Sedimentology and compositional characteristics of siliciclastic and associated sediments in Ruvu basin: implication on paleo-depositional environment, provenance, and tectonic setting

ABSTRACT

The paleo-depositional environment of the Ruvu basin ranges from alluvial, floodplain, low- and high-energy fluvial channels, and deltaic sub-environments of Ngerengere beds, Tanga beds, and Jurassic Msata formations. Amboni limestones that overlap the Karoo sequences, Bagamoyo Formation and Sakura Formation were probably deposited in a shallow marine environment. Trace elements show that the sediments were deposited in oxic environments with a V/Cr ratio of 0.85 and Ni/Co ratio of 2.9 to anoxic environments with a V/Cr ratio >4.3. Low Cr, Ni, Co, and V, high Y/Ni and Zr/Cr ratios of up to 0.83 and 3.6, respectively, and low Ti/Zr ratios ranging from 0.004 to 0.01 indicate a contribution from a felsic source, and few ratios >1 show some contribution from a mafic source. This is supported by the diverse enrichment in light rare earth elements (LREEs), small negative Eu anomalies and modest heavy REEs reflecting a dominantly granitic source. The provenance is probably from the Usagaran mobile belt and Proterozoic Mozambique mobile belt. The cross-plot between Th and Ta from mid-Jurassic to Cretaceous sediments indicates that the basin was essentially developed as a passive continental margin; however, discriminant factor plots of the Karoo indicate the existence of a rift setting.

KEYWORDS:

• Sedimentology
• depositional environment
• tectonic setting
• provenance
• discriminant factor
• rift setting

1. Introduction

The Ruvu Basin is a rift basin, formed in the Permo-Triassic to early Jurassic in relating to the break-up of Gondwanaland (Delvaux, 2001; Mpanda, 1997). The basin is characterized by two major lithological successions; the first succession comprises the siliciclastic sediments, while the second succession composed of chemical to biochemical sedimentary rocks (Kent, 1954; Kent et al., 1971; Kent & Pyre, 1973). Some information has been reported on lithological characteristics of some formations including Karoo Formation (Figure 1). Detailed interpretation of the sedimentological data obtained from lithological logging of the core/cutting samples and outcrops enable the interpretation of the depositional environments (Li et al., 2020), from which the sediments were deposited. To infer the source of sediments that contributed to the siliciclastic lithologies, sediment geochemistry is used as an important tool to pinpoint the source of the sediments (Das & Haake, 2003; Moosavirad et al., 2011) and the physio-chemical condition of the depositional environments. Also, based on available information, it has not been clearly explained if both lithological successions were deposited in the same tectonic setting; using geochemical plots, it is possible to understand the tectonic setting (Das & Haake, 2003; Feng & Kerrich, 1990; McLennan & Taylor, 1991; Moosavirad et al., 2011) that leads to the depositional environments of the two major lithological successions.

Figure 1. Geological map of Ruvu basin (modified after Kent, 1954).

The Ruvu basin structural development and basinal characteristics are very similar to the Gulf of Suez, Benue Trough (Benkhelil, 1989; Nwachukwu, 1972; Ofoegbu, 1984; Olade, 1975) and the Viking Graben (Badley et al., 1988; Beach et al., 1987; Marrett & Allmendinger, 1992), for example, all of which are major rift-related hydrocarbon provinces. Despite similarities with rift-related hydrocarbon provinces elsewhere, only two deep wells – Makarawe-1 and Kiwangwa-1 – have been drilled with hydrocarbon shows (Bofin & Pedersen, 2017; Kajato, 1986; Kidston et al., 1997; Komba et al., 2015; Zongying et al., 2013). Both wells were dry holes, with the presence of hydrocarbon shows at Makarawe-1 (Mahanjane, 2014; Mbede, 1991; Zongying et al., 2013). Being a potential basin for hydrocarbon exploration and a missing link on paleo-physio-chemical environments and tectonic setting, the study focus on interpretation of sedimentological data and geochemical analysis of outcrops, borehole core, and cuttings as an important tool in understanding depositional environment, lithic characteristics of the rocks, physio-chemical condition of the paleo-environment, and tectonic setting, which are important factors during characterization of the source rock, reservoir and seal rock as an important element of the petroleum system.

