Structural and stratigraphic evolution of the Anza rift, Kenya

ELSEVIER

Tectonophysics 236 (1994) 93-115

Structural and stratigraphic evolution of the Anza rift, IQenya William Bosworth a,*, Chris K. Morley b~l aMarathon Petroleum Egypt, Ltd., P.O. Box 52, Maadi 11431, Egypt b Amoco Production Company, P.O. Box 3092, Houston, TX, 77253, USA Received 27 July 1992; accepted 3 May 1993

Abstract The Anza rift is a large, multi-phase continental rift basin that links the Lamu embayment of soqlthern Kenya with the South Sudan rifts. Extension and deposition of syn-rift sediments are known to have commenced by the Neocomian. Aptian-Albian strata have, thus far, not been encountered during limited drilling campaigns and, in at least one well, are replaced by a significant unconformity. Widespread rifting occurred during the Cenomanian to Maastrichtian, and continued into the Early Tertiary. Marine waters appear to have reached the central Anza rift in the Cenomanian, and a second marine incursion may have occurred during the Campanian. As no wells have yet reached basement in the basinal deeps, the possibility exists that the Anza rift may have initiated in the Late Jurassic, in conjunction with extension to the south in the Lamu embayment and to the north in the Blrle Nile rift of Sudan. Structural and stratigraphic evolution in the Anza rift followed a pattern that has now been inferred in several rift settings. Early phases of extension were accommodated by moderately dipping faults that produced large strata1 rotations. Sedimentary environments were dominantly fluvial, with associated small lakes and dune fields. Volcanic activity is documented for the early Neocomian, but its extent is unknown. This initial style of defplrmation and sedimentation may have continued through several of the earliest pulses of rifting. By the Late Cretaceous, a new system of steeply dipping faults was established, that produced a deep basin without significant rotation of strata in the north, and only minor rotation in the south. This basin geometry favored the establishment of largt, deep lakes, which occasionally were connected to the sea. The older basins were partly cannibalized during thq sedimentary in-filling of these successor basins. Early Senonian volcanism was encountered in one well, and reflqction seismic evidence suggests that one or more thick, regionally extensive igneous sills were intruded, probably du?ing the Early Tertiary. The change in rift style from early, strongly rotational, shallow basins to late, non-rotational, deep basins has been observed in the southern Gulf of Suez/northern Red Sea, the Southwestern Turkana/northern Kenyan rift, and at Anza. It therefore takes place in rifts in variable tectonic settings, with a wide range of volcanic activity and, presumably, with different driving mechanisms. The shift in deposition in each case is away from early rift-bounding faults toward the half-graben flexural margins, further in-board to the upper structural plate. This suggests at least some component of regional simple shear in the deformation history of the rifted lithosphere, either via broad shear zones or at discrete detachment surfaces.

* Correspondence to: W. Bosworth, P.O. Box 1228, Houston, TX 77251-1228,USA. ’ Present address: University of Brunei Darussalam, Gadong 3186, Brunei Darussalam. 0040-1951/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved ssD10040-1951(94)00025-5

the geophysical data acquired in a program such as KRISP, the role of older extensional events in shaping the present structure of the East African lithosphere must be considered. This is particularly important with regard to old rifts that crosscut the study area, or parallel or underlie the modern rift valley. In Kenya, both these geometries are known to exist. The northern Kenyan rift valley, in the vicinity of Lake Turkana, is paralleled by several inactive rift basins of Oligocene and Miocene age (Cerhng and Powers, 1977; Morley et al., 1992). This earlier extension resulted in thinning of the crust at the position later occupied by Lake Turkana. Seismic refraction studies undertaken during KRISP have revealed that the depth to the Moho

The Phanerozoic history of Central and East Africa has been dominated by the processes and products of continental rifting. Rifting events are known to have occurred in the Early and Late Permian, Early-Middle Jurassic, Late Jurassic, Early Cretaceous, Late Cretaceous, and Early Tertiary (Burke and Whiteman, 1973; Lambiase, 1989; Bosworth, 1992), and extension probably continues today in at least some segments of the Neogene East African rift system. This young rift is commonly considered critical for fo~ulating generalized models of continental extension, as typified by the various aspects of KRISP discussed in this volume. In order to properly assess

AFRICAN

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Fig. 1. Location of the Anza Rift within the West and Centrat African Rift Systems. Basins and strike-slip faults are depicted at their time of maximum development, which regionally varies from Early Cretaceous to Pafeogene. After Bosworth &XX?), Cuiraud and Maurin (19921, Genik (1992) and Taha (1992).

