Volcaniclastic Sedimentation in and Around an Ordovician Subaqueous Caldera, Lower Rhyolitic Tuff Formation, North Wales

Volcaniclastic sedimentation in and around an Ordovician subaqueous caldera, Lower Rhyolitic Tuff Formation, North Wales

WILLIAM J. FRITZ Department of Geology, Georgia State University, Atlanta, Georgia 30303 M. F. HOWELLS \ A. J. REEDMAN } British Geological Survey, Bryn Eithyn Hall Aberystwyth, Dyfed SY23 4BY Wales, United Kingdom S.D.G. CAMPBELL J

ABSTRACT

The Lower Rhyolitic Tuff Formation of Ordovician age in North caldera collapse; primary ash-flow tuffs totaling 500 m in thickness ac- Wales records the collapse, infilling, and subsequent resurgence of a cumulated in the northern part of the caldera, and a single outflow tuff unit volcanic caldera with an original diameter of about 15 km. This vol- up to 70 m thick extended at least 30 km to the northeast of the caldera canic center controlled patterns of volcaniclastic sedimentation, pro- (Howells and others, 1986). viding enough topographic relief for both a shallow-lagoon deposi- In the northeast, the outflow tuff is represented by the lowest member tional basin within the caldera and a source of sediment derived from of the Lower Crafnant Tuff Formation, which Howells and others (1973) the rim. Within the caldera, sediment consists of tuffaceous laminated interpreted as a submarine ash-flow tuff. This submarine ash-flow tuff siltstone and immature, coarse-grained, volcaniclastic sandstone con- comprises a complete ash-flow tuff with crystals and lithic clasts concen- taining plane beds, ripple cross-laminations, symmetrical wave ripples, trated at the base and a vitric dust-tuff at the top. Brachiopod shells and and hummocky cross-stratification. Coarse-grained, matrix-sup- siltstone clasts, the latter incorporated while unlithified, are found in the ported, conglomerate layers and layers of ash-flow tuffs are also pres- base of the flow, and the tuff is both underlain and overlain by marine ent. These sediments accumulated in shallow water above fair- siltstone. Minimal erosion or reworking of the top of the tuff suggests that weather wave base. Conglomerate units represent debris flows from the flow was emplaced below wave base in relatively deep water. the caldera rim, a nearby shoreline, and elevated areas associated with Howells and others (1986) indicated that the LRTF caldera also resurgent domes. developed, at least in part, within a shallow-marine environment. In the Sedimentation outside of the caldera consisted of deposition of south, the intracaldera tuff sequence rests on the deeply eroded surface of background suspension and volcanic-ash suspension, and turbidite an older ash-flow tuff, but to the north, it overlies littoral deposits, includ- deposition on a pyroclastic apron. The outer margin of the apron was ing magnetite and ilmenite-rich beach placer, plane-bedded sandstone, dominated by fine-grained suspension and turbidite deposition, pillowed basalt, shallow-marine sandstone, and siltstone (Howells and whereas the inner margin of the apron contains hummocky others, 1986). In the center, the intracaldera ash-flow tuffs transgress a cross-stratification and other evidence of reworking by episodic storm series of northeast-southwest-trending fault scarps from which mass grav- waves. Local highs with associated shallow-water sedimentation ex- ity flows were generated to form megabreccias at the base of the caldera isted outside the caldera. sequence. The caldera collapsed incremently during ash-flow eruption Even though deposited in high-energy marine environments, all with the greatest subsidence, -500 m, in the north. The absence of wide- sedimentary rocks are both texturally and mineralogically very imma- spread erosional breaks in the tuff sequence indicates that tuff ture. This textural immaturity differs from the typical very mature accumulation was almost continuous and that subsidence kept pace with marine sediments and was caused by rapid depositional rates and a deposition. Lenses of reworked tuff low in the sequence, which are of very local volcanic sediment source. local occurrence, contain brachiopods. There is no evidence of the exis- tence of a substantial volcanic edifice (Howells and others, 1986). INTRODUCTION The intrusion and extrusion of rhyolite domes followed the cessation of the main episode of ash-flow volcanism and caldera collapse (Campbell In this report, we examine and discuss the patterns of sedimentation and others, 1987) and was accompanied by resurgence of the caldera fill within and surrounding an Ordovician shallow-water caldera following and the establishment of small islands, particularly in the northern part of the main episode of ash-flow volcanism and caldera collapse. the caldera. A period of rapid deposition of volcaniclastic sediments de- The Lower Rhyolitic Tuff Formation (LRTF) of Ordovician (Cara- rived from the intracaldera tuffs then ensued. doc) age crops out in northern and central Snowdonia, North Wales (Fig. The processes and characteristics of the sediments in the variously 1), and comprises a thick sequence of sedimentary and volcanic megabrec- reworked sedimentary facies of the LRTF are summarized in lithofacies cias, acidic ash-flow tuffs, and volcaniclastic sedimentary rocks (Howells codes (Tables 1 and 2). These codes are modified from both those and others, 1986). The sequence records the collapse, infilling, and subse- proposed for fluvial environments by Miall (1978) and those from ter- quent resurgence of a large asymmetric caldera with an original diameter restrial volcaniclastic environments by Fritz and Harrison (1985) and of — 15 km. A minimum of 60 km3 of ash-flow tuff was erupted during Smith (1987). These codes should be used as a key to the various log

Geological Society of America Bulletin, v. 102, p. 1246-1256, 10 figs., 2 tables, September 1990.

1246

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sections presented in this paper and as a summary of the various processes and environments of deposition that we envisage for the reworked facies oftheLRTF. All rocks in the LRTF have been affected by several episodes of alteration, including hydrothermal alteration and low-grade burial and regional metamorphism (Campbell and others, 1987). Despite this metamorphism, the sedimentary rocks exhibit well-preserved sedimentary structures, especially in the coarser-grained facies. In both the intracaldera and extra-caldera sections, the sediments are predominantly volcanogenic (typically -30% Q, