NEW MEXICO ELEVATOR BASINS-3: Elevator basin models-implications for exploration in central New Mexico

The Trementina model (Fig. 16) is typified by the Des Moinesian and Missourian strata in the Trementina sub-basin of the Tucumcari basin (Fig. 3).

The long dimension of this type of elevator basin is oblique to the margin of the adjacent uplift. As such, the basin cuts into the flank of the uplift. Rate of basin subsidence was greater than the rate of sedimentary accretion in the basin.

The location of the basin in relation to the adjacent uplift resulted in sediment being shed into the basin from three sides. Alluvial fans and fan deltas were formed along those three sides of the basin. The basin was open to the epeiric Pennsylvanian sea on the fourth side. This geometry allowed only limited interchange of water between the elevator basin and the adjacent shelf of the central part of the Tucumcari basin.

Sediments with high percentages of organic carbon were deposited in quiet, deep waters along the basin axis. This resulted in the accumulation of thick sections of shales with highly elevated levels of TOC.

Because subsidence rates exceeded sedimentary accretion rates along the axis of the basin, the bathymetrically deepest part of the basin remained below sea level for long periods of time. Marine algal organic matter was incorporated into the shales along with woody and herbaceous terrestrial organic matter derived from the alluvial fans and uplifts that bordered the seaway. With these types of organic matter present in the shales of the basin fill, the Trementina model is characterized by source rocks capable of generating both oil and gas upon maturation.

Perhaps the most appropriate modern analogs for the Trementina model are the fjords of the Norwegian and British Columbia coasts. Deposition and circulation in fjords is summarized by Friedman and Sanders,⁴⁸ Richards,⁴⁹ Strom,⁵⁰ Calvert and Price,⁵¹ and Gross et al.⁵² The water column in many fjords is characterized by a fresh, oxygenated upper part and a saline, anoxic lower part.

The fresh waters are derived from streams that drain the adjacent highlands. These flow over more dense saline marine waters. A salinity and density stratification is created in which the lower, dense, saline layer becomes anoxic if organic productivity is high.

Organic-rich muds are deposited in many fjords. Levels of TOC can exceed 5%.⁵¹ This setting has resulted in the highly elevated levels of organic carbon that are present in the Trementina sub-basin. As previously discussed, TOC exceeds 9% in Canyon (Missourian) shales in the Trementina sub-basin.

Cuervo model

The Cuervo model (Fig. 17) is typified by Des Moinesian strata in the Cuervo sub-basin of the Tucumcari basin (Fig. 3).

This type of elevator basin is elongate parallel to the bordering uplift. The basin separates the uplift from a broad area of shallow shelf deposition. The rate of basin subsidence was normally greater than the rate of sediment accretion in the basin. As a result, this type of elevator basin acted as a trap for sediment derived from the uplift.

Deposition of coarse clastic sediments was mostly limited to the elevator basin. Relatively little sand and gravel bypassed the basin and was transported onto the shelf. These coarse sediments are deposited in alluvial fans and fan deltas that border the adjacent uplift. Carbonates and shales are deposited on the adjacent shelf.

In the absence of a large influx of siliciclastic sediment, reefs grew on the shelf and along the shelf margin of the basin opposite the uplift. These depositional conditions prevailed except for brief periods when sediment accretion rate exceeds subsidence rate and coarse siliciclastics in alluvial fans or fan deltas prograded across the elevator basin and onto the shelf.

The Precambrian basement may have been exposed along the shelf side of the basin during early phases of basin development. Where this occurred, alluvial fans were also formed along the shelf side of the basin and coarse siliciclastics may dominate the lower part of the basin fill along the shelf side of the basin.

Waters in the bathymetrically deeper parts of the elevator basin were anoxic. Fine-grained sediments with high percentages of organic matter were deposited and formed thick sections of black shales with elevated levels of TOC were formed. Marine algal matter was incorporated into the shales along with woody and herbaceous terrestrial organic matter derived from the alluvial fans and the uplift that bordered the basin. With these types of organic matter present in shales of the basin fill, the Cuervo model is characterized by source rocks capable of generating both oil and gas upon maturation.