W. Bosworth, C.K. Morley / Tectonophysics 236 (1994) 93-115

decreases from about 35 km beneath the central Kenyan rift axis, to roughly 20 km at Turkana (Mechie et al., 1994). The present Moho surface can therefore be interpreted to represent the cumulative effects of young rifting superimposed

95

on the previously thinned crust in the north and “un-thinned” crust in the south (Morley et al., 1992; Morley, 1994). The picture in the northern Kermyan rift is further complicated by a cross-cutting Mesozoic

Fig. 2. Reflection seismic coverage and well control in the Anza Rift. Locations of seismic profiles shown in figudes are given by bolder lines.

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basin, the Anza rift (Reeves et al., 1987; Bosworth, 1992). The Anza rift is a component of a continent-scale rift system that extends from the Sirte basin in Libya, through the Mesozoic basins of Chad and Sudan, to the Lamu embayment in southern Kenya (Fig. 1). Reflection seismic data indicate that the Sudan rifts extend at least as far south as Mongalla, which has the characteristics of a pull-apart basin (Bosworth, 1992). Seismic data and exploratory drilling that will be discussed in this paper demonstrate that the Anza rift extends beneath and west of the Mt. Marsabit Neogene volcanic cone (Fig. 2). Mesozoic syn-rift sediments may be present beneath the Omo delta at the northern end of Lake Turkana, but on the western shore of the lake, young footwall uplift has exposed the basement complex and no record of Mesozoic rifting remains. Although the exact nature of the connection between the Anza rift and the South Sudan rifts is unknown, analogy with other segments of the rift system and the geometry of the Mongalla basin suggest that it is probably linked by a large-scale transcurrent fault located near the Kenya/Sudan border (Fig. 1; Bosworth, 1992). Following the discovery of hydr~arbons in the Muglad and Melut basins of the South Sudan rifts (Schull, 19881, several consortia of oil companies intensified exploration efforts in the related Mesozoic and Early Tertiary rift basins of western, central, and eastern Kenya. Reflection seismic data and a limited number of exploratory wells therefore became available for the stratigraphic and structural interpretation of the Anza rift. Prior to this time, the rift was only known from interpretation of potential-field data, shallow boreholes and a few very small outcrops of Jurassic, Cretaceous and Miocene strata (Reeves et al., 1987). We will present examples of the new seismic data, and will outline the structural and stratigraphic/ paleoenvironmental evolution of the Anza rift as it is presently understood. A more detailed description of lithofacies encountered during drilling is presented elsewhere (Winn et al., 1993). Based on the interpretation of the Anza rift and the younger Oligocene Southwestern Turkana basins, we will comment on how continental rifts in general evolve, and on the

236 (1994) 93-I I5

significance of this to the East African rift system.

2. Regional geology The Anza rift is divided into two physiographic provinces by the large volcanic cone at Mt. Marsabit (Fig. 2). Northwest of Marsabit, in the area of the Chalbi Desert (Fig. 31, much of the Mesozoic structure is concealed beneath Late Tertiary/ Recent volcanic rocks (Charsley, 1987; Key, 1987; Greene et al., 1990, and seismic acquisition is difficult. Some outcrops of Miocene sandstone of the late rift-fill are present in this area. Southeast of Marsabit the surface basalt flows taper to zero, and are replaced by Quaternary sediments underlying generally flat bushland. Seismic data in this area is greatly improved, although occasional shallow basalt flows are still present in the subsurface in some areas. A few scattered outcrops of Jurassic and Cretaceous sediments are present in the areas north and southeast of Marsabit, largely in the vicinity of the Matasade Horst (Figs. 2, 3). East of Matasade, the Anza rift turns south and joins the broad Lamu emba~ent, which continues south to the Indian Ocean passive continental margin. The southernmost Anza rift is an area where basins of several different ages intersect and are superimposed. The oldest basins, such as the Mandera Lugh, are of Karoo age (Carboniferous(?)/Triassic) and comprise an older phase of continental rift development, which is capped by an extensive continental sag basin of PermoTriassic age. Subsequent basins have buried the Karoo to at least 6 or 7 km in places. The Jurassic is marked by a marine incursion related to another phase of continental rifting, which finally led to the separation of Madagascar from East Africa in the Middle Jurassic. A triple junction may have been formed, with the N-S-trending failed arm running through the present-day Lamu emba~ent and the southern Anza rift (Reeves et al., 1987; Greene et al., 1991). The main evidence for this is the occurrence of Jurassic shallow water carbonates on the Matasade Horst (Figs. 2, 3). As the age of these carbonates are

W. Boworth, C.L Morley / Tectonophysics 236 (1994) 93-l 15

SLICKENLINES

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Fig. 3. Mesozoic outcrops in the area of the Anza Rift. Only the limestones at Matasade have been paleontologically dated (probable Bathonian to Oxfordian). The outcrops rimming the Chalbi Desert are Miocene fluvial sandstones. The stereograms at Bule Burgaba are lower hemisphere, Schmidt net projections of slickenlines and poles to small-scale faults, contoured at 7% and 16% (black) per 1% area. The rose diagrams at Matasade and the Chalbi Desert show strike of joints.

