Hiwegi Formation
From Wikipedia, the free encyclopedia
Grit Member
Fossil Bed Member
Kibanga Member| Hiwegi Formation | |
|---|---|
| Stratigraphic range: Early Miocene ~ | |
| Type | Formation |
| Unit of | Rusinga Group |
| Sub-units | Kaswanga Point Member
Grit Member Fossil Bed Member Kibanga Member |
| Underlies | Kulu Formation |
| Overlies | Rusinga Agglomerate |
| Thickness | 57 m (Waregi Hills) |
| Lithology | |
| Primary | volcanic tuffs, clastic sediments |
| Location | |
| Region | Lake Victoria |
| Country | Kenya |
The Hiwegi Formation is a geological formation on Rusinga Island in Kenya preserving fossils dating to the Early Miocene period. The Hiwegi Formation is known for the well preserved plant fossils it preserves, which indicate a tropical forest environment that underwent wet and dry periods. The middle members of the formation in particular indicate a brief period in which conditions were notably dryer with a more open environment compared to older and younger units. Some of the formation's fauna, such as an early ancestor of the modern aye-aye and a chameleon of the genus Calumma, link Miocene East Africa to modern day Madagascar.
_Mary_Douglas_Nicol_Leakey_(1913-1996)_and_her_husband_Louis_Seymour_Bazett_Leakey_(1903-1972).jpg)
The first discovery of fossil material by western researchers was documented in the early 20th century in the form of a British colonial report on the East Africa Protectorate. Excavations for fossils began in the 1930s with the work of Louis Leakey, followed by the British-Kenya Miocene Expedition. A major factor in the exploration of Rusinga Island was the discovery of several specimens originally assigned to the stem-hominoid Proconsul (now regarded as a distinct genus, Ekembo). Further interest for the formation was created due to the presence of ample non-primate fossil mammals, as well as a vast collection of fossil plants.[1]
Geography
Outcrops of the Hiwegi Formation are found across Rusinga Island, an island at the eastern end of Lake Victoria within Kenyan territory. The island itself is situated close to the mouth of the Winam Gulf. The Hiwegi Formation has outcrops across the islands, in particular in the area north-east of Mbita Point. Additional outcrops are found around the mountain Lugongo in the center of the island, Waregi Hills in the east as well as near Kiahera and Kaswanga on the islands north-western shore.
Geology and Stratigraphy
The Hiwegi Formation is part of the larger Rusinga Group. During the Miocene the sediments now forming Rusinga Island were deposited on the flanks of the Kisingiri Volcano, which had formed in the early Miocene. Two hypotheses seek to explain the volcano's influence on the sediments. Drake et al.[2] hypothesize that the Hiwegi Formation, alongside the Kiahera and Rusinga Agglomerate were deposited during early eruptions of the Kisingiri Volcano, prior to a period of silence during which the Kulu Formation was deposited and a second period of eruptions later. Bestland et al. (1995)[3] meanwhile suggest that the Kisingiri Volcano experienced up to three periods of activity, with the strata of the Hiwegi Formation being deposited in the second and third period of volcanic activity. The formation generally underlies the younger Kulu Formation and overlies the Rusinga Agglomerate. However at Waregi Hills, in the east of the island, the formation overlies the Ombonya Beds which are found nowhere else on Rusinga. The sediments in this region are overlain by the Kiangata Agglomerate and the Lunene Lavas.[1]
The formation is divided into four members.
- Kaswanga Point Member
- The oldest member of the Hiwegi Formation, it consists primarily of air-filled tuffs and other units reworked from tuffaceous material. The fact that the member is composed of volcanic material indicates that the nearby Kisingiri Volcano was particularly active at the time the unit was deposited. Besides the tuffs this member also consists of fossil bearing mudstone which yielded fossil leaves in the Kiahera Hill fossil site, the sedimentology of which indicates deposition as part of a debris or mudflow following a volcanic eruption.[4]
- Grit Member
- The Grit Member is the second oldest member of the formation and underlies the Fossil Bed Member. It is primarily composed of tuffaceous sandstones and conglomerates consisting of pebbles in a sandy matrix. The layers of the Member indicate a fluvial or ponded mode of deposition which contrasts with the volcanic deposition of the Kibanga and Kaswanga Point Members. Palaeosols are found intercalated into these sediments. This member, as well as the overlying fossil bed member, were likely deposited in floodplain channels during flashfloods and mudflows. Additionally, they show signs of dryer conditions leading to salt hoppers and mud cracks.[1]
- Fossil Bed Member
- The Fossil Bed Member shows no differences in sedimentology compared to the Grit Member and was distinguished from the former on the basis of fossil material being absent in the older unit.[5] Due to this lack of distinguishing features, the two groups are put together by Michel et al. (2020).
- Kibanga Member
- Overall the Kibanga Member is dominated by airfilled tuffs much like the Kaswanga Point Member and is interbedded with sandstone, conglomerates and palaeosol. The fossil leaf bearing units of the Kibanga Member show a mix of interbedding layers of sandstone and mudstone. The mudstone was likely deposited during periodical wet seasons when ponds formed, followed by dry seasons creating desiccation cracks which are fond atop some of the mudstone layers.[4]
K–Ar dating suggests a mean age of 17.8 million years for the formation. which correlates with the Burdigalian stage of the Miocene. In accordance with this method, the sediments of the Hiwegi Formation are thought to have been deposited over a short period of time, approximately 500.000 years. However dating of the formation has been met with difficulty due to the loss of certain minerals due to Diagenesis, the absence of other minerals important for dating and the effects of the Kisingiri volcano, which erupted through Precambrian sediments. Using Ar-Ar dating rather than K-Ar dating yields different results, still correlating to the Burdigalian but with a longer timespan dating from 20 to 17 million years ago.[4]