A modern analog for the Cuervo model is the shelf of Guatemala and Honduras. Deposition in this setting has been summarized by Sellwood⁵³ and Ginsburg and James.⁵⁴ Some differences exist between the Cuervo sub-basin and this modern analog. There was probably a somewhat greater amount of clastic influx in the Cuervo sub-basin.

Although a deep channel is present between the siliciclastic shoreline and the shelf reefs, this channel appears to be of constructional origin in the case of Guatemala and Honduras but is of tectonic origin in the Cuervo sub-basin. Also, the Cuervo sub-basin was bordered by an extensive epeiric sea south of the sub-basin. On the shelf of Guatemala and Honduras, the seaward side of the bank of fringing reefs is characterized by a precipitous dropoff of several hundred meters into the Caribbean.

Estancia model

The Estancia model (Fig. 18) is typified by Morrowan through Virgilian (Early to Late Pennsylvanian) strata in the Perro sub-basin of the Estancia basin (Fig. 3).

This type of elevator basin is elongate parallel to the adjacent uplift. The basin separates the uplift from a broad area of shelf deposition. Unlike the Cuervo model, however, rates of sedimentary accretion generally equaled or exceeded rates of basin subsidence. As a result, siliciclastic sediments derived from the bordering uplift generally infill the basin as fast as it subsides.

Basin fill is mostly nonmarine and consists of alluvial and deltaic conglomerates and sandstones, and paludal shales and coals. At locations where sediment input was temporarily decreased the basin became inundated for short periods and thin limestones were deposited in either marine or lacustrine environments. Large volumes of sediments were transported across the basin in alluvial and deltaic systems and were deposited on the shelf where they intertongue with marine limestones and shales. Percentage of siliciclastic sediments decreases with distance from the uplift.

Woody terrestrial organic matter was incorporated into alluvial and deltaic shales and coals deposited in the basin fill. With these types of organic matter present in basin fill sediments, the Estancia model is characterized by gas-prone source rocks. No modern analogs have been identified for the Estancia model.

Variations from the models

Other elevator basins associated with the ancestral Rocky Mountains will undoubtedly have variations from the three models described above.

In all basins, the relationship between the rate of subsidence and the rate of sedimentary accretion may change during the life of the basin. A basin that initially subsides quickly may at first form a deep depositional trough whose subsidence outpaces the capacity of the alluvial systems on the adjacent uplifts to fill in the trough.

If subsidence slows later in the tectonic history of the basin, the alluvial systems may eventually overtake the basin and fill in the basin with sediment derived from the adjoining uplift. This has happened to all of the elevator sub-basins in the Tucumcari basin because the alluvial red beds of the Abo formation marked the last infilling of depressions associated with each of the sub-basins. In the Carrizozo basin, the last phase of infilling is represented by the evaporites and shallow-marine siliciclastics of the Yeso formation.

A model that has not emerged from the studied basins involves a basin that cuts into the flank of the uplift but where sedimentary accretion rates equal or exceed the rate of basin subsidence for most of the period of basin formation. These basins may be characterized by alluvial fans that rim the basin and feed an axial fluvial system that discharges at one end of the basin.

Although sand and conglomerate reservoirs may be plentiful in the basin fill, source rocks are limited to coals or shales associated with the axial fluvial system. These source rocks would be characterized by gas-prone terrestrial organic matter. Because the basin fill may be dominated by the alluvial fans that rim the basin, the volume of source rocks would be significantly less than what is found in the three models discussed above.

Summary

Elevator basins of the southern ancestral Rocky Mountains are long, narrow, and structurally deep troughs bounded by high-angle faults. These basins are formed along the flanks of ancestral Rocky Mountain uplifts. They are typically 20-50 miles long and 5-15 miles wide.

Bounding faults have vertical offsets that can exceed 5,000 ft. These bounding faults separate the basins from the adjoining ancestral Rocky mountain uplifts and also separate the basins from wide areas of shelf marine deposition.