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variously considered Middle or Late Jurassic (discussed below), their exact relationship to Middle Jurassic rifting is uncertain. During the Cretaceous, continental sedimentation dominated in most of the Anza basin. At least one marine incursion occurred, as shown by the Cenomanian section in the N’dovu-1 well and discussed below. The Sirius-l well west of Mt. Marsabit suggests that rifting was initiated in the northern Anza no later than the Early Cretaceous (Neocomian). In the central and southern Anza rift only Late Cretaceous sediments have been reached by drilling, due to the thickening of the Upper Cretaceous and Tertiary section toward the southeast. Rifting continued in the Anza rift during the Tertiary, and a thick sequence (locally in excess of 3000 m) of Paleocene-Miocene fluvial and lacustrine sediments was deposited. During the Early Miocene, a marine transgression deposited carbonates just southeast of the Anza rift at the Wal-Merer-1 well. This marine sequence is capped by Miocene-Quaternary coarse fluvial deposits. A shallow unconformity (300-1000 m), angular in the south but locally parallel in the north and central regions, separates syn-rift Anza strata from the youngest flat lying post-rift deposits of Late Miocene-Recent age.

3. Reflection seismic data The location of seismic data used in our interpretation of the Anza rift is shown in Fig. 2. Most of the data is 60-fold Vibroseis 2 utilizing four vibrators, although experimental dynamite lines were also acquired in attempts to penetrate shallow basalt layers. Fig. 4 is a regional line through the central Anza basin southeast of Mt. Marsabit, crossing the N’dovu-1 well. The weil was drilled near the crest of a gently NE-dipping fault block, and penetrated more than 4000 m of a Campanian to Early Tertiary section. Seismic data qual-

’ Vibroseis is a registered trademark of Conoco Inc.

236 (1994) 93-115

ity in the Anza rift south of Matasade is generally good, although lines located southeast of N’dovu1, as at the Hothori-1 location (Fig. 51, show a strong reflection at two-way travel times of about 2.5-4.5 s (approx. 4.5-7 km), and little else below (see also Bosworth, 1992, fig. 9). The cause of this reflection is interpreted to be a large diabase sill, although no wells have been drilled deep enough to confirm this. In general, the “sill” reflection cuts across some faults, and is offset by others. Some faults are interpreted to have more offset in the lower Tertiary section than at the inferred sill. These relationships suggest that the possible sill is syntectonic in origin and of Early Tertiary age. Basalts encountered during drilling of the Sirius-l and Bellatrix-1 wells west of Mt. Marsabit (Fig. 2) have been dated by K-Ar whole-rock analyses as early Neocomian (130 f 7.0 Ma), Campanian (82.9 + 4.1 Ma) and Late Miocene (5.6 f~ 1.9 Ma). Late Cretaceous volcanism is known to have been relatively limited, based on the large number of wells that reach this part of the stratigraphic section. The area1 extent of Early Cretaceous surface volcanism, however, is unknown. The Matasade Horst separates the main body of the Anza rift from the shallower, parallel Kaisut basin (Fig. 2). Figs. 6 and 7 are representative of data from the Kaisut basin. Fault blocks are strongly rotated, by up to about 45” when converted to depth. Several well-defined unconformities are present in the seismic data. The Kaisut-1 well reached the uppermost strata in the seismic package below the number 2 unconformity (Fig. 6), which is roughly the Tertiary/Cretaceous boundary. Unconformity 4 at the top of the synrift section, is undated at Kaisut but is believed to be Miocene to Recent. On seismic line C in Fig. 6, the post-unconformity 2 seismic sequence thickens to the southwest. The lower sequence thickens to the northeast. Between these two periods of rifting, the main border fault flipped from the northeast side of the basin (at Matasade) to the southwest. This corresponds to a structural reorganization in which accommodation zones were eliminated and the overall geometry of Kaisut was simplified (see details in Bosworth, 1992).

