, but also from structural relief along faults.

In general areas of smooth, broad contours ("low-frequency" anomalies; Slide 13) correspond to deep basement (Slide 14) and areas of very irregular contours ("high-frequency" anomalies; Slide 13) correspond with shallow basement or shallow volcanic cover (Slide 14). The depth-to-basement map was interpreted from the aeromagnetic data (Slide 14) and shows depth contours on the inferred basement surface. Faults are inferred where basement depth contour values change abruptly. Areas of volcanic cover are shown with a pink hachure pattern. The shallowest depths to basement are less than 2,000 ft while the deepest basinal areas are shown to be in excess of 34,000 ft. Note that basement in the onshore areas is consistently shallower than in the offshore areas. Also, the east-northeast-trending structural arch seen in the Gracias a Dios platform at the top Cretaceous structural level (Slide 15) appears to have, in part, a corresponding feature at the basement structural level (Slide 14), and at least some of these structures may be related to the thrust-fold structure observed on Texaco Line 80-68 (Slide 6). Deep basinal areas and intervening structural arches are present in the Mosquitia basin. To the north along the present-day shelf margin that separates the Gracias a Dios platform and the Tela basin a series of basement uplifts and deep troughs are suggested by the interpretation (Slide 15). The relationship of these abrupt structures to the broader structures to the south, including the onshore structures, is unclear and should be investigated.

In summary (Slide 16), a reconnaissance study of the available geological and geophysical data from the Gracias a Dios platform and Mosquitia basin offshore eastern Honduras has yielded the following preliminary results. An Eocene basin is recognized offshore which has characteristics that suggest compressional reactivation of extension structures (a mild inversion event). Inferrence from onshore geology and from the plate-tectonic context suggest that the Eocene basin probably evolved in a transtensional environment and was deformed sometime in the Neogene by transpression. The sense of shear and timing of the inferred strike-slip deformation offshore and any relationship to documented strike-slip motions onshore along the Guayape fault are currently unknown but should be a focus of future study. The Eocene basin has not yet been documented onshore but may be exposed in the Punta Patuca area. The Cretaceous strata underlying the Gracias a Dios platform and Mosquitia basin appear to have been subjected to intense shortening similar to that which is seen in the Montañas de Colon area onshore. The structural arch underlying the Gracias a Dios platform is intruded by highly magnetic plutons and some intrude the sedimentary section and are therefore relatively young (late Cretaceous or younger). Volcanic flows or sills are documented in the magnetic data offshore and appear to be similar to Cretaceous and Tertiary volcanics described onshore. Therefore, the Mesozoic and Cenozoic stratigraphy and main structural elements of eastern Honduras extend with apparent continuity offshore. Any major structural boundaries within the Chortis block should recognize this continuity. A crustal boundary between the Chortis block and oceanic terrane to the south probably exists along a trend parallel to the Hess Escarpment in northern Nicaragua and offshore along the southern margin of the Nicaragua Rise.

References Cited and Uncited
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Carpenter, R.H., 1954. Geology and ore deposits of the Rosario Mining District and the San Juancito Mountains, Honduras, Central America: Bull. Geol. Soc. Am., V. 65, pp 23-38.

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Dupré, W.R., 1970. Geology of the Zambrano quadrangle, Honduras, Central America (unpl. Master's thesis): University of Texas, Austin, TX, 128 p.

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Emmet, P. A., 1986, Major structures and base metal potential of central and western Honduras clarified by radar analysis and new geologic compilation: Geological Society of America Abstracts with Programs, v. 18, p. 594.

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Last modified Sept. 27, 2002