2. Methodology

Borehole and outcrop logging data were used to synthesize the sedimentological and compositional characteristics. Lithological logs of Makarawe-1 and Kiwangwa-1 boreholes from unpublished drilling reports were used to extract lithic characteristic of the rocks and their stratigraphic relationship. 28 samples collected from different formation along the litho-column were analyzed by X-Ray Fluorescence (XRF). For XRF analyses, 12 g of each milled sample material were mixed with 3 g Lico wax and hard-pressed with the applied pressure of 25 tonne by a hydraulic press to form a powder briquette. The briquettes were analyzed by a PANalytical Magic-X Fast Simultaneous X-Ray Fluorescence spectrometer equipped with Rh-tube. Geochemical data obtained were analyzed using GCDKit 4.2 and TecSand software to infer the tectonic setting of the basin. Th v/s Ta was plotted using GCDKit 4.2, while the arc-rift-collision relationship was plotted using TecSand software based on discriminant Factor 1 and Factor 2 of higher silica. Selection of whether to apply discriminant factors of high silica or low silica approach was assumed based on a sedimentological description of the lithologies, and provenance information was obtained from the analysis of trace and REE elements. This is because some of the major elements results including %SO₂ were missing from available data. Also, Min Tab 17 statistical tool was used to analyze the trend of geochemical data.

3. Result and discussion

Description of the lithologies from core/cuttings and outcrops is presented below. During geochemical analysis 20 elements analyzed including Ba, Co, Cr, Rb, Sr, Ta, Ti, Th, Ti, U, V, Y, Zr, La, Pt, Nd, Eu, Yb, La, and Lu and their results are presented in Table 1 and their corresponding ratios were calculated and are presented in Table 2.

Table 1. Trace elements and rare earth elements (REE).

Sample ID	Ba	Co	Cr	Rb	Sr	Ta	Ti	Th	Ti	U	V	Y	Zr	La	Pt	Nd	Eu	Yb	Lu	Lu
S1	279	18.9	138	79.2	278	0.7	4300	6.6	0.45	2.9	113	24.3	93	31	7.8	28.5	1.44	3.1	0.4	0.49
S2	362	17.5	102	112	242	0.1	5500	10.9	0.59	3.2	129	26	107	45	10.9	38.6	1.74	1.9	0.3	0.35
S3	34	36	63	82.8	299	0.6	4000	8.2	0.46	2.1	89	20.4	76	29	7.4	27.9	1.26	1.8	0.3	0.42