W.Boswurth,C.K Modey / Tectomphysies236 (1994) 93-115

Extension estimates for Kaisut are very high, between 43 and 86% on the modelled sections in Fig. 6 (13= 1.43 to 1.8% Bosworth, 1992). This extension and a~ompanyiug crustal thinning resuited in isostatically driven upiift following the first phase of rifting, and bevelhng of several kilometers of sediment, perhaps during the Aptian-AIbian as discussed below. Much of this reworked Kaisut sediment may have been deposited in nearby depocenters of the main Anza trough. This is suppled by the presence of Early Cretaceous (Aptian-Albian) palynomorphs within the Late Cretaceous sections of the Dumaf and N’dovu-1 weifs (J. Steiumetz, pers. commun., 1989). Rifting and subsidence occurred in many of the Central and East African “Mesozoic” rift basins in the latest Cretaceous and Early Tertiary, as was the case in the Anza basin. This was, however, also a period of wrench-faulting and basin inversion. Structures of this type are restricted to specific basins, but occur sporadically both in outcrop and the subsurface from western Somalia to north-central Sudan (Bosworth, 1992). The hanging-wall strata above the normal fault on the southwestern end of Fig. 7 are interpreted to have been folded during Late CretaceousJ Early Tertiary wrenching, with most deformation occurring between the time of unconformities 3 and 4. Large faulted anticlines are also evident in Figs, 4 and 5, such as the Hothori structure. The Campanian section at Hothori is known to contain over-pressured shales, and shale diapirism may be responsible for the anticlin~l structures. Alternatively, wrenching may have initiated and localized the diapirism. Lack of Tertiary units in outcrop, and insufficient paleonto~ogic control in the subsurface, prevent constraining the age of the strike-slip defo~ation more tightly than Early Tertiary.

4. Implications of Mesozoic surface outcrops Outcrops of pre-Miocene sedimentary rocks in the vicinity of the Anza rift basin are limited to a few exposures southeast of Mt. Marsabit, and one

99

small hill north of Marsabit. Sedimentary outcrops in the Chalbi Desert west of Marsabit, formerly thought to be Upper Jurassic-Cretaceous (Geological Map of Kenya, 1987) are now believed to be of Miocene age. The largest pre-Miocene exposures are at Matasade, on a horst block between the Kaisut basin and the central Anza trough [Figs. 2, 3). This area was mapped by R.G. Dodtson, and is described in an unpublished report (Kenya Geological Survey Report No. 93). Dodsion reported the occurrence of Iimestones contain&g probable Oxfordian fossils, capped by quartzitic sandstone. Mapping by us and colleagues (P. Bouisset and MS-C. Bernet-Rollande, pers. commun., 1985) suggests that an unconfo~i~ exists between the limestones and the sandstone, which may therefore be considerably younger than Late Jurassic. The limestones include reefal material, coquinas and ooiite beds. and are therefore dominantly shallow marine in origin. Reexamination of the fossil remains indicates that they may be as old as Bathonian, but these results are not definitive. South of Matasade, reddish brotin sandstone with red shale and siltstone is found in exposures at Dogogicha (Fig. 3). Similar sands~ne and minor mudstone outcrop in several smaft hills at Kubi Dakhara, near Merti (Fig. 3). Matheson (19711 interpreted these rocks to be of probable Jurassic age, based on their proximity to Matasade. Field relationships suggest that they are at least older than the Late Tertiary Merti beds (Matheson, 1971). As none of the Kubi Dakhara or Dogogicha lithologies have yielded fossils, and in light of the broad 8ge range of similar units now known from the subsurface, the Jurassic age assignment must be treated cautiousty. North of Mt. Marsabit, a small hill of reddish quartz sandstone and pebbly sandstone outcrops at Bule Burgaba (Fig. 3). The sandstone is cut by numerous small-scale normal faults. Analysis of slickenlines and fault planes gives an average extension direction of N35”E. The Geological Map of Kenya (1987) and an unpublished report by R.G. Dodson and F.J. Matheson (Kenya Geological Survey Report No. 94) list these sandstones as Upper Jurassic-Cretaceous and Juras-

sic(?), respectively, but they are also without fossil control. The supposed Mesozoic sedimentary outcrops of the Anza region would offer more help in paleoenvironmental and tectonic interpretations if their ages could be more precisely defined. Until this occurs, only a few very general statements can be made. The marine limestone outcrops at Matasade demonstrate the presence of a seaway connection, probably in the late Middle to early Late Jurassic. Given the final separation of Madagascar and India from East Africa by 157 Ma (Veevers et al., 1980; Rabinowitz et al., 1983), the interpretation that the carbonates reflect lingering marine conditions along the failed arm of a triple junction (Reeves et al, 1987; Greene et al., 1991) appears reasonable. The Matasade limestones are located on the footwall of the border fault to the southern Kaisut basin. During periods of active rifting, Matasade should have experienced uplift and erosion. It seems unlikely, therefore, that the Kaisut basin was actively extending during deposition of the Matasade limestones.

