Bol. Soc.Geol. Mexicana, Tomo XL, Nos. 1 y 2, 1979

Geologic Report on Upper Cretaceous Coal-Bearinc Rocks, Rio Escondido Basin,Coahuila, Mexico

http://dx.doi.org/10.18268/BSGM1979v40n1a2

Earle F. McBride* and Kyle C. Caffey**

*Comisión Federal de Electricidad, Series Técnicas, No. 3.
**The University of Texas, Austin, Texas.

Introduction

In northern Mexico and adjacent parts of Texas, deposition of platform and basinal carbonate rocks of Early Cretaceous age was followed by the widespread deposition of terrigenous clastics during Late Cretaceous time (Weidie et al., 1972). Detritus, derived chiefly from volcanic and shallow intrusive rocks exposed in the area of the present Sierra Madre occidental, was transported by rivers several hundred kilometers eastward and deposited in marine, deltaic, and fluvial environments along the margin of the ancestral Gulf of Mexico. Deltaic deposits, some of which are coal bearing, were deposited over broad areas and are preserved today in areas (basins) that subsided more than adjacent regions or in areas that escaped erosion following Laramide folding and faulting (Figure 1). Regional stratigraphic analyses show that deltas prograded in general from west to east, and that the main loci of deposition shifted eastward with time (Figure 2).

Following a summary of the chief characteristics of the deltaic sequences of each basin, this report treats the stratigraphy of the coal-bearing and adjacent formations in the Rio Escondido Basin and presents recommendations to aid future coal exploration operations.

Late Cretaceous basins, northern Mexico and vicinity

Ojinaga Basin

The lithostratigraphy and biostratigraphy of the Ojinaga Basin have been described by Arenal (1964) and Wolleben (1965, 1967, 1968); a generalized stratigraphic section is given in Figure 3. The Ojinaga Formation includes marine shelf and prodelta shale deposits; the San Carlos Formation in chiefly delta-front and distributary-channel sandstones and lower delta-plain deposits, including some coal beds; and the El Picacho Formation is delta plain and fluvial shale and sandstone, including some red beds. Non-marine fossils are present locally in the El Picacho Formation. The preserved stratigraphic sequence in the basin has a maximum thickness of 1600 m.

Big Bend Graben

Big Bend National Park and vicinity in Texas contains Late Cretaceous fluvial-deltaic rocks that are up to 650 m thick (Figure 3); these rocks are preserved as erosional remnants in a large graben. The Pen Formation is sparsely fossiliferous gray shale, locally with septarian siderite concretions of shelf and prodelta origin. The overlying Aguja Formation is sandstone, shale and rare coal beds that formed as delta-front and lower delta-plain deposits. The succeeding Javelina Formation, representing fluvial and upper delta-plain deposits, is chiefly yellow, maroon, and green clays and local channel-fill sandstones. Dinosaur bones and petrified wood are present locally. Stratigraphic details of these formations are given by Hopkins (1965) and Maxwell et al. (1967).

FRONTISPIECE A. Part of core GN-I04 showing prodelta deposits of the Upson Formation. The following features are visible: intensely bioturbaled muddy sandstone, (a); weakley fissile shale in which burrowing has disrupted primary laminations, (b); sandstones beds with scattered burrows, (c). The sandstones beds are frontal splay deposits that increase in thickness and in number upward. The thinner sandstones beds have largely losl their identity because of intense bioturbation.

Río Escondido Basin

Stratigraphic units in the Río Escondido basin (Figure 3) are similar to those in the adjacent Sabinas basin, except that red beds have not been recognized in the Escondido Formation.

FRONTlSPIECE B. Part of core GN-80 bis showing the following features: prodelta shale of Upson Fm., (a); shale clasts at base of distributary-channel sandstone (San Miguel Fm.), (b); foreset layers of trough cross-beds, (c); laminated sandstone with abundant transported plant debris, (d); current-ripple lamination (e); structureless sandstone (abandoned channel event), (g); basal shale of the Olmos Fm. (h).

The stratigraphy of the basin is described in detail in the body of this reports.