S4	267	16.9	85	99.7	186	1.6	6100	11.3	0.45	2.8	145	26.1	99	41	10.3	38.8	1.71	2.5	0.4	0.53
S5	425	12.9	73	96.7	211	0.1	3500	10.4	0.47	2.2	89	14.3	89	32	6.9	25.6	1	1.5	0.2	0.36
S6	615	8.4	83	84.9	139	0.1	2600	9.1	0.4	2	57	22.6	107	32	9.7	38.8	1.93	2	0.3	0.39
S7	343	12.2	103	91.1	957	0.4	4100	12	0.6	2.9	93	12.5	59	26	6.6	24.6	0.92	1.6	0.2	0.24
S8	247	22	100	76.3	579	0.1	3700	13.2	0.66	1.9	93	10	47	19	5.1	19	0.69	1.3	0.2	0.21
S9	305	11.2	85	75.5	844	0.7	3700	11.2	0.57	2.5	95	12.9	56	26	6.7	24.7	0.94	1.5	0.2	0.27
S10	337	17.1	89	98.9	567	0.1	3600	10.8	0.6	2.2	89	12	60	29	7.2	26.2	0.92	1.3	0.2	0.1
S11	800	12.7	79	94	445	0.1	3100	9.6	0.46	2	69	14.5	94	32	7.6	27.8	1.05	2	0.2	0.38
S12	432	18.2	85	97.5	336	0.1	2400	12.6	0.57	2.8	69	15.7	71	36	8.3	29.7	1.1	1.9	0.2	0.27
S13	460	15.4	95	106	686	0.2	4100	13.1	0.55	2	97	11.4	56	27	6.5	23	0.82	2.3	0.2	0.35
S14	445	17.4	124	118	460	0.1	1900	12	0.58	2.1	67	18.5	52	36	8.5	31	1.21	2.4	0.3	0.5
S15	472	14.7	72	106	910	0.1	3300	13.7	0.51	2.6	88	15.9	52	39	9.2	32.5	1.12	1.7	0.2	0.41
S16	385	11.8	104	105	467	0.1	3800	14.7	0.55	2	106	15.4	53	32	7.6	27.7	1.04	2.6	0.3	0.31
S17	446	22.9	158	122	246	0.1	2800	16	0.65	2.1	74	17.6	68	45	10.4	37.1	1.27	2.8	0.3	0.44
S18	254	11.6	100	75.9	355	0.5	3300	10.4	0.37	2.2	73	31.1	38	41	9.8	36.9	1.66	3.8	0.4	0.46
S19	458	16.7	130	122	392	0.1	3200	14.3	0.52	2.8	79	18.8	69	39	8.9	32	1.19	3.6	0.3	0.52
S20	321	14.6	135	58	527	0.8	3600	10.1	0.46	2.2	86	14.9	85	22	6	22.9	0.94	2.3	0.3	0.46
S21	437	18.3	113	125	331	0.5	4800	16	0.57	2.4	112	17.9	78	40	9.4	33.1	1.24	3.9	0.3	0.47
S22	275	20.9	132	111	122	0.2	5200	12.9	0.54	1.9	106	16.2	59	39	8.9	31.7	1.17	1.4	0.3	0.55
S23	309	17.3	113	92.1	610	0.1	2100	8.5	0.47	1.7	61	18	72	31	7.2	26.3	1.06	2.5	0.3	0.47
S24	371	22.3	143	105	444	0.1	2800	10.1	0.53	2.1	79	17	69	31	7.4	26.6	1.03	1.9	0.2	0.19
S25	280	17.9	117	86.6	713	0.1	1600	9.1	0.54	2.2	63	12.8	67	26	6.1	22.2	0.85	1.5	0.2	0.05
S26	306	18.8	145	89	486	0.1	1300	7.6	0.48	2	49	13.2	58	28	6.5	23.4	0.9	2.1	0.2	0.14
S27	282	25	113	89.9	723	0.1	1600	10.1	0.46	2	47	14.3	62	29	6.7	23.9	0.91	1.5	0.2	0.41
S28	437	15.7	138	105	427	0.2	3200	12.1	0.49	2.1	78	15.1	108	31	7.3	26.3	1.01	2	0.2	0.22

