Abstract
The Altar basin found in the north western Sonora of Mexico, is a known subsidiary basin forming the present inactive part of the Colorado River delta. The sedimentary records of this basin show how the delta upgraded over a late Miocene. A structurally distinct aquatic basin at the northern end of the Gulf of California is found. Our interpretation of outcrop data, and information from exploratory wells, as well as analog seismic lines of Petróleos Mexicanos (PEMEX), and magnetic and gravity explorations from various sources pointed to the existence of three sedimentary sequences, which can be related at regional scale and have a thickness of about 5 km at the basin deposits center. The lower sedimentary sequence is a shale unit that represents open marine conditions. It changes into a thick sequence of inter stratified sandstone, mudstone and siltstone in the next sequence which then changes in turn into poorly combined sand. Other outcrops of a sand, the cut and fill succession that is evident along the coast of Sonora are testimony with sequences being the sub-aqueous and the sub-aerial parts of the Colorado delta. A contact at the base of the sequence where pre-marine continental deposits are not found, and where the marine sequence overlay crystalline basement, is thought to be tectonic transport along a top to the northwest detachment fault. The Altar basin, therefore, became inactive as result of the westward shift in the location of tectonic activity from the Altar fault to the Cerro Prieto fault, coupled with the changes in the course of the Colorado River during the Pleistocene period.
Introduction
The Colorado River delta largely occupies the depression called the Salton trough. It comprises the Imperial Valley which is found Southern California and the Mexicali Valley found in the northern Baja California. The trough is known to be traversed by active faults that are part of the southern San Andreas Fault system and the Gulf of California. The Colorado delta is structurally restricted and lies across a major plate boundary found between the Pacific and North American plates (Lonsdale, 499-521). However, currently, no processing of the geological structure and history of the whole Colorado delta has been availed because some areas are still not clearly known.
We, therefore, will use some data from onshore wells and seismic reflection profiles and from magnetic and gravity surveys, to try and establish the sedimentation and structure that is associated with this fault plate margin and its relationship to the geological history of Colorado River. The Altar basin has a common history with the other basins within the Salton trough and, therefore, forms a basis of the basin formation in this tectonic environment. Each of these basins found in this area must have experienced the effects of three main geological events that frequent this area and which includes; a period of extension and subsidence, a stage of marine sedimentation and a later stage of basin filling.
These three events have largely contributed to the growth of the Colorado River delta and to some extent by deposits of alluvial soil from the basin margin. Where there are time differences in the occurrence of these three events, the beginning and the end of this sequence would differ from basin to basin and that would also depend on their positioning to major depositing centers and on the complexities of the delta. The formation of the Colorado River delta divided the Altar basin and the northern gulf from the basins that are located to the north and west within the trough of Salton. In the north- central part of the trough, the growth of the delta produced environments that were occasionally flooded by effluents from the Colorado River.
Besides sharing the structural control of the delta, the basins are also known to share an early stage that is characterized by marine deposits. However, it is thought that marine situations may have continued on both sides of the delta because the rifts continued to widen and diminish with time. The Altar basin, which is located east of the active modern delta, is a good example of an extinct basin, whereas the Laguna Salada which is found to the west is still an active basin. This difference exists though both basins are the result of changes in the strengths of tectonic deformation. Exploratory seismic reflection explorations and the drilling of hydrocarbons by oil companies and geothermal resources have been used to produce large quantities of data, which for many years, for one reason or another remained unpublished, apart from some provisional reports (PEMEX, 57; Guzmán, 94-100).
We are, therefore, going to interpret data from the Altar basin. Additionally, we will use the gravity and magnetic differences to interpret the structure of the Altar basin. We would also study the sedimentation of outcrops that are found along the western margin of the Altar basin to gain some knowledge of the sedimentary environments for the uppermost part of the Colorado delta. Then, we would contrast this model of the structure and stratigraphy of the Colorado River delta to the other basins that are located to the west and northwest in the Salton trough. We will also discuss the tectonic involvements of these results.Stratigraphy of the Colorado River
Well, logs that were found in the Altar Desert and the eastern part of the Mexicali Valley indicate that there are three primary sedimentary sequences, which in some cases overlie on the crystalline basement. The scant paleontological data that was available from well samples of the older series helped to document the paleo environments and to determine the age of deposits fund in the basin. However, the age correlation between these wells was not possible to determine at this stage because age differences are considerably minimal.