Sabinas Basin

Roebeck et al. (1956) record about 1000 m of Upper Cretaceous terrigenous clastic rocks in the Sabinas Basin (Figure 3), where stratigraphic units were studied in outcrop and open pit coal mines. The marine shelf and pro-deltaic Upson Clay is overlain by the San Miguel Formation, chieny delta·front and distributary channel sandstones. The overlying Olmos Formation, interbedded shale and sandstone, contains several meters of coal-bearing strata of delta-plain origin at its base. The succeeding Escondido Formation is composed rnostly of marine shale and siltstone bul includes some shallow marine or delta-front sandslone. The upper part of the Cretaceous sequence in the western part of the Sabinas basin is about 250 m of red claystone and siltstone of upper delta-plain facies. Roebeck et al. (1956)placed the red beds in the Muzquiz formation, but because the red strata interfinger with normal nonred rocks of Escondido type, the Muzquiz is best treated as a member or tongue of the Escondido Formation. The Muzquiz red beds resemble red beds in the Difunta Group to the south.

Figure 1. Map of northern of Mexico and adjacent part of Texas showing the location of preserved Late Cretaceous basins.

Figure 2 Map showing the inferred shoreline position during Early Campanian and Early Maestritchtian time. 

FRONTISPIECIE C. Part of core GN-104 showing upper deltaplain deposits of the Olmos Formation. Visible features include: fluvial sandstone with granule-size lag deposit just above the base, (a); parallel laminations, (b); gradiations from sandstone into overlying overbak shale, (c); coaly shale of probable backswamp deposit, (d); overbank shales, (e); sandstone of probable lacustrine origin that grades both upward and downward into carbonaceous shale, (f).

La Popa-Parras Basins

The Parras and La Popa basins differ from the other basins described here in several ways. They contain the thickest clastic sequence (6000 m of the Difunta Group, Figure 3), they record four major cycles of delta progradation and marine transgression, they contain evaporitcs in the Jurassic sequence (Minas Viejas Fm.), they lack coal deposits, and they underwent the strongest degree of Laramide folding. Stratigraphic and sedimentologic data are given by McBride (1974) and McBride et al. (1974; 1975).

Deposits of the Parras and La Popa basins interfinger at their junction, but each basin was fed sediment from the west by separate rivers. Eastward progradation of deltas at least 200 km is recorded in the Parras basin. Prodelta and delta-plain facies in the Difunta Group are hundreds of meters thick (Figure 3) because of the high rate of basin subsidence.

Delta-plain deposits are lenticular varicolored siltstone and claystone beds that include repetitious upward-coarsening and upward-fining lake deposíls. locally including soil zones that are interrupted by distributary-channel sandstone beds from 200 to 600 m wide and 3 to 10 m thick. Larger, rnultistory fluvial sandstone are present only locally.

Delta-front sheet sandstone units, 3 to 15 m thick and up to 20 km wide, are characterized by parallel laminae and lesser trough crossbeds, Ophiomorpha, and transported oysters.

Delta-platform deposits are characterized by bioturbated clayey siltstone beds with marine molluscs thal are intercalaled with laminated sandstone beds introduced by flood discharge, many of which foundered to form ball-and-pillow structures. Sorne sand was carried down the prodelta slope as turbidity currents to produce fysch like interbeds of turbidites and bioturbated background mudstone beds that contain foraminifers, Exogira, and ammonites.

Delta-flank deposits include rnudstone, oyster reefs, marine-mud and sand-falt deposits, and rnixed terrigenous sand-oolite deposits.

Shelf-deposits include rare, thin glauconite beds, phosphatic limestone beds, and 9 lenticular biostromes. The latter are up to 300 m thick and 19 km long, and are red-algal packstones that contain sparse rudists and other frame builders. Several biostromes are marginal to evaporite diapirs that may have been active during Late Cretaceous time.

Figure 3. Generalized stratigraphic sections of Late Cretaceous basins, northern Mexico and adjacent part of Texas. Diagonal lines indicate red beds; unpatterned areas are shale.

Evidence for deltaic deposition

The interpretation of a deltaic origin for the major part of the Upper Cretaceous clastic rocks discussed here is based on 1) a comparison of facies of the Cretaceous rocks with modern (Holocene) deltaic facies (eg., Fisher et al. 1969; Broussard, 1975), 2) a comparison of facies of the Cretaceous rocks with ancient rocks interpreted as deltaíc in origin, and 3) the limitations of possible environmental interpretations that can explain the vertical succession of facies and formations in each basin.