5. Basin stratigraphy Our stratigraphic analysis of the Anza rift is based on the results of seven wells, complemented by data from the northernmost Lamu embayment (Figs. 1, 2). The stratigraphic sequence and corresponding tectonic events are summarized in Fig. 8. It must be emphasized that crystalline basement has not been reached by any of the wells drilled in the Anza basin. Four wells have been drilled in the main depocenter of the central and southeastern Anza rift. AI1 four reached total depth in Late Cretaceous sediments, ranging from Cenomanian to Maastrichtian in age. Hothori-1 penetrated the thickest section of the Anza wells, with a total depth of 4390 m. The lower 1200 m are assigned to the Campanian, and consist of immature, often red-colored sandstone, mudstone and shale, and dark gray lacustrine shale with thin sandstone beds. These are overlain by several packages of sandstone and siltstone, separated by minor dis-

conformities or unconformities, all of probable Early Tertiary age. The uppermost 600 m of sandstone and mudstone correspond to the unfaulted seismic sequence in Fig. 5 and are Upper Miocene to Recent. N’dovu-1, drilled northwest of Hothori-I (Fig. 21, encountered a much thinner Tertiary section. Palynological data suggest that the top of the Early Maastrichtian/ Campanian is no deeper than 940 m. The section above this is undated, and there is no significant lithologic break at this point (R. Winn, pers. commun., 1991). The postrift strata at N’dovu correspond to 0.3 s on the seismic data (Fig. 4), or about 330 m. If these strata are Miocene and younger, then the Early Tertiary section is 600 m or less thick. The lower part of the thick Upper Cretaceous section at N’dovu-1 is of probable Cenomanian age. This section consists dominantly of medium gray to black shales that contain rare dinoflagellams. The dinoflagellates may be present due to contamination from drilling muds (R. Winn, pers. commun., 1991). However, based on the lithologic association, and the frequency of marine incursions in the nearby Lamu embayment, we interpret the Cenomanian at N’dovu-1 as deep-water marine, grading up section to brackish and lacustrine conditions. The Campanian/ Maastrichtian section at N’dovu is composed of siltstone, shale and generally immature, poorly sorted sandstone. Duma-1 was drilled north of Matasade on the western flank of the main Anza trough (Fig. 2). The well reached a total depth of 3333 m in Campanian shale, siltstone and poorly sorted sandstone. The upper Campanian/ Maastrichtian section contained rare dinoflagellates that may represent a second period of Late Cretaceous marine invasion or brackish conditions. The Cretaceous is capped by approximately 300 m of Tertiary sandstone and conglomerate, probably of Miocene age and younger. The Early Tertiary appears to be absent at Duma. Northwest of Mt. Marsabit, the Tertiary and Late Cretaceous sections are relatively thin, and older sediments are within economic drilling depths. At Sirius-l, the base of the penetrated sedimentary section consists of 56 m of limestone, capped by approximately 150 m of well-sorted

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H? Bosworth,C.K Morley/ Tectonophysics236 (1994) 93-115

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SOUTH lb) Fig. 6. Seismic lines crossing southern Kaisut basin. (a) Migrated data, line C. (b) Structural interpretations, links A, B, and C. Unconformities are indicated by bold lines and number. Based on the results of Kaisut-1, Unconformity 2 is adproximately the Cretaceous/Tertiary boundary. Unconformity 4 is at the top of the syn-rift section. Locations of lines are given id Fig. 2.

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W. Boswortk, C.K h4orley/ Tectorwphysics 236 (1994) 93-l 15

Fig. 8. Composite stratigraphic section based on wells drilled in the Anza Rift.

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236 (1994) 93-115

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W. Boaworth, C.K. Morley / Tectonophysics 236 (1994) 93-115

Fig. 9. Paleogeographic reconstructions of the Anza Rift. Only wells with information pertinent to the specific time interval are located on the reconstructions. The basin was dominated by siliciclastic sedimentation during the Cretaceous except for some carbonates encountered in the Sirius-l well during the Neocomian. Details are provided in the text.

sandstone. These units are believed to be of Neocomian to Barremian age. This section is overlain by Cenomanian and younger shale, sandstone and siltstone. Aptian-Albian sediments are believed to be eroded from the Sirius-l section. Bellatrix-l was drilled in a structurally lower position than Sirius, and reached total depth in the Cenomanian. The Cenomanian here included some coal beds. The top of the Cretaceous section, of probable Maastrichtian age, consists of sandstone red beds and conglomerate at Sirius and Bellatrix. The total Late Cretaceous section at Sirius-l is about 1400 m thick, and increases to at least 1800 m at Bellatrix-1. The post-cretaceous section in this northwestern area consists of a thick undated basal sandstone (750 m at Sirius, 1325 m at Bellatrix), 180-200 m of Miocene basalt, and 40-60 m of younger sand and gravel. In the Kaisut basin, which is roughly on strike with the Sirius and Bellatrix wells, the Kaisut-1

well reached a total depth of 1450 m in Early Tertiary or latest Cretaceous quartzitk: sandstone and minor indurated shale. Tieing this well to seismic data, however, indicates that a section of steeply rotated, high-velocity sedimemts lies beneath the well (Figs. 6, 7). This loklter section probably correlates in part with the Senonian strata at Sirius-l. Alternatively, some of these high-velocity sediments may be older, highly indurated Aptian-Albian or Jurassic strata.