Table 2. Ratios of trace elements and rare earth elements (REE).

Sample ID	Zr/Ti	Nb/Y	Nb/Yb	Th/ Yb	La/Th	Zr/Rb	(Zr +Rb)/Sr	Zr/Cr	Ti/Zr	Y/Ni	Cr/Ni	Ta/Yb	Ba/Co	Sr/Ba	Rb/K	V/Cr	Ni/Co	Zr/Ti
S1	0.7	0.8	3.5	2.1	4.7	1.2	0.62	0.67	0.005	0.52	2.9	0.23	14.8	1	0.0048	0.8	2.49	0.022
S2	1.2	0.9	3.5	5.7	4.2	1	0.9	1.05	0.006	0.49	1.9	0.05	20.7	0.67	0.006	1.3	3.04	0.019
S3	1.1	0.7	4.9	4.6	4.3	0.9	0.53	1.21	0.006	0.29	0.9	0.33	0.94	8.79	0.0041	1.4	1.97	0.019
S4	1	0.8	5.2	4.5	4	1	1.07	1.16	0.005	0.57	1.9	0.64	15.8	0.7	0.0051	1.7	2.72	0.016
S5	1	0.6	0.5	6.9	3.4	0.9	0.88	1.22	0.005	0.47	2.4	0.07	33	0.5	0.0043	1.2	2.36	0.025
S6	1	0.8	0.1	4.6	4.2	1.3	1.38	1.29	0.004	0.92	3.4	0.05	73.2	0.23	0.0037	0.7	2.92	0.041
S7	0.8	0.8	4.9	7.5	3.5	0.6	0.16	0.57	0.01	0.23	1.9	0.25	28.1	2.79	0.0043	0.9	4.43	0.014
S8	0.8	1	1.7	10.2	3.2	0.6	0.21	0.47	0.014	0.15	1.5	0.08	11.2	2.34	0.0034	0.9	3.05	0.013
S9	0.9	0.7	6.3	7.5	3.4	0.7	0.16	0.66	0.01	0.28	1.9	0.47	27.2	2.77	0.0036	1.1	4.08	0.015
S10	0.9	2	1.4	8.3	3.6	0.6	0.28	0.67	0.01	0.24	1.8	0.08	19.7	1.68	0.0047	1	2.91	0.017
S11	0.7	0.5	0.3	4.8	3.6	1	0.42	1.19	0.005	0.43	2.4	0.05	63	0.56	0.0041	0.9	2.65	0.03
S12	0.7	0.7	0.2	6.6	3.3	0.7	0.5	0.84	0.008	0.36	2	0.05	23.7	0.78	0.0045	0.8	2.38	0.03
S13	0.6	0.6	3.4	5.7	3.3	0.5	0.24	0.59	0.01	0.26	2.2	0.09	29.9	1.49	0.0048	1	2.86	0.014
S14	0.8	0.6	0.1	5	4.3	0.4	0.37	0.42	0.011	0.35	2.4	0.04	25.6	1.03	0.0046	0.5	3.02	0.027
S15	0.9	0.5	1.6	8.1	3.2	0.5	0.17	0.72	0.01	0.37	1.7	0.06	32.1	1.93	0.0055	1.2	2.89	0.016
S16	0.6	1	1.2	5.7	3.7	0.5	0.34	0.51	0.01	0.32	2.1	0.04	32.6	1.21	0.0052	1	4.14	0.014
S17	0.6	0.7	0.1	5.7	3.5	0.6	0.77	0.43	0.01	0.28	2.5	0.04	19.5	0.55	0.006	0.5	2.72	0.024
S18	0.6	0.9	2	2.7	4.5	0.5	0.32	0.38	0.01	0.83	2.7	0.13	21.9	1.4	0.0063	0.7	3.22	0.012
S19	0.5	0.6	0.1	4	3.5	0.6	0.49	0.53	0.008	0.41	2.8	0.03	27.4	0.86	0.0058	0.6	2.74	0.022
S20	0.7	0.7	4.3	4.4	4.5	1.5	0.27	0.63	0.005	0.4	3.6	0.35	22	1.64	0.0032	0.6	2.54	0.024
S24	0.8	1.1	0.7	5.3	4.1	0.7	0.39	0.48	0.008	0.29	2.5	0.05	16.6	1.2	0.0044	0.6	2.6	0.025
S25	0.9	4	0.3	6.1	3.7	0.8	0.22	0.57	0.008	0.25	2.2	0.07	15.6	2.55	0.0045	0.5	2.92	0.042
S26	0.6	1.4	0.1	3.6	4.2	0.7	0.3	0.4	0.008	0.25	2.8	0.05	16.3	1.59	0.0045	0.3	2.78	0.045
S27	0.9	0.5	0.7	6.7	3.6	0.7	0.21	0.55	0.007	0.29	2.3	0.07	11.3	2.56	0.0048	0.4	1.94	0.039
S28	0.8	0.9	1.2	6.1	3.5	1	0.5	0.78	0.005	0.31	2.8	0.1	27.8	0.98	0.0042	0.6	3.09	0.034
AVE.	0.8	0.9	2	5.7	3.8	0.8	0.5	0.71	0.01	0.38	2.3	0.13	24.5	1.61	0.0047	0.9	2.9	0.021

3.1. Sedimentological analysis

Based on sedimentological results, six formation were encountered constituting clastic to non-clastic lithologies, these formation include, Sakura (Kiwangwa) formation, Kipatimu (Kipumpwe) formation, Bagamoyo formation, Amboni limestones, Msata formation, and the oldest karoo formation, which overlay on the metamorphic basement of the Mozambique mobile belt.

3.1.1. Sakura formation

This formation has unconformity from 0 to 35 m as observed at Makarawe-1 borehole where Neogene sediments overlay the Cenomanian sediments. From 35 m to 515 m, Marl sequence are medium grey to bluish-grey, soft, sticky, few pyrite cubes seen throughout this section. These marls were probably deposited in shallow shelf environment resulted from Cenomanian transgression (Mweneinda, 2014).

3.1.2. Kipatimu formation

Kipatimu formation of Neocomian is estimated to have a thickness of approximately 200 m and is the oldest Cretaceous rocks outcrop exposed 16–19 km west of Chole. They were first described by Stockley (1943) as massive, red and green variegated shales, frequently silty, with occasional cross-bedded friable fine sandstone beds (Mweneinda, 2014).