The basic stratigraphic pattern that each basin has in common is that marine shale (e.g. Ojinaga, Pen, Upson, Parras Formations) is overlain by several meters ar tens of meters of sandstone (e.g. San Carlos, Aguja, San Miguel, basal Cerro del Pueblo), which is succcedcd by interbedded shale and sandstone (El Pichacho, Olmos, Javelina, upper Cerro del Pueblo Formations) that in placcs contains coal, non-marine fossils (tand plants, dinosaurs), varigated strata including red beds, and lenticular sandstones. Such a regressive sequence can form only by progradation of a strand plain (nonbarrier island coast), barrier-island coast, or deltaic coast. Detailed examination of the facies shows that only the progradation of a delta explains all the specific rock typcs and fossil types. Not all the facies of the Upper Cretaceous clastic rocks are strictiy deltaic; some are marine-shelf deposits and some are fluvial deposits. However, the shelf facies generally grade upward into prodelta shale, and delta-plain deposits grade upward into fluvial sandstones and shale; both are the result of regression (basin-ward migration of the shoreline) and progradation (outbuilding of the shoreline).

The stratigraphic successions in the Ojinaga, Big Bend, Rio Escondido and Sabinas basins record only one major regressive sequence, whereas the Parras-La Popa basins record four major regressive (and intervening transgressive sequences). A greater rate of sediment supply to the Parras-La Popa basins and their faster rate of subsidence is a major reason for the different histories. The Ojinaga may have had more than one regression, but erosion has removed the evidence.

General stratigraphy of upper cretaceous rocks, Rio Escondido Basin

General Relations

The Upper Cretaceous formations exposed in the Rio Escondido Basin are, from bottom to top, the Upson, San Miguel, Olmos, and Escondido. Lithic characters of the Upson, San Miguel and Olmos are known from cores taken by the C.F.E., but the Escondido Formation has not yet been cored except possibly in the far south. The Upson and Escondido formations have been dated as Santonian-Campanian and Maestrichtian respectively, but because the Upson-San Miguel-Olmos accumulated during regression, they all are diachronous units. In any location, such as a core hole, the Upson is older than the San Miguel, which in turn is older than the Olmos. However, because of diachroneity, the Olmos at the location of core 46 is olderthan the Upson at the location of core ED-80, although there is no fossil evidence to prove this interpretation. Because deltas prograded eastward, each formation is progressively younger in an eastward direction (Figure 3A).

Environmental interpretation shows that the Upson Formation contains prodelta and probably shelf deposits, that the San Miguel contains delta-front and distributary channel deposits, and the Olmos contains delta-plain and alluvial plain deposits. Lithic characteristics of each formation are summarized io the following pages; stratigraphic details of the Upson, San Miguel and Olmos are shown on the stratigraphic cross sections that accompany the report. The environmental facies terms instead of the formation names have been used in the cross sections.

Upson formation

The Upson Formation is composed chiefly of dark gray shale, locally slightly calcitic, that contains scattered blebs of pyrite. Lenticular beds of sandstone less than 1 m thick occur at scattered positions in the fotmation. These are frontal splay deposits that formed when sand spilled over distributary-mouth bars during flood episodes. Sorne of these sandstones can be correlated over distances of 2-3 km. The shale is poorIy fissile. because the original preferred orientation of clay minerals (illite and chlorite) was destroyed by bioturbation, ie., by burrowing animals. Shale beds that escaped extensive bioturbation have good fissility.

Figure 3A. Seaward migration of depositional environments of a typical delta showing formation equivalents in the Rio Esoondido basin.

Figure 3B. Block diagram of meandering-river flood-plain and channel deposits.

The gray color of the shale is imparted by minute pieces of land plant debris, unresolvable organic matier, and pyrite. Terrigenous silt content in the shale ranges from 0 to 30%; it probably was deposited during flood and storms events and was subsequently mixed with clay minerals during bioturbation. In addition to the nearly ubiquitous trace fossils, Inoceramus pla es (calcite prismatic layer only) and a sparse group of other molluscs are present locally. Shale was not examined for microfossils.

