"Rock of the Month # 114, posted for December 2010" ---

The term peperite refers to the confusing rocks with complex textures formed when hot volcanic rock (lava) impinges on water or wet sediment. Boiling water and fracturing of plastic, water-saturated coastal, deltaic or lake sediments will inevitably produce a complex result, and a challenge to interpretation.

Peperites are not reported frequently in the literature, although it is possible that they have been overlooked, and described in other terms. The tops of flows may be described as "autobreccia" or "flow-top breccia", overlooking the intrusive (invasive) mode of formation (Rawlings et al., 1999). Peperites have been recognized in rocks of Archean to Cenozoic ages, involving a wide chemical range of both volcanic and sedimentary components. First described in France, in Oligocene interactions of basalt with calcareous lacustrine sediments (see Jones, 1969), they often involve mixtures of intermediate to mafic volcanics with cogenetic volcaniclastic sediments (e.g., Branney and Suthren, 1988).

The writer is not yet 100% convinced that the rock shown here is peperite, but help is at hand! Several fine reviews of volcanic textures, including or focusing on peperites, have been published in the past 20 years (e.g., McPhie et al., 1993; Skilling et al., 2002; Gifkins et al., 2005). Further examples occur in Archean greenstone belts (Houle et al., 2008), Paleozoic andesite sills (Awdankiewicz et al., 2004) and Miocene volcanic sequences (Harris et al., 2006). The morphology of peperite clasts may reflect the paleodepth of formation of these rocks (Hudak et al., 2004).

The regional geology of Tigrai and the wider Ethiopian region has been explored in cycles, some of the classic work being that of Blanford (1870), Dainelli and Marinelli (1912), Merla and Minucci (1938), Dainelli (1943a,b,c.d), Mohr (1962), and Garland (1978, 1980).

Wilson and Pavlish (2005) documented aspects of the geology and stratigraphy of a 100 km² area north of the Ethiopian town of Adigrat, just south of the border with Eritrea. This area of the Ethiopian Highlands, west of the east African rift valley, is underlain by Precambrian bedrocks, including felsic intrusive rocks (diorite, granodiorite, granite and pegmatite), schist, slate and marble. The basement is overlain unconformably by Mesozoic nonfossiliferous clastic sediments (sandstone and siltstone, plus local conglomerate, shale, coarse grit and orthoquartzite). Some show evidence of soft sediment deformation. Ironstones (gritty rocks rich in Fe oxides) are developed along planar fractures, affecting large volumes of rock. Thin cap rocks to the Mesozoic sequence have been variably described as siltstone, paleosol or peperite. The bedrock sequence is completed in Tertiary magmatism, largely the extensive Oligocene basalt flows associated with the Rift Valley, plus some prominent plugs of trachyte and gabbro. The occurrences in the Adigrat district can be summarized as follows (Wilson and Pavlish, 2005):

"The top of the Mesozoic succession is commonly marked by a thin but variable resistant cap rock found in the upper few metres of many of the vertical valley walls. The underlying sandstone is often extensively reddened. A good example is seen in the north wall of the head of the Ambeset valley, some 2.5 km east of Fatsi. Here the sandstone sequence is ended by 5 m of variegated maroon and grey-white silty sediments capped by a hard 1-m layer of coarse pisolithic laterite. In the majority of instances this "rim-rock" sequence is plausibly interpreted as a paleosol, a mixture of maroon and cream-coloured silty sediments, and local evidence of bleaching along fractures or joints. In some localities, however, it may be a peperite, an intimate mixture of siltstones invaded by an iron-rich (?) basaltic lava, possibly formed in a lacustrine setting as mafic magma was injected into unconsolidated wet sediments. The inferred lava appears in places aphyric, or porphyritic, or dotted with rounded amygdales infilled by (?) zeolites".

Three polished thin sections (Wilson, 2006) of these rocks sampled contacts between two lithologies in intimate contact: a dark maroon to blackish rock, apparently invaded by a pale cream-coloured silty rock, with sharp contacts. The dark rock, identified as an iron-rich lava like basalt or andesite, is now very rich in opaque Fe oxides and lesser goethite. This rock is vuggy, with numerous jagged cavities, containing granular strained quartz, and traces of fibrous chalcedony infilling small vugs. The pale lithology is a very fine-grained massive rock, grain size generally <5 microns, with scattered larger grains dominated by rounded to angular quartz, seldom >100 microns in diameter. The thin section is thought consistent with the field identification of a peperite (a hot lava interacting with wet sediment, generating by explosive, steam-filled mixture an intricate blending of rapidly-cooled lava and baked sediment). The two lithologies have sharp contacts, the pale silt having a faint but definite fabric of aligned oxidized wisps, cross-cut by the dark lava. The margin of the lava appears vesiculated, incorporating small rounded blebs of the fine sediment.