Similarly, to Kipatimu member identified in the northern part of the adjacent Mandawa basin, the Kipatimu formation (Kent et al., 1971) at Ruvu basin also constitutes mudstone to silty mudstone whose silt content varies from 0 to 50%, laminated to weakly laminated siltstone containing silt to very fine sand grain sizes and laminated to bedded sandstone whose grain size ranges from fine to very coarse sand with a varying silt content from 0 to 20%. In some locality, across stratified sandy siltstone with their grain size ranging from silt to coarse sand with occasionally mud clasts, often upwards fining sequences. It also has a varying degree of bioturbation, with some shell and coral fragments. The Kipatimu Beds appear to be estuarine brackish-water beds with marine intercalations (Kent et al., 1971; Stockley, 1943).

3.1.3. Bagamoyo formation

The formation comprises of Argillaceous sediments (shales, claystone, and limestones with minor sandstone) deposited from Aptian to Cenomanian and was penetrated by Makarawe-1 at a depth of 757−1190 m and kiwangwa-1 at 865–2576 m (Mweneinda, 2014). Minor outcrops are on the roads between Bagamoyo, Lugoba, and Msata and these exposures are discontinuous, which are sandy limestone, calcareous sandstone with brachiopods, and orbitolina limestone. This formation shows the characteristics of the shallow marine environment, which resulted from late Aptian transgression (Kent et al., 1971).

3.1.4. Amboni Limestone

Amboni Limestone is ranging from 80 m to 340 m thick. It is a poorly fossiliferous and dense well-bedded rock with partly oolitic (having a primary porosity 15%) or pisolitic, deposited from Bajocian to Callovian bases on very limited molluscan evidence (Kent et al., 1971; Kent & Pyre, 1973). Mapping indicates that the Amboni Limestone overlaps the thick Karroo onto Basement. These limestones were possibly deposited in a shallow marine environment (Haldemann, 1959). Few shreds of evidence have been reported.

3.1.5. Msata formation

Msata formation is recognized by three major facies associations (Kabohola, 2017; Kent et al., 1971)) that have been interpreted based ten sub-lithofacies summarized below. The first facies association is characterized by both matrix- and clast-supported inclined polymictic conglomerate and calcareous sandstone, the second facies association containing interbedded laminated shale and calcareous siltstone/sandstone, and the third facies association characterized by bioclastic matrix-supported conglomerate.

Matrix-supported conglomerate (Figure 2(a)

Figure 2. Msata formation. a) Matrix-supported conglomerate with clastic, granitic gneiss (black circle), mudstone (black arrow), precipitated calcite (red arrow) and pebble imbrications (black circle). b) Clast supported conglomerate with clastic, granitic gneiss (blue arrows), fossils fragments; brachiopods (black arrows) and gastropods (red arrow). c) Bioclastic matrix-supported conglomerate, it is highly weathered. d) Massive calcareous sandstone; brachiopods fragment (black circle). e) Fossiliferous calcareous sandstone with different fossils fragments like shells of brachiopods, bivalves, and gastropods. f) Planar bedded calcareous sandstone.

), these have clasts ranging from pebble to cobble size, highly indurated, sub-angular to subrounded, dominated by granitic gneiss from the basement and precipitated calcite. Their matrix contains fine to coarse-grained sand, sub-rounded to sub-angular grains of quartz, calcite, feldspar and micas. They are moderate to poorly sorted and cemented with calcite.Mostly massive with tabular to lenticular structure and sharp surface contact, also shells of brachiopods, bioturbation, and pebble imbrications are found in these conglomerates. Their colour is moderate orange-pink. These conglomerates are characteristics of unidirectional high-energy flow deposits.

Clast-supported conglomerate (Figure 2(b)) have clasts ranging from pebble to boulder size, which is highly indurated, subangular- rounded, limestone, fine-grained sandstone and clasts from the basement (gneiss; Kabohola, 2017; Kent, 1954; Kent et al., 1971). They are massive, tabular to lenticular structure, and sharp surface contact, while their colour ranges from medium-light grey to greyish yellow-green. These conglomerates are characteristic of unidirectional high-energy deposits.

Bioclastic matrix-supported limestone conglomerate (Figure 2(c)), these have clasts ranging from pebble to boulder-sized dominated with limestone rip-ups and reworked greenish fine-grained sandstone having a massive, geodes and traces of fossils, and their colour range from medium-light grey to greyish yellow-green. These are likely to be unidirectional high-energy flow deposits.

Massive calcareous sandstone (Figure 2(d); Kabohola, 2017; Kent et al., 1971), these are massive sandstone characterized by medium- to coarse-grained sand, moderately to well sorted and their composition being dominantly quartzo-feldspatho-lithic and cemented with calcite. Their colours are moderate orange pink and are typical of unidirectional medium- to low-energy flow deposit.