The complete thickness of the Upson was not penetrated in C.F.E. cores in the Rio Escondido Basin. The maximum thickness penetrated is about 70 m in the northern area, but the Upson probably is in excess of 200 m thick.

The Upson is overlain conformably by the San Miguel Pormation. Siltstone and very-fine sandstone beds increase in abundance in the upper part of the Upson until a predominantly sandstone sequence of beds thicker than 1 m is encountered. In this generally gradational sequence the top of the Upson is arbitrarily placed at the base of the lowest sandstone bed thicker than 1 m where sandstone beds prevail over shale beds.

San Miguel Formation

The San Miguel formation is chiefiy fine to medium grained sandstone with some shale interbeds, but because it locally includes frontal-splay deposits, distributary-channel, and channel-mouthbar deposits, it varies considerably in thickness and character. Interbeds of sandstone and shale a meter or so thick are common at the base and give way upwards to thicker sandstone beds with rare thin shale interbeds. In some cores shale beds are absent and 20 m of clean sandstone are present. Thus, the lower part of the formation is a sequence of beds that generally thicken upward and also coarsen upward; this sequence is typical of prograding regressive sand bodies. The upper part of the San Miguel is of variable character, but commonly shale interbeds interrrupt the sandstone layers, and the sandstone packages from 1 to 3 m thick tend to become finer grained upward. These upward-fining sequences were formed by deposition in distributary channels.

Burrows, including Ophiomorpha, occur in the lower part of the formation in some wells. Placers up to 20 cm thick of oyster shells and sandstone pebbles are present in a few cores; these are thin transgressive deposits that forrned when active deltaic deposition had shifted temporarily to a different area. They have not been found in wells spaced only 1 km apart. Ophiomorpha burrows probably formed during short-term shut-downs of deltaic deposition. In Cretaceous rocks, Ophiomorpha were formed by a crustacean that lived in very shallow water in normal marine salinity. Beds a few decimeters thick that are intensely mottled by burrows 2-3 mm wide are present in a few cores; their significance is unknown.

Parallel lamination and structureless bedding are the most common stratification types in sandstones. Large-scale cross beds are present locally, but generally are impossible to identify in the srnall diameter cores that are available. Most sandstone beds have abrupt lower and upper contacts.

In the cores exarnined the San Miguel ranges in thickness from 3 to 30 m; it generally is from 8 to 15 m thick.

Sandstones that have not been bioturbated have framework (sand and coarse silt grains) cornpositions of 35 to 40% quartz, 25 to 30% feldspar and 30 to 35% volcanic rock fragments and locally derived clay clasts. Most sandstones have secondary porosities of 15 to 25% that was produced by removal by dissolution of calcite cernent and some feldspars by acid subsurface waters. These sandstones now are weakly cemented by authigenic kaolinite and locally by authigenie chlorite. Clay clasts in some sandstones also have been chloritized and they now are green and resernble glauconite. Sorne sandstones retain all their previous calcite cement and have essentially no porosity.

Petrographic study of sandstone samples to establish diagenetic (post-depositional) events and their relation to coaI beds is in progress.

Olmos Formation

The Olmos Formation is cornposed of dark gray, cornmonly carbonaceous shale with interbedded sandstone beds from 1 to 5 m thick and local coal beds. The maximum thickness penetrated by C.F.E. cores is 63 m, but it probably exceeds 100 m total thickness. The lower part of the formation is composed of delta·plain, marsh, swamp, and bay deposits and thin distributary and fluvial channel deposits; the upper part is chiefly alluvial plain deposits of fluvial sandstones and overbank/backswamp shales (Figure 3B).

Channel-fill sandstones commonly both decrease upward in grain size and in thickness of beds; also, many sandstones have a basal lag conglomerate layer a few centimeters thick of shale and coaly clasts, and pass upward from parallel or cross-lamination to current-ripple cross-lamination. Burrows are rare, but are present in a few beds. Sandstone is similar in composition to that in the San Miguel, except coal clasts and plant debris are more abundant in the Olmos sandstones.

Shale beds contain abundant minute plant fragments, local plant-root impressions, and, because of only moderate bioturbation, are moderately fissile.

Coal beds range in thickness from partings to about 2 m. Stratigraphic details of the coal beds are described later in the report.