6. Pal~n~~nrnen~l evolution Given the Anza extremely grids are with the

inte~~~tio~

and basin

the size and structural complexities of rift, well control must bq considered sparse (Fig. 2). The existing seismic sufficient to delineate fault patterns, exception of the Mt. Marsabit region

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and for two-way travel times greater than that of the regional “sill” reflection (Figs. 4, 5). Uncertainties in the timing of faulting in different parts of the basin reflect the position with respect to the nearest wells, quality of microfossil picks in these wells, and degree of lateral continuity of the main seismic packages. Accepting these limitations, a paleoenvironmental interpretation for the Anza rift, starting with the oldest known sediments at Sirius-l, is presented in Fig. 9. A more accurate understanding of the evolution of this basin must await drilling to depths beyond those of economic interest.

236 (1994) 93-115

worked Aptian-Albian palynomorphs in Late Cretaceous strata (N’dovu-1, Duma-1, Bellatrix-1, Sirius-l) supports this interpretation. 6.3. Late Albiad?) Renewed faulting in the Anza rift resulted in at least local footwall uplift and erosion of the older rift-fill, as was the case at the Sirius-l location. This may have been the culmination of a period of uplift along the southwestern flank of the rift, from Sirius to Kaisut. 6.4. Cenomanian

6.1. Neocomian The Neocomian section has only been penetrated by Sirius-l, in the western Anza rift (Fig. 2). The oldest strata at Sirius-l are carbonates of probable lacustrine origin, overlain by eolian sandstone. The well was probably drilled near a lake edge that receded and permitted the transgression of dunes (D. Stone, pers. commun., 1989). The Kaisut basin, along strike to the southeast, may also have been active in the Neocomian. 6.2. Aptian -Albian The Neocomian sediments in the Sirius-l well are unconformably overlain by Upper Cretaceous strata. The Aptian-Albian section is, therefore, not known from any part of the Anza rift proper. In the northern Lamu embayment, at Wal-Merer1, the Aptian-Albian is represented by marine strata. The northern extent of marine conditions is not known, but it is possible that some of the Anza trough may contain a similar section, The Cretaceous rifts in southern Sudan were sites of lacustrine sedimentation in the Albian-Aptian, resulting in widespread deposition of source rocks (Schull, 1988). Oil shows were encountered in Sirius-l, and it is possible that Aptian-Albian lacustrine source rocks may be present down-dip or in other undrilled deep parts of Anza. Based on the history of the Lamu embayment, Bosworth (1992) inferred that the Aptian-Albian was a period of rifting in at least the eastern Anza province. The widespread occurrence of re-

The central Anza basin contains several hundred meters of Cenomanian(?) deep marine shale, as at N’dovu-1. The rift was therefore open to the Indian Ocean via the Lamu embayment. in the northwest, the Cenomanian rift-fill sequence is characterized by coarse elastic fluvial deposits. Input of the elastics is believed to have occurred dominantly by transport from the northwest along the axis of the basin. 6.5. Campanian Coarse fluvial sandstone, overbank deposits and episodic lacustrine shales occurred throughout most of the Anza rift during the Campanian. In the central Anza basin (N’dovu-1 and Duma-0, reworked Early Cretaceous sediments, perhaps derived from erosion of the Kaisut basin, were deposited. In the southeast, I-&hori-1 penetrated more than 300 m of a Lower Camp&an lacustrine section, and, based on seismic interpretation, the overall Campanian thickness increases significantly in this direction. 6.6. Maastrichtian Microfossils and traces of anhydrite at Duma-1 suggest that a brief marine incursion may have occurred in the Maastrichtian. Coarse fluvial clastic deposits continued to dominate the rift system, and fault activity was reduced. This resulted in the burial of rift structures (transfer faults, accommodation zones), and the development of a through-going axial drainage system.

