Lignites and Origin of Porcellanites

On the relationship between Lignites and Porcellanites in Trinidad, West Indies.

Within the various stratigraphic units of Trinidad (Fig.1) are found a number of lignite and porcellanite deposits. These have been the subject of discussion since Wall and Sawkins (1860) first described them and potentially commercial lignites were described by Cunninghan Craig (1904) ,Guppy (1877) and Suter (1951). Porcellanites are of economic importance in southern Trinidad where hard rocks that can be used as aggregate are rare. They have been and continue to be mined for use as aggregate in road foundations and construction.
Lignite beds have been described in the Telemaque Member of the Manzanilla Formation, the Upper Morne L’Enfer and Erin Formations, while porcellanites are confined to the Erin , Talparo and Upper Morne L’Enfer. Based on observations at a number of outcrops in south and central Trinidad there are a number of explanations for their origin.

Volumetrically, the thickest lignites are found in the Telemaque Member of the Manzanilla Formation. In the North Soldado Field, in the Gulf of Paria individual beds up to 30 feet thick have been reported. In outcrop these lignite beds are generally about 1’, in the Upper Morne L’Enfer and Erin Formations, while in the Forest Reserve Field beds up to 10’ have been encountered in the subsurface.

At an outcrop of the Telemaque member of the Manzanilla Formation in the Forres Park area, are exposed four coarsening upwards cycles each capped by a thin ( less than 1’ thick) lignite bed(Fig 2, Plate 1). The base of these lignite beds are clearly erosional (Plates 2 & 3) an internally the lignite shows sedimentary structures suggesting they were resedimented from nearby deposits. Also of note no rooted zone is present .

At Guapo Bay, the area closest to the Vessigny Beach Facility has a lignite bed 1-2 ‘ thick exposed that is overlain in places by a porcellanite The lignite bed is dominantly organic material with leaf imprints (Plate 4) and tree trunks in random orientations and growth positions (Plates 5a & 5b).

The lower contact of these beds is variable, in places it is clearly erosional (Plate 6a-b) and in others the underlying claystone beds have rooted zones (Plate 6c-d).

The lignite rich bed grades upwards (Plate 7) into parallel bedded claystone (Plate 8) and siltstones that have numerous leaf impressions (Plate 9 a-b) parallel to the laminations. The only difference between these beds and the underlying lignite being the clay content.

The second and thickest lignite unit (12’) at Guapo Bay (plate 10) is a composite unit made up of 3 beds with lower erosional contacts

Within all of the beds the lignites are of parallel laminated (plate 12) organic material, dominantly of leaf and grass origin.

The upper most bed grades upward (Plate 13a) into a parallel laminated leaf rich claystone (plates 13b-c). Leaves appear to be Rhizophora (Plate 13d)(red mangrove).

The Upper and Lower Morne L’Enfer outcrop at Quoin Cliff and Frank Bay are also dominated by parallel laminated lignites about 1 – 2’ thick (plates 14a – b) with no rooted zones, that are overlain in places by bioturbated claystones (Plate 14c) of are found in small channels (plate 14d).

Another lignite bed a short distance to the south also has no woody fragments or rooted zone (Plate 15), suggesting that the material has been transported from the point of accumulation and redeposited in a quieter environment (back barrier lagoon?). The unit is capped by a clay that probably represents a flooding event. Overlying the clay is a fairly massive sand that is in turn overlain by a coarsening upward sand-siltstone sequence.

It is only within the Erin Formation on the western side of Quoin Cliff that we see a thin lignite bed with tree trunks in growth positions (Plates 16 a – b).

At Chatham (White Cliff) there are a number of lignite beds, the outcrop closest to the old hotel exposes a large channel complex (Plate 17 a-c) about 200’ in width. The base of the lowest channel rests on a thinly layered (2’) lignite in which plant material is visible as are occasional woody fragments. At the eastern end of the channel, another channel has eroded into the margin. Sediments at this edge are lignitic and were probably redeposited in a slack water area after being eroded from elsewhere. Further east is another outcrop with a strike section of a lignite. The base of the outcrop is a pure claystone with NO rooted zone. There is an abrupt change upwards into a ligntic claystone 2’ thick, another abrupt change to pure hard lignite 1’ thick and finally back into another lignitic claystone.

Two possible origins will be reviewed for the lignites observed in outcrops at Guapo Bay, Chatham, Quoin Cliff and Frank Bay. The first involves an insitu origin within mangrove and sedge swamps developed behind estuarine sand barrier bars and spits While the other involves erosion and resedimentation. Figures 2 a-d illustrate lignite development within a mangrove swamp probably during a transgressive event.

Kosters and Suter (1993) show that coals preferentially landward of the shoreline (Fig. 3).
What are these lignites composed of? G.W. Halse (1937) reported a Mr. McCallum had analysed the lignites at Cedros Point and showed that they are made up of:
Water (loss on drying to 1300C) – 5.3%
Water & Organic matter (loss on ignition) – 16.1%
Organic matter by diference – 10.8%
Sulphur – 1.86%

We are really looking at a carbonaceous shale and what we have seen is that in places the ‘lignites’ grade upward into organic rich claystones.

Porcellanites are found only in the formations that have lignite beds and traditionally have been thought to have originated by lignites catching fire and baking the overlying clays into a hard reddish rock. Wall and Sawkins (1860) were the first to suggest that combustion of lignites burnt the overlying shales. Halse (1937) described in his paper ‘Burning cliff at Cedros’ a fire that was first observed in 1933 and was still burning in 1937. The burning area fairly small (10’ x 30’) indicates that “ he has never observed a naturally burnt lignite.” “it is the shales themselves either bituminous or in a transition stage between bituminous or carbonaceous that have spontaneously ignited and burnt” probably due to heat generated by the oxidation of sulphides.
The combustion of the ‘lignites’ may not generate enough heat to bake clays. However, clays retain heat, and any combustion may cause high temperatures because of slow heat dissipation.