Massive fossiliferous calcareous sandstone (Figure 2(e)), these massive sandstones characterized by fine to medium-grained sand, sub-angular to sub-rounded, moderate sorted and dominated with quartzo-feldspatho-lithic in their composition. They contain shells of brachiopods, bivalves and gastropod and their colour are moderate orange-pink. They are a typical characteristic of unidirectional medium- to low-energy flow deposit.

Course stratified calcareous sandstone (Figure 2(f)) have medium- to coarse-grained sand, moderately sorted and dominantly quartzo-feldspatho-lithic in composition (Kabohola, 2017; Kent et al., 1971). They are lamination due to change in grain size where sand is interlaminated with thin mica layer, and also they are planar bedded with parting lineation and moderate orange-pink in colour. These sandstones are characteristics of the unidirectional medium- to low-energy flow deposit.

Convolute stratified sandstone (Figure 3(a)

Figure 3. Msata formation. (a) convolute calcareous sandstone, (b) calcareous nodules, (c) fine stratified calcareous sandstone, and (d) finely laminated shale (black arrows) interbedded with sandstone/siltstone (red arrows).

) is moderate orange-pink sandstone having fine- to medium-grained, moderate sorted, sub-angular to sub-rounded texture. Convolute bedded with oolitic-pisolitic limestone and are characteristics of the unidirectional medium- to low-energy flow deposit.

Fine stratified calcareous sandstone (Figure 3(c)), these sandstones have medium- to fine-grained sand showing moderately to well-sorted texture. They are planar bedded with parting lineation, a trace of fossils like brachiopods, a trace of plant and have moderate orange-pink colour (Kabohola, 2017). They are typical characteristics of the unidirectional medium- to low-energy flow deposit.

Finely laminated shale (Figure 3(d)), these are moderately brown finely laminated shales constituting mainly clay to silt size and are characteristic of low-energy depositional environment. The last facies is the Interbedded laminated shale with sandstone/siltstone where the main grain sizes are clay to silt size. They are moderate brown to greyish orange, parallel laminated, weakly laminated and fossils content. These were deposited in a low-energy depositional environment.

The facies associations of Msata formation indicate a deposition environment of sediments in terrestrial settings involving fluvial deposits (alluvial to channel), and marginal marine setting of a deltaic deposit (Figure 4).

Figure 4. Stratigraphic section of the Msata succession showing features, facies association and depositional environment adapted from Kabohola (2017).

3.1.6. Karoo formation

Deposition of Karoo sequence is related to episodic periods of regional uplift and erosion resulted in the accumulation of over 10,000 m of siliciclastic sediments in the coastal basin including Ruvu basin. In the Ruvu Basin, the Karoo sequence intersected from the drilled wells (Makarawe-1 and Kiwangwa-1 well) includes the older Karoo formation of Permian which also crop out to the south-west of the basin, Tanga beds of Triassic and the lower Jurassic Ngerengere beds which show distinct megacycle of fining upward. The deposition of Karoo lithofacies was controlled by the development of grabens and half-grabens where the main axis of accumulation was probably farther to the east in the graben corresponding to the Ruvu trough. The Karroo sediments are predominantly terrestrial clastics deposited in a variety of continental environments ranging from low-energy fluvial, with lacustrine and deltaic facies, interbedded with occasional marine incursions (Kent, 1954; Kreuser, 1995; Kreuser et al., 1990).

Makarawe-1 well penetrated 2,465 m of the Tanga Beds ranging from clean and sorted, medium- to coarse-grained quartz sandstone with interbedded claystone deposited in a relatively high-energy braided stream environment to coals and carbonaceous shales developed in a floodplain and deltaic environments. Kiwangwa-1 (Haynes et al., 1987; SSI, 1987) penetrated 352 m of the Tanga Beds comprising interbedded medium- to coarse-grained sandstones and shales.

Ngerengere beds overlay the Tanga beds where it is intersected with Kiwangwa-1 (Haynes et al., 1987; SSI, 1987) well dominated with well-sorted medium- to coarse-grained quarzitic sandstones interbedded with shales, claystone and dark grey carbonaceous claystone. Ngerengere beds are equivalents to Nondwa formation of Mandawa basin.