W. Boworth, C.K Morley / Tectonophysics 236 (1994) 93-115

6.7. Terti~~ The Tertiary history of the Anza rift was complex and displays considerable along-strike variation. Lower Tertiary deposits are unknown from the area northwest of Mt. Marsabit, vary from 0 to 600 m east of Marsabit, and thicken to over 3000 m in the southeast. In the northwest and southeast parts of the rift, Miocene age units reach 600 m thickness, while in the central rift they are commonly less than 300 m. Throughout the Tertiary, deposits are predominantly coarse fluvial elastics interspersed with overbank and episodic lacustrine deposits. There is one intriguing piece of evidence that suggests that during the Miocene, the Southwestern Turkana basins of the Neogene Kenyan rift were linked to the coast, most probably through the Anza basin and Lamu embayment. This evidence is a late Burdigalian fossil marine whale that was found in the Turkana fluvial grits near Loperot (Mead, 1975). Mead speculated that the whale strayed into a fluvial system that ran into the sea some distance east of the Turkana area. The river system had to be substantial because the whale was about 6 m long. Lower Miocene carbonates 600 km to the southeast in the WalMerer-1 well define roughly the maximum distance the whale had to traverse in fresh or brackish waters to reach its final destination. Sometime following deposition of the Late Cretaceous Marehan Sandstone in eastern Kenya, the eastern Anza province underwent strike-slip faulting, associated folding and minor basin inversion. The detailed timing and kinematics of this deformation are not known. The final geologic activity in the area has been related to development of the Neogene East African rift system, with outpourings of flood basal& and building of the Marsabit cone (Key, 1987).

7. Synthesis and rift model The Anza rift began ac~m~ating continental lacustrine and fluvial/eolian sediments by Neocomian times. Reflections evident on seismic pro-

111

files, but deeper than existing well control, allow for the possibility that parts of the Anza basin may have been actively extending and receiving syn-rift sediments in the Jurassic (Greene et al., 1991). The presence of Upper Jurassic strata within the Lamu emba~ent at least as far north as the Garissa-1 well support this interpretation (Coffin and Rabinowitz, 1988). The Aptian/ Albian was a period of’ significant extension and subsidence in the South Sudan rifts (Schull, 1988) and the Lamu embaymcnt (Coffin and Rabinowitz, 1988). Strata of this age have, thus far, not been reached in the central and southeastern Anza rift and, in at least ,one well in the northwest, were eroded or are missing through non-deposition. By the Cenomanian, most of the Anza basin was active, and marine waters had reached the area east of Mt. Marsabit. The thickest rift-fill recorded by drilling was &posited in the Campanian, represented by more than 3000 m of fluvial, deltaic and lacustrine shale, siltstone and sandstone. This period of rifting continued to the Maastrichtian in the central Anza rift, and into the Earty Tertiary in the southeast part of the Anza rift and the northern Lamu ambayment. Most of the basin is capped by a few hundred meters of unfaulted Miocene to Recent sandstone, conglomerate and basalt, or by the thick volcanic cone at Mt. Marsabit. The structural evolution of the Anza rift can be summarized by a three-step model (Fig. 10). Early basin geometries are strongly asymmetric, with half-graben cross-sections that change direction of tilt episodically or periodically along strike. Strata display large rotations and are cut by moderately to shallowly dipping fauits. In the Anza basin, these early structures are only visible in the shallow basins northwest of Mt. Marsabit and at Kaisut, and were active in the Early Cretaceous and possibly the Jurassic. Following this early phase of complex faulting, basin-bounding faults gradually coalesced and eliminated or minimized the alternating half-graben geometry. This resulted in large, more laterally continuous basins. This process of fault and basin reorganization was not complete at Kaisut until the 1 test Cretaceous or Early Tertiary, and theref Bre involved several episodes of rifting (Fig. 8).

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Morfey / Teclonophysics 236 (1994) 93-115

The asymmetric geometry of early phases of rift development suggests that thinning of the upper crust may be offset laterally from thinning at the base of the crust or base of the lithosphere (Wernicke, 1981; Bally, 1982). If this inference is correct, then late thermal subsidence could similarly be offset from early, structurally controlled basins. The third phase of rifting would then occur as basins with a high ratio of subsidence to extension, shifted away from the early basins and farther within the upper plate of phase 2 (Fig. 10). The main Anza basin of the central Anza rift (Fig. 3) appears to occupy this position during the Late Cretaceous and Early Tertiary. Basins of this type are bounded by high-angle, largely nonrotational faults such as the Lagh 3oga1, which accommodate great volumes of sediment with only minor crustal extension.

Some aspects of the structural style and evolutionary sequence we have described for the Anza rift have been observed in other active and ancient rift systems. Reorganization of early rift structures has been recognized in the East African rift system. Even in the low-extension Tanganyika rift, the early random half-graben arrangement appears to be reorganizing in favor of a dominant west-side, east-dipping bounda~-fault system (Morley, 1988; Bosworth, 1989). Thus in the future, a single dip province about 700 km long may emerge. In the northern Kenyan rift there are also suggestions from seismic lines (e.g., fig. 7 in Morley et al., 1992) that earlier, more random Sunday-fault orientations later gave way to a dominant east-dipping orientation that extends about 450 km along strike. Bosworth (1987) suggested that linking of east-dipping boundary-faults may also be occurring in the central Kenyan rift. Recent field work by Strecker et al. (1990), however, indicates that the N~a~a-N~ur~ subbasin may be countering this regional trend, with a late shift to a west-dipping boundary-fault. This discussion illustrates a potential probIem with interpreting rifting kinematics and mecha-