Therefore, we would base our relationship with lithostratigraphic criteria because we realized that these faces could be literally diachronous and could have represented a more diverse and complex assembly of sedimentary environments than the ones suggested here. Basement rocks were thought to have bottomed in late-Cretaceous to the period around early-Tertiary granitic rocks formation. The crystalline basement in wells was found to be a grey-greenish biotite granodiorite, with plagioclase, potassic feldspar, and quartz. The crystalline basement in well another well was found to consist of a biotite-mus- covite-hornblende-garnet-bearing granitic rock, with quartz and K-feldspar.
The rock has mildly granoblastic texture and was highly fractured and was also found to contain hematite as fracture fill, which could have portrayed deformation at and under the basin detachment fault. The reported K-Ar ages in K-feldspar concentrates varied from 79±5 Ma to 61±5 Ma (PEMEX, 1985). Although these reported ages could have had significant margin errors. We were confident though that these three wells terminated in late-Cretaceous to the period around early Tertiary granitic and metamorphic basement rocks and not in late-Cenozoic magmatic intrusions. However, the second well that was tested could have ended in a young volcanic intrusion. The magnetic high west and southwest of the second well and the gravity high around it may have reflected mafic magmatism in the area covered by the basin.The Tectonic History of the Colorado River
It is unclear how the Colorado River became formed because there have been many different indications to different aspects. Colorado is one of the places that has been tectonically active and thus, experiences different concepts when it comes to the formation of rivers. The conclusion about the Colorado River is that it experienced all the four aspects that comprise the mechanism of River formation. The history of the river has been suggested to be related to the reversal of the flow of drainage to the Southeastern area as it emanated from the Northeastern area. What happened is the Basin and Range experienced a crustal expansion that affected the position of the South and East. The position of the two was affected in terms of high they were, which is premised upon their elevated position. Before the expansion or rather extension happened, the east and west were elevated and existed in a highland that allowed the flow of drainage into the Plateau.
Therefore, because of the changes that occurred in the position of elevation, there was a reversal of flow of the very drainage. From there on, there are a few mixed theories on how the river took shape. It was strongly suggested that the extension caused the dissipation of the river into the Grand Canyon at the West because of the limestone that surrounds this area. The evidence for such a theory is that the river carries limestone as it flows, and the source could be the area of the Grand Canyon. The explanations continue to shift as the flow as the flow of the river is exploited. It was suggested that the river could have experienced a small drainage from the Grand Cliffs. The evidence is that the deposits that the river has can be traced to mountains that are full of rock.
However, the smallness of the amount of deposit can be accrued to the fact that the drainage flow was small after the extension occurred. When the extension happened, there have been suggestions that a large river may have extended up to where the Grand Canyon was. More evidence points to the relation of the Colorado River with the Grand Canyon after the extension happened. The gravels that are located in the southern part of the River. The gravels can be traced to the Grand Canyon and have no origin from anywhere else around the environment. The sediment has been used as evidence of the extension and the relationship it has with the Colorado River in the scope of the Grand Canyon. The Gulf of California
Without a doubt, the Gulf of California would be denoted as a recent marine basin, and it is traced sandwiched between the Baja California peninsula and the Mexico mainland. That would be attributed to various factors with the element that its estrangement took place in recent years being among them (Scott, 45-47). The reality that the Gulf of California remains to be active to date explains why it has over the years been termed as the largest active Pacific-North America. In addition, various studies have undergone to discover various rift-structures within the Gulf of California. By illustration, the north-trending dip-slip and the sinistral strike-slip all of which would be traced from the two stages. The two stages referred to include the proto-Gulf that were bridging from -12.5 – 6 Ma while the second stages lasted from -6 Ma to modern times where it was termed as the modern-Gulf stage (Scott, 45-47). It was during the Proto-Gulf era that the strain mentioned was distributed in uncertain manner between the dextral San Benito and Tosco –Abreojos faults all traced to the California Peninsula. Thanks to the comprehended knowledge about the Gulf of California, it would factually be held that a trench, andesite chain, as well as ignimbrite all co-exist therein. Substance to remark herein is the fact that some zones ended to be weak consequential to strike-slip faulting and inter-plate. The concept of a gulf was initially used to refer to the early period of extension in the Gulf of California, an analog to other volcano-tectonic rift zones associated with active trench-arc system. Consequently, the term proto-gulf has been used as a synonym for the presence of early basins and marine incursion in the gulf area of California (Scott, 45-47).