In 2003 another lignite fire (caused by a man made fire) burnt for 3 months before being extinguished (Plate 19).

During this time overlying clays were baked and converted to a reddish semi hard rock (Plate 20).

At Vessigny (Union Industrial Estate) a number of porcellanites were re-exposed and eventually removed by site construction activities. At the site of the former Tobago dam a thin (4’) porcellanite bed was examined, (Plate 21) and no lignite bed was present. Two types of porcellanite were present, a more massive but fractured rock and a brecciated rock that in places looked as if it was deposited by water.

Leaf imprints were common in the porcellanite, (plate 22) which was overlain by a grey black clay rich in leaves and was sulphur stained in places.

Plate 23 shows a number of stacked sand and clay filled channels underlain by lignite and no nearby porcellanite.

On the Vessigny coast at Guapo Bay the porcellanite (Plate 24) closest to the beach facility is weathered and relatively soft in the cliff face, it is however quite hard in the outcrop which extends into the sea with an azimuth of 2800. It is overlain by southwesterly dipping lignite, sand and claystone.

Closer examination of the lignite-porcellanite contact(Plate 25a) is variable, to the left it appears to be an erosional or slumped one while to the right it appears transitional. The lignite is fairly silty with abundant carbonaceous material and overlies the porcellanite. The basal contact with the underlying claystone (Plate 25b) is abrupt.

At Union Industrial Estate the outcrop shows mostly well bedded rock (Plate 26) with zones of bedded but fractured rock.

Well preserved leaf imprints (Plate 27) are present , in addition there are rare Thalassanoides preserved, no lignite is present at this outcrop.

“Red Cliff” (Plate 28) represents one of the largest and most continuous outcrop of porcellanite in the country.

Plates29 (a) & (b) are a strike view of outcrop showing arrangement of rocks. From the base to the top of the outcrop is dominantly rocks consist of a silty claystone, overlain by a thin lignite, a crossbedded sand and finally by a well bedded porcellanite. The base of the porcellanite is very uneven and in places it may represent the original sedimentary contact. No ash bed is present, and the rocks appear well bedded with no sign of collapse of the overlying rock.

Plates 30 a & b are a close up of the contact between the porcellanite and underlying sands. A thin grey layer separates the two beds (ash or weathering effect ?), above it the porcellanite is thinly layered and becomes more massive to the top. The sands show no signs of being exposed to heat.

At Quoin Cliff Plate 31a, is a view of the porcellanites looking in a westerly direction. The colour change observed from a dark red-orange at the base to a pinkish-red at the top and represents the gradual change from hard porcellanite to ‘unbaked’ clay rich rock with abundant leaves at the top. The contact with the upper beds is gradational. Within the procellanite are present poorly preserved leaves. There is one fragment of lignite within the porcellanite , however no other lignite bed is present either above or below the porcellanite, thus there is no source of fuel for fires to bake the clays. It is possible that the abundant leaves within the clay provide the fuel source. The colour variations (Plate 31b), within the hard rock most likely represent variations in the original mineralogy of the clays and not scorching by fire.

Three possible origins of porcellanites will be discussed based on the examples shown. 1) Ignition and burning of lignite beds in the near surface (Plates 34 a-d), in a manner similar to the fires previously described. The burning lignite bakes the overlying clays and as the lignite is consumed by the fires the overlying baked clays collapse to form a rubble bed. Later deposition of sands and clays buries this ‘porcellanite’ rubble bed. As a result one should find that there should have been no baking of the overlying beds, the production of a porcellanite rubble bed, an ash layer and baked lower beds. Because of the availability of oxygen these porcellanites should be extensive.

Lignites are deposited, buried and later re-exposed by erosion. A fire then burns the lignite which bakes the overlying clays that either collapse into a breccia or subside as undisturbed layers(Plates 35 a-b) . The depth below the ground surface and proximity of the water table will limit the depth of burning by reducing oxygen supplies and thus limiting porcellanite development. There are reports of wells (ACD-1 Kugler 1996) encountering porcellanite in the subsurface. Another expectation is where the clay layers above and below the lignite is baked and separated by a thin ash layer.

One expectation from the previous origins that the porcellanites should exhibit decreasing amounts of baking away (Plates 36 a-c) from the burnt lignite (similar to that of contact metamorphism). No known research has been done to see if there is any mineralogical changes from base to top of the beds. Bristow 1969 analysed rocks from the Cedros area and found that in thin section the porcellanite could be described as a scoriaceous clay that was “Partially melted … almost wholly recrystallized. Fine grained euhedral needles of mullite… in a matrix of cristoballite or pale brown glass”. Other minerals include hematite and tridymite. Variable amounts of alunite and small amounts of high temperature silicates may also be present. A maximum temperature of 700C suggested by the abundance of alunite , while cristoballite, mullite and tridymite are high temperature silicates, stable in temperatures in excess of 1000C.

Diagenetic changes due to movement of water in the subsurface (Plate 36). What is seen is that the degree of porcellatite development decreases toward the top of the bed where it passes into unaltered leaf rich claystones. This is probably the case where NO lignite beds are in proximity to the porcellanite.

Questions for future consideration

Why are there no porcellanites in the Manzanilla Formation ?
Are mineralogical changes associated with the burning or gradational or abrupt to unaltered rock?
Why do we not find ash beds ?
Can the thicknesses of lignites seen today account for the volume of porcellanites that exist ?
Why no baked sands?

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