nisms based solely on general basin geometries, as might be interpreted from potential-field data. Our interpretation of rift evolution suggests that asymmetric processes, i.e., half-graben formation, commonly lead to symmetric bulk geometries (full-graben), due to temporal changes in fault kinematics. This process can affect entire basins, not just the points of overlap of bounding faults at a~ommodation zones as has been previously documented (Bosworth, 1985; Rosendahl, 1987; Morley, 1988). The relative roles of low- and high-angle faults may vary from rift to rift, not only due to evolutionary processes as we have discussed for the Anza rift, but also according to physical properties of the lithosphere at the time of rift initiation. Morley (1988, 1989), for example, described the western branch of the East African rift as being dominated by high-angle, high-subsidence/ low-rotation boundary-faults, while the northern Kenyan rift displayed relatively low-angle, high-e~ension bounda~-faults. There appeared to be some correlation between heat flow and the boundary-fault angle, the lower angle faults being associated with higher heat flows. The linkage between the thermo-mechanical properties of the lithosphere and fault orientation may ultimately be related to strain rate (e.g., Kusznir and Park, 1987). Pre-existing structures and fabrics play similarly important roles, as for example in the Triassic/ Jurassic rifts of the eastern US, where shallow-dipping Paleozoic thrust faults control the orientation of many border faults (Ratcliffe and Burton, 1985; Ratcliffe et al., 1986). Smith and Mosley (1993) and Hetzel and Strecker (1993) have suggested that the orientation of major faults in the central Kenyan rift are also controlled by pre-existing structure, specific~ly Late Proterozoic to Early Paleozoic shear zones and the boundary between the Archean Tanzania Craton and the Mozambique mobile belt. The reflection seismic, well and outcrop data available for north-central Kenya have, thus far, not demonstrated any pronounced control of Neogene structures by the older Anza rift. Perhaps this is not surprising, given the apparent difference in extension directions for the two systems. However, one first-order feature of the

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W, Bosworth, C. K Morley / Tectonophysics 236 (I 994) 93-l 15

a

COMPLEX

SUB-BASINS

CCOMMODATIO

50

I

MANTLE

UTHOSPHERE

I-

-

_

KM

b

BASIN

c

ISOSTATIC THERMAL

REORGANIZATION

UPLIFT SAG

POLARITY

REVERSALS

ABANDONED

Cr

BAS

Fig. 10. Stages in the kinematic evolution of continental rifts, based on the Anza model: Rifts may initiate as gentle sags, but very quickly develop asymmetric half-graben cross-sections. This is attributed to movement on a deta~bment system, or zone of decoupling within the middle to lower crust. The polarity of rift segments in many cases alternates along strike, as in the earIy history of the Kaisut basin. At higher strains, this mechanically inefficient arrangement of alternating border fauhs reorganizes, with elimination of faults that dip in one direction and linking of border faults of the same polarity. This process occurred during the intermediate phases of rifting at Kaisut. Crustal thinning leads to &static uplift and development of regional utrconformities in the final stages of rifting. Uplift results in abandonment of detachments and either late-stage high-angle normal faulting or formation of new detachments with new breakaways along the old rift axis. Late thermalty driven subsidence is offset toward the hanging wall plate due to the regional simple shear distribution of crustal thinning. The late, relatively nonrotation~l subsidence of the Anza basin proper is inferred to occupy such a position relative to Kaisut, and the Turkana basin of the young Kbnyan Rift may be, in part, an analogous response to Oligocene/Early Miocene rifting in basins to the west (Southwestern Turkana rifts).

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East African rift system, the presence of the Kenya and Ethiopia “domes”, may be related to the pre-existing Mesozoic rift. The topographic saddle between these two domes is centered on Lake Turkana, which corresponds to the area where the Anza and Kenyan rifts cross. This relationship may be coincidental, or it may reflect a lithosphere- or mantle-scale structure that warrants further examination by the techniques utilized in KRISP.

Acknowledgements

A large group of people contributed to the work on the Anza rift, both within our own companies and others. We would especialiy like to thank Marie-Claire Bernet-Rollande, Rich Bosher, Patrick Bouisset, Ron Day, Bob Harper, William Kerekgyarto, Joseph Lambiase, And& Maurin, Ron Nelson, Denise Stone, Bill Wescott, Robert Winn and Gregory Wood for their comments and suggestions. Comments by R.A. Johnson and an anonymous reviewer helped improve the manuscript. We thank members of the Kenya National Oil Company for their assistance in Nairobi, and Amoco and Marathon for permission to publish this paper.

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