Elevator basins were sites of source rock deposition. Thick sections of interbedded sandstones and shales were derived from sediments eroded from the adjacent uplifts and deposited in the elevator basins. Shales in elevator basins generally have higher organic content than shales on adjoining shelf areas. TOC can exceed 9% in some of the basinal shales. Greater depth of burial in the basins has led to higher levels of organic maturation than when compared to adjacent shelf areas.

Three models of petroleum systems in elevator basins have been developed from study of elevator basins in the southern ancestral Rocky Mountains. These models are defined by the interaction of two tectonic and depositional variables. One variable is the orientation of the basin relative to the adjoining uplift. Is the basin elongate parallel to the uplift, or does it cut obliquely across the margin of the uplift? The second variable concerns the relationship between the rate of sedimentation in the elevator basin and the rate of subsidence of the elevator basin.

If the basin is elongate parallel to the margin of the adjoining uplift, then the basin separates the uplift from a broad area of shelf deposition. Sands and muds derived from erosion of the uplift are deposited in the elevator basin. If the rate of subsidence exceeded the rate of sedimentary accretion in the basin, then the basin is characterized by marine deposition and acted as a trap for sediment eroded from the uplift. Fringing reefs may have been deposited on the shelf edge opposite the uplift.

Shales in the basin are source rocks of oil and gas. Sandstones in the basin may be reservoir. Hydrocarbons may also have migrated up the bounding basinal faults and be trapped in carbonate reservoirs formed by the fringing reefs on the shelf edge. A modern analog for this type of elevator basin is present along the shelf of Guatemala and Honduras.

If the basin is elongate parallel to the uplift and the rate of sedimentary accretion in the basin exceeded the rate of subsidence, then the basin became filled with sediment derived from the uplift. These sediments were also carried across the basin and deposited on the adjoining shelf. The basin is characterized by nonmarine sediments, and source rocks are principally nonmarine shales and coals. Reservoirs may be alluvial sandstones in the basin as well as bypass sandstones deposited on the adjoining shelf.

If the basin is elongate oblique to the uplift, then it is incised into the uplift flank. This type of basin is bordered on three sides by the uplift and on only one side by an area of shelf deposition. If the rate of basinal subsidence exceeded the rate of sedimentary accretion, then the basin is characterized by deposition of marine oil-prone shale source rocks along the basin axis. Reservoirs are alluvial sandstones and conglomerates deposited along the flanks of the adjoining uplift. Modern analogs for this type of basin are the fjords of the coasts of Norway and British Columbia.

Acknowledgments

Thanks to Ben Donegan of Donegan Resources and Jack Frizzell of Enrich Oil Corp. for sharing their ideas and extensive data on the Carrizozo basin area. Robert Gunn of Gunn Oil Co. discussed his recent gas discovery of Rhombochasm field in a Texas elevator basin. Charles Reynolds of Geological Associates Inc. and Gene Woodard of Woodard Energy freely discussed their ideas on tectonics and petroleum geology of the Tucumcari basin, and Woodard supplied several seismic reflection lines from the basin. Steve Cather and Charles Chapin of the New Mexico Bureau of Mines and Mineral Resources provided invaluable discussion of Pennsylvanian tectonics in New Mexico. Bob Lindsay, Donegan, and Reynolds reviewed the manuscript. Chapin, director emeritus of the New Mexico Bureau of Mines and Mineral Resources, coined the term elevator basin. Leo Gabaldon drafted the original illustrations.

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The author

Ronald F. Broadhead is principal senior petroleum geologist and associate director at the New Mexico Bureau of Mines & Mineral Resources, a division of New Mexico Tech and adjunct faculty at New Mexico Tech. He has worked as a petroleum geologist with the former Cities Service Oil Co. His recent work has included the petroleum geology of the Brushy Canyon formation in southeast New Mexico, and the Tucumcari and Estancia basins as well as the McGregor Range of south-central New Mexico. He has a BS degree in geology from New Mexico Tech and an MS degree in geology from the University of Cincinnati. E-mail: [email protected]