In the study conducted by Kent et al. (1971) on outcrop exposures of the coastal basin observed the presence of Karroo facies of bedded feldspathic calcareous sandstone with occasional some oolitic limestone bed and shales near Ngerengere in fault contact with Basement gneisses (Aitken, 1959; Hennig, 1913). Kent et al. (1971) noted that the lower part of the bedded feldspathic carbonaceous sandstone where a uniform series of cross-bedded feldspathic sandstones which suggests rapid deposition.

3.1.7. Synthesized stratigraphy of the basin

The describe formation above (Aitken, 1959; Haldemann, 1959; Kabohola, 2017; Kent et al., 1971; Kent & Pyre, 1973; Mweneinda, 2014; SSI, 1987; Stockley, 1943) enabled to synthesize the stratigraphic column of the basin (Figure 5). However, detailed information form multidisciplinary research such as seismic, petrophysics, sedimentology, geochemistry, geochronology, and sequence stratigraphy will help to generate a general stratigraphy of the basin which is currently Missing.

Figure 5. Synthesized lithostratigraphy of the basin.

3.2. Geochemical analysis

The distribution of trace and rare earth element (Table 1) may give information on provenance, depositional conditions and tectonic setting (McLennan et al., 1993). One of the most reliable indicators of sediment provenance is the REE patterns. They are highly resistant to fractionation during weathering and diagenesis and easily preserved in terrigenous sediments (McLennan et al., 1993; Mclennan, 2018; Taylor & McLennan, 1985). Due to nearly quantitative transfer of REE from the source region to the depositional site (largely as suspended rather than dissolved load), the terrigenous sediments should reflect the average composition of the source region (Condie, 1993).

3.2.1. Provenance of the sediments

Based on the chondrite, normalized REE patterns of the Kipatimu formation show a diverse enrichment in LREE, small negative Eu-anomalies, modest heavy-REE slop (Figure 6)

Figure 6. Average chondrite-normalized REE values for the siliciclastic sediments of the Ruvu basin. Chondrite values adapted from McLennan et al. (1980).

, possibly reflect a granitic source region for the deposited sediments (McLennan et al., 1993). Low compatible elements such as HREE and the relative enrichment LREE and Th, which are incompatible elements that concentrate in the residual liquid but not easily included in the crystal structure of rock-forming minerals also indicate more felsic average provenance (McLennan et al., 1993; Mclennan, 2018; Taylor & McLennan, 1985). The La/Th ratios of the Kipatimu formation support the source of sediments being from granitic/felsic provenance (McLennan et al., 1980), which falls between La/Th = 3.2 and La/Th = 4.7 (Table 2). The trace element geochemistry and its origin in the siliciclastic sediments of the Ruvu Basin sampled from Sakura, Kipatimu, and Msata formation give a wide understanding of the source rock composition, maturity, grain size variations, and provenance of the sediments where different elemental ratios and concentration relationship can be used. Based on the relative abundance of different trace elements, the provenance of the sediments can be inferred where different anomalies related to different provenance. For example, in felsic rocks Zr, Th, and La are more abundant, while the ultramafic and mafic rocks have scarce (Feng & Kerrich, 1990).

Low concentrations of Cr, Ni, Co, V, and Sc observed in fine-grained sediments reflect sediments were originated from felsic rocks (Condie, 1993; Taylor & McLennan, 1985).

The Zr/Rb ratios can be used as a tool interpretation of the relative grain size of the siliciclastic sediments, while the disparities in the ratio between siliciclastics and carbonates can be interpreted from (Zr + Rb)/Sr ratios. Higher Zr/Rb ratios averaged 2.7 were encountered in Msata Formation (Figures 7 and 8), which reflect more coarse-grained units, while lower ratios were characteristics of more clayey lithologies of Sakura Formation, Bagamoyo formation, and Makarawe shales. Sr is normally related with carbonates mostly biogenic carbonates. Low (Zr + Rb)/Sr ratios (Table 1) generally reflect carbonate-rich units (Dypvik & Harris, 2001; Taylor & McLennan, 1985). The lower Zr/Rb and (Zr + Rb) /Sr ratios of 0.8 and 0.5, respectively, in Sakura, Bagamoyo, and Makarawe formation, suggest more fine-grained and carbonate-enriched sediments, which differ from Msata formation which is characterized by higher Zr/Rb ratios and low (Zr+Rb)/Sr.

Figure 7. Distribution of (Zr + Rb)/Sr ratio versus samples.

Figure 8. V/Cr against elemental index plot.