The base of the Olmos Formation is placed at the top of the sandstone immediately below the lowest coal in the sequence, or at the top of the sandstone at the base of a predominantly shale sequence several meters. thick. The San Miguel-Olmos contact is a diachronous surface that corresponds with a facies change, i.e., seaward progradation of a delta-plain/delta-front interface (Figure 3A).

Escondido Formation

The Escondido Formation, which overlies the Olmos Formation, is exposed along the Rio Bravo where it contains four mappable members (Figure 4), and has a total thickness of 300 m (Cooper, 1970). The formation is chiefIy fossiliferous shale, burrowed mudstone and siltstone, but contains numerous sandstone beds in the lower half. Glauconitic shale and impure limestone make up the upper member, which is overlain at a bored surface by limestone of the Paleocene Midway Group. Prodelta shale and delta-front sandstone of a minor regressive event are present, in the lower part of the Escondido Formation, but interdeltaic and marine shelf deposits make up the bulk of the formation.

Detailed stratigraphy of Upper Cretaceous rocks, Río Escondido Basin

Northern Area: Dip Sections

The zone of exploration situated near Piedras Negras covers an area running approximately 120 km north-south and 40 to 50 km east·west. Within this area, numerous wells have been drilled and cores taken for evaluation. For this study, data was obtained by examining the full diameter cores using a 10x hand lens and the unaided eye. Rock characteristics were recorded on a standardized logging form, and these were subsequently used to construct stratigraphic cross sections.*

The Rio Escondido Basin can be divided into two principal study areas. The northern part of the basin contains more control well and was studied in greater detail than the southern part of the basin.

Cores along three east-west transects were examined to provide stratigraphic control in a direction that is parallel with the present structural dip, and which coincides approximately with the paleoslope into the Cretaceous ofean basin.

Three different types of dip sections were constructed for each transect. The first type is a section showing the depth to the major coal beds found in the cores. This type of dip section shows that there are several major coal beds present in the northern area and that these coals offlap eastwards into the basin. Next, the dip sections were constructed to show the present structural attitude of the coals and the major facies recognized in the cores from each well location. The third type of dip section is a reconstruction of the sedimentary facies and it shows probable correlations of sedimentary packages found in the basin.

* All log data are on file at C.F.E. office, Piedras Negras.

Figure 4. Generalized composite columnar section of type Escondido Formation, Rio Grande Embayment.

Figure 5 shows dip section A-A' with the depth to the major coals plotted for each well location. In Zone "D" a coal bed, 2 m thick, can be correlated a distance of approximately 4 km. Overlying this are several other coals which offlap eastwards into the basin, i.e.: they are higher in the stratigraphic column in an eastward direction. Because of a lack of major structural elements (faults, folds) in the Rio Escondido Basin, the coaIs are progressively younger in an eastward direction. This corrrelation indicates that there are several distinct sedimentary packages along this transect, a conclusion supported by other dip-oriented cross sections.

The three major sedimentary depositional facies recognized in the cores are the shales and sandstones of the prodelta facies (Upson Fm.), the thick laminated and crossbedded sandstone of the delta-front facies (San Miguel Fm.), and the sandstones, shales, and coals of the delta plain and fluvial facies (Olmos Fm.) Note that the majority of the cores in this section do not penetrate through the delta-front facies into the prodelta facies. Therefore, the varying thickness of the delta-front sandstone is unknown. Because of the lack of this valuable information, a "net sand map" could not be constructed for the area. Such a map would have defined the loci of maximum sedimentation. AIso note that cores from well ED-197 contains no coal because coring was begun in the delta-front facies rather than in the delta-plain facies (which contains the coal beds).

When the section is correlated on the coal beds, the basin fill can be seen to consist of several distinct packages. Each of these packages consists of the prodelta, delta-front and delta-plain facies. These packages offlap into the basin, with the most eastward package being found higher in the stratigraphic column than those packages found west-ward. The delta-plain facies of each package contains coal beds that can be correlated within that package, but the coal beds are separate and isolated from the coals found in the other packages.

Of the other two dip sections, B-B' is located 8 km north of A-A', and D-D' is located 4 km south of A-A'. These dip sections were chosen to illustrate lateral changes in the depositional facies of the northern area. The "depth to coal" dip sections of B-B' (Figure 6) and D-D' (Figure 7) show that the same pattern is present over the whole northern area. Large sedimentary packages, containing their corresponding coal beds, offlap eastwards into the basin. The "depth to coal" dip section D-D' (Figure 7) also shows the approximate location of the different depositional facies found in the cores for each sedimentary package. Parts of four and possibly five or more separate packages are recognized in this dip section.