This is consistent with the Stratigraphy and structure of the Altar basin of North West Sonora with published ages of the oldest marine deposits cropping out in the northern Gulf of California and Salton trough areas as being younger than 7.4 Ma (Quin and Cronin, 234-238; Dean, 196-204). However, the possibility of an older age for the initial marine incursion cannot be completely ruled out. Paleontological evidence of reexamined microfossils of mid-Miocene age identified in some localities with late- Miocene to Pliocene marine sequences, and differences in chronological distribution of key microfossil species.
Some geologists, however, still maintain the argument for a somehow older marine incursion in the northern gulf region. Additionally, ongoing biostratigraphic studies of cutting samples from a PEMEX well in the upper Tiburón basin, suggested the presence of mid-Miocene marine microfossils.Discussion
Correlation of the marine shale sequence across the Altar basin is based on the presence of this unit in the wells that cut through the basement at depth. We believe that the sequence is also present under other wells as suggested by a shale interval in the lowermost 100 m in the wells. The thickness of the sequence in the wells that cut crystalline basement is about 600 m. This thickness could be the minimum estimate for the total thickness of this sequence, because it clearly continued at depth in other wells in its distinctive seismic pattern .These stratigraphic relationships strongly suggest that the marine sequence has a widespread distribution in the Altar basin.
Correlation of the marine mudstones of the sequence toward the Yuma basin is a possibility because a similar unit had been reported in the exploration well. In the Yuma basin, a marine upper-Miocene sequence is about 490 m thick and grades down the section to a tuffaceous sandstone and coarse-grained conglomeratic sandstone. These stones, in turn, overlie a predominantly volcanic unit, dated at 16 Ma, and older continental clastics and volcanic intrusive. In the Altar basin, the mid- to early-Miocene volcanic and sedimentary section is unavailable in a number of wells that penetrated in the crystalline basement. This indicated that more than 900 m of pre-marine volcanic and sedimentary rocks were preserved in the Yuma basin a few kilometers north of wells, but are absent in the Altar basin.
This difference in the late-Cenozoic stratigraphy suggests that the Yuma basin and the Altar basin were separated basins prior to the initial marine incursion in the northern Gulf of California in the late-Miocene period. Our provisional biostratigraphic dating favors a late-Miocene age for the sequence, which is consistent with a synchronous marine incursion in the northern Gulf of California and the Salton trough areas. However, the presence of early- to mid- Miocene marine microfossils needs to be explained, and thus an even older age for proto-Gulf marine deposits cannot be ruled out.Conclusion
Analysis of geophysical data, suggested that this structurally distinct basin contains three major sedimentary sequences, which could be traced across the Altar basin and record the growth of the sub-aqueous part of the Colorado delta as it upgraded into the marine gulf since late Miocene to early Pliocene. The lower sequence recorded a late-Miocene to early-Pliocene marine incursion in the northern Gulf of California. A minimum thickness of 600 m of marine shale in the Altar basin suggested that open marine conditions existed prior to the arrival of detritus carried by the Colorado River into northwestern Sonora.
This is consistent with a regional marine transgression in the northern gulf and was also concurrent with localization of Trans tensional deformation in the gulf area at about 6.2 Ma. Our data did not allow the dating of the first arrival of Colorado River sediments to the Altar basin. However, we considered that the Colorado River reached this area at about the same time as in the southwestern Salton trough. It is probably believed that both areas were still adjacent in Pliocene time, and thus the age range for the arrival of the Colorado River into the south west Salton trough is likely the age of the base of sequence in the Altar basin. Sea-level changes may produce variations to shallower sedimentary environments, like delta front and tidal flats settings. High-frequency interstratification of sandstone, siltstone and mudstone, with bell-shaped electro sequences, which contain shallow water microfossils and mollusc shell fragments, are consistent with this interpretation.
The Altar fault and the detachment of the fault may have been linked to the Trans tensional domain of the southern San Andreas Fault in late Miocene to Pliocene. The long-term aim should be to make a comprehensive three- dimensional model of the transformation of the different sedimentary environments of the Colorado River delta, in order to understand how, and when, the delta formed and was deformed. The delta, however, is a repository of information on its responses to tectonism, long-term climate changes, sea-level changes, and transient events such as catastrophic floods and hurricanes.
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