Awdankiewicz,M, Breitkreuz,C and Ehling,B-C (2004) Emplacement textures in late Palaeozoic andesite sills of the Flechtingen-Roblau block, north of Magdeburg (Germany). In `Physical Geology of High-Level Magmatic Systems' (Breitkreuz,C and Petford,N editors), Geol.Soc. Spec.Publ. 234, 253pp., 51-66.

Blanford,WT (1870) Observations on the Geology and Zoology of Abyssinia. Made during the Progress of the British Expedition to that Country in 1867-68. Macmillan and Co., London, 487pp., reprinted in 2005 by Adamant Media Corporation in the Elibron Classics Series.

Branney,MJ and Suthren,RJ (1988) High-level peperitic sills in the English Lake District: distinction from block lavas, and implications for Borrowdale Volcanic Group stratigraphy. Geol.J. 23, 171-187.

Dainelli,G (1943a) Geologia Dell'Africa Orientale, Vol. I. Il Progresso delle Conoscenze. Reale Accademia d'Italia, Rome, 464pp. (in Italian).

Dainelli,G (1943b) Geologia Dell'Africa Orientale, Vol. II. L'Imbasamento Cristallino e la Serie Sedimentaria Mesozoica. Reale Accademia d'Italia, Rome, 704pp. (in Ital.).

Dainelli,G (1943c) Geologia Dell'Africa Orientale, Vol. III. La Successione Terziaria e i Fenomeni del Quaternario. Reale Accademia d'Italia, Rome, 748pp. (in Ital.).

Dainelli,G (1943d) Geologia Dell'Africa Orientale, Vol. IV. Tavole. Reale Accademia d'Italia, Rome, map case with 10 sheets (in Ital.).

Dainelli,G and Marinelli,O (1912) Resultati scientifici di un viaggio nella colonia Eritrea. Firenze, 573pp. (in Italian).

Garland,CR (1978) Adigrat. Ethiopian Mapping Agency, 1:250,000 scale.

Garland,CR (1980) Geology of the Adigrat area. Geological Survey of Ethiopia Memoir 1, 51pp.

Gifkins,C, Herrmann,W and Large,R (2005) Altered Volcanic Rocks: A Guide to Description and Interpretation. CODES (Centre for Ore Deposit Research), University of Tasmania, 275pp.

Harris,AC, Bryan,SE and Holcombe,RJ (2006) Volcanic setting of the Bajo de la Alumbrera porphyry Cu-Au deposit, Farallon Negro volcanics, northwest Argentina. Econ.Geol. 101, 71-94.

Houle,MG, Gibson,HL, Lesher,CM, Davis,PC, Cas,RAF, Beresford,SW and Arndt,NT (2008) Komatiitic sills and multigenerational peperite at Dundonald Beach, Abitibi greenstone belt, Ontario: volcanic architecture and nickel sulfide distribution. Econ.Geol. 103, 1269-1284.

Hudak,GJ, Newkirk,TT, Drexler,H, Odette,JD and Hocker,SM (2004) Neoarchean peperites in the vicinity of Fivemile Lake, Vermilion district, NE Minnesota. Abs. 50th Annual Meeting, Institute on Lake Superior Geology, vol. 50 part 1, 161pp., 84-85, Duluth, Minnesota.

Jones,JG (1969) A lacustrine volcano of central France and the nature of peperites. Proc.Geol.Assoc. 80, 177-188.

McPhie,J, Doyle,M and Allen,R (1993) Volcanic Textures: a Guide to the Interpretation of Textures in Volcanic Rocks. Centre for Ore Deposit and Exploration Studies, University of Tasmania, 198pp.

Merla,G and Minucci,E (1938) Missione Geologica nel Tigrai, volume primo. Reale Accademia d'Italia, Rome, 363pp. plus 2 map sheets at 1:250,000 scale (in Ital.).

Mohr,PA (1962) The Geology of Ethiopia. University College of Addis Ababa Press, printed by Il Poligrafico in Asmara, 268pp.

Rawlings,DJ, Watkeys,MK and Sweeney,RJ (1999) Peperitic upper margin of an invasive flow, Karoo flood basalt province, northern Lebombo. S.Afr.J.Geol. 102, 377-383.

Skilling,IP, White,JDL and McPhie,J (2002) Peperite: a review of magma-sediment mingling. J.Volc.Geothermal Research 114, 1-17.

Wilson,GC (2006) Petrographic study of rock types from the Gulo Makeda area, eastern Tigrai province, northern Ethiopia. Turnstone Geol.Serv.Ltd. Report 2005-12, for Simon Fraser University, Vancouver, vi+74pp., 22 February.

Wilson,GC and Pavlish,LA (2005) Field geology of Gulo Makeda area, eastern Tigrai province, northern Ethiopia. Turnstone Geol.Serv.Ltd / University of Toronto field report, 12pp. and 10 photographs, 11 July.