The various dip sections show that the northern area is free of structural complications and that the basin fill consists of offlapping sedimentary packages. These packages each contain prodelta, delta-front and delta-plain facies. Within the delta-plain facies of each package are coal beds that are correlatable over varying distances within that package. The thickest coal is consistently found in the western part of the area. In the east, the sedimentary packages contain numerous coaIs that are thinner than their western counterparts.

Northern Area: Strike Sections

Two strike sections were constructed in the northern study area. Strike section S·S', which covers the full length of the northern area, is located 6 km east (basinward) of T-T'. For each strike section a "depth to top of delta-front facies" diagram was constructed. The top of the delta front (the top of the first thick sandstone below the oldest coal) can be used as a datum for further correlation. Cross sections were also constructed showing the facies recognized in each core and the lateral correlation of the facies using the top of the delta-front facies as a datum. These strike sections show that the basin is not structurally complicated and that a datum is present that can be used to make detailed correlations of the facies recognized in the cores. The sections also show that the thickest coals are found in the western part of the basin and that the correlation of coal beds in a strike direction is much more difficult to make. A possible explanation of this difficulty in correlation is that the distributary channels are dip-oriented, so that the distributary-channels sandstones interrupt the lateral continuity of the delta-plain deposits.

Figure 5. Section A-A' showing depth to coal in strike section.

Figure 6. Dip section B-B' showing depth to coal.

Figure 7. Dip section D-D' showing depth to coal.

The variance in the depth to the top of the delta-front facies in each core is caused by the regional dip in the area and the particular location of the wells in the strike section. The tap of the delta-front facies is essentially a horizontal surface now and therefore was a horizontal surface at sea level during Cretaceous time.

There is a lack of data on the lateral changes in thickness of the delta-front sandstone because the majority of the cores did not penetrate through the entire delta-front into the prodelta facies. However, in the wells that did penetrate into the prodelta. a complex pattern is hinted at by the presence of delta-front sandstones that are overlain by prodelta shales. Other wells (ED-44, ED-182, ED-183) show delta-front sandstones which have major shale beds which differ radically from adjacent wells ((Ed-1. Ed-51. Ed-36) which have very thick delta-front sandstones with no shale beds. The areas of the delta front, receiving large amounts of sediment, built thick clean sandstone sequences, while marginal delta-front areas received intermittent pulses of sediment moved by current and wave transport. However, across the 17 km that the section extends, there are no interdistributary bay facies recognizable in the cores. In all cores the delta-front facies is present, but in varying thickness.

The lowermost coal beds present in strike section T-T' (wells ED-37 to ED-44) at first look as if they correlate. However, it is entirely possible that they are separate beds or beds that split into two beds (ED-6). AIso note that the coals present in wells ED-21, Ed-16, and ED-11, are found slightIy higher in the section than the coals in adjacent wells (ED-37, ED-6). The coals present 10 or more meters higher in the delta-plain facies are divided by distributary-channel facies and can be traced laterally, with confidence, only 1 or possibly 2 km (ED-16, ED-11, ED-6). The thickest and most laterally extensive coals present in the area are always found just above the delta-front facies (i.e., just above the San Miguel Fm.).

In Figure 8, which shows the "depth to the top of delta-front" for strike section S-S', the depth to the delta front is a function of the well site and of the regional dip. The section extends more than 25 km and has delta-front facies present in every core that reached a depth that would penetrate it. Again, no interdistributary bay facies are recognized in the cores.

The delta-front facies ranges in thickness across the area as in T-T'. Again, the areas of the delta front near the mouths of distributary channeis received large amounts of sediments and correspondingly the delta-front sandstones in these areas are thicker than on the marginal areas.

The coal beds in S-S' are laterally discontinuous and are correlatable only over a distance of 1 or 2 km because the coaIs are divided lateraIly by the distributary-channel facies (ED-110, ED-94, ED-91). The division of the coal beds laterally by the distributary channels is shown in map view in Figure 9. Because distributary channels are dip oriented, they tend to compartmentalize the coals. During lateral migration of distributary channels, and also during regressive progradation, they locally eroded coal beds and removed them from the site.