Higher Y/Ni, Zr/Cr of up to 0.83 and 3.6, respectively, and lower Ti/Zr ratios ranging from 0.004 to 0.01 generally indicate a contribution from a felsic source supporting the Rare Earth Elements results (Dokuz & Tanyolu, 2006; Ishiga et al., 1999). But, these ratios can be altered due to weathering in the source and transport of the sediments and diagenesis processes in the depositional environment. The higher Ba/Co ratio of up to 73.21 suggests that sediments derived from a felsic-granitic source (Cullers et al., 1988).

3.2.2. Paleo-chemical environment

Also, the ratios of V, Ni, Cr, and Co were used to interpret the redox condition of the paleoenvironment where average V/Cr ratio of 0.85 and Ni/Co ratio of 2.90 were obtained, these values are characteristics of oxic environment and a V/Cr ratio >4.3 has been reported at Makarawe shales indicating Anoxic environmental condition. The trend of V/Cr ratio indicates the more in oxic condition (Chen et al., 2019; Jones & Manning, 1994; Nagarajan et al., 2007) prevailed during deposition of Msata formation whose facies associations belongs to an alluvial, fluvial channel, delta fronts and delta top (Figure 8).

3.2.3. Tectonic setting

According to Mpanda (1997) and Kapilima (2003), the Ruvu basin is a rift basin, related to the break-up of Gondwanaland in the Permo-Triassic to early Jurassic. The rift formed a north-easterly trend which extended into the offshore zone and developed as a spreading centre (Morley & Ngenoh, 1999) which eventually formed the West Somalia Basin leaving the Ruvu Basins as a failed arm forming a half-graben.

Subsequently, from the Mid-Jurassic the Ruvu basin developed essentially as a passive continental margin. The Arc-Rift-Collision discriminant plot (Das & Haake, 2003; Moosavirad et al., 2011; Roser & Korsch, 1986) of the Karoo sandstone (Figure 9

Figure 9. Tectonic setting discriminant factor plot of the Karoo sandstone.

.) indicated that in a plot of discriminant factor 2 versus Discriminant factor 2 most of the discriminant values falls in a rift setting. From mid-Jurassic to Cretaceous, a plot of Th versus Ta discriminants was plotted (Figure 10.)

Figure 10. Tectonic setting discriminant of mid-Jurassic to Cretaceous sediments.

and indicates that the values fall into a passive continental margin setting.

4. Conclusion

Considering the sedimentological and geochemical aspect, the rocks of the Ruvu basin were deposited in a various depositional environment ranging from deep marine, shallow to fluvial depositional environment. Deltaic environments have been shown in the upper part of Msata formation where the middle and lower show channel and alluvial depositional environment. The Kipatimu formation shows blackish-water estuarine environment with marine intercalations, it also has coal fragments indicating the terrestrial source of the material. Marl of the Sakura formation is related to the transgression event between Albian and Cenomanian that created a shallow marine environment. Other formation includes Amboni limestone which is characterized by oolitic and pisolitic limestones which are inferred to be typical of the shallow marine environment due to the presence of limited molluscan evidence. The Karroo sediments are predominantly terrestrial clastics deposited in a variety of continental environments ranging from low-energy fluvial, with lacustrine and deltaic facies, interbedded with occasional marine incursions.

Based on geochemistry analysis, it shows that the sediments originated from multiple sources ranging from felsic rocks or their equivalent metamorphic rocks most granitic rocks to mafic rocks of either Proterozoic Usagaran mobile belt or Neoproterozoic Mozambique mobile belt. Higher Y/Ni, Zr/Cr, lower Ti/Zr ratios, a diverse enrichment in LREE, small negative Eu-anomalies, and modest heavy-REE possibly reflect a dominantly granitic source. The presence of the clasts of granitic gneiss in matrix and clast supported conglomerates of Msata Formation add to evidence of the contribution of this granitic gneiss as a source of the sediments. Although the high concentrations of Cr, Ni, Co, and V have been reported on some clayey lithologies indicating mafic source, while their ratios indicate that the sediments were deposited in oxic environment especially Msata formation and Blackish water environment for Makarawe shales. According to the tectonic setting discriminant plot, it shows that the basin falls in rift setting, which later were transformed to a passive continental margin setting.

Acknowledgments

The Authors appreciates the support provided by Tanzania Petroleum Corporation (TPDC), Department of Geology of the University of Dodoma and other stakeholders.

Disclosure statement

No potential conflict of interest was reported by the author(s).