Note that the coal beds in transect S-S' are more numerous but thinner than the coal beds in transect T-T'. In the cores available for study, the delta-plain and fluvial facies is uninterrupted by transgressive sequences. As one moves upward in a core, once above the top of the delta-front facies, no marine facies are encountered. Any transgressiye deposits above the delta-front facies lie above the units sampled in the northern area. The only transgressive deposits found in the cores are in the delta-front facies. These transgressive units are usually less than 20 cm thick and consist of marine shells and burrows in a sandstone matrix. They represent lag material that accumulated during reworking of deltaic deposits by waves, and also the burrowing activity of marine organisms that occupied the area during times of no deltaic sedimentation.

Southem Area

The southern area of Rio Escondido Basin receiyed less attention in this study because the wider spacing between wells gave less core control. The average spacing between wells in section R-R' is 2 1/2 km and the spacing in Z-Z' is approximately 5 km compared with an average spacing of 1 km between wells in the northern area. The two strike sections were constructed in order to study the depositional packages found-in this part of the basin and to help reconstruct the depositional history of the entire basin.

Figure 8. Strike section S-S', datum on ground Ievel.

Figure 9. Facies of a lobate delta. Sand reaches the delta front through many small distributaries and is shaped by waves into a Iaterally continuous delta-front sand (facies C1). Progradation of the sand produces a sheet or      deposit.

Figure 10, a cross section showing "depth to top of delta front" for R-R', shows that the top ofthe delta-front facies is essentialIy a horizontal surface and can be used as a datum for correlation of the facies found in the cores ofthe section. When this strike section is tied into the strike section T-T', the top of the delta-front facies is correlatable between the two sections. This correlation of the delta-front shows that the same deltaic packages continue from the northernmost weIl of T-T' (GN-5) to the southernmost well of R-R' (G-3), a distance of 45 km.

Too few wells penetrated completely through the delta-front facies into the prodelta facies to know the major loci of deposition, but the deltafront facies is present in all cores but one (G-3), in which coring stopped above the delta-front facies. Sorne vertical stacking of the delta-front sandstone is recognized in wells F-5, F-4, and F-1. Minor transgressive units were deposited on top of the delta-pIain. Any major transgressive units in this area are above the cored intervals of this section.

There are numerous coal beds present in the delta-plain facies of this section. However, the wide spaciog of the wells makes correlations of these coal beds unreliable. The thicker of these coal beds are in well F-2. These coal beds are not found 2½ km away in the flanking cores. Also in this area, fluvial distributary channels divide the coal beds into "compartments". Control is not adequate to identify these compartments.

Laterally there is no continuous delta-front facies correlatable between the wells, and there is no delta-front facies present whatsoever in the two southernmost wells (GN-133 and G-126). Facies change laterally from prodelta to delta-front and in some cores also to the delta-pIain facies. Any coal beds found in the cares will also be discontinuous across the 5 km spacing between wells.

Figure 12. Block diagram showing principal sedimentary facies of the Modern Niger Delta.

Figure 13. Distribution of swamp and marsh environments on Mississipi deltaic plain.

The cores in the southern part (south of section R-R') of the study area do not contain thick coals, and the area does not look like it will be a productive area, Nevertheless, additional cores, those originally planned for this area plus cores farther east and south, should be taken in order to enable the major deltaic facies to be determined. There are too few cores available at present to permit a proper interpretation of the facies and too few to evaluate the coal prospects.

 
Figure 14. Diagram showing the inferred origin of coal-bearing cyclic packages. Each cyde is the product of a major episode of deltaic deposition.

References

Arenal, e. Rodolfo del, 1964, Estudio geológico para localización de yacimientos de carbón en el área Ojinaga-San Carlos, Estado de Chihuahua, Mexico: Asoc. Mexicana de Geologos Pet., v. 16, p.121-142.

Broussard, M.L., 1975, Deltas, models for exploration: Houston Geol. Soc., 555 p.

Coleman, J.M., 1966. Recent coastal sedimentation, central Louisiana coast: Louisiana State Univ., Coastal Srudies Ser. 17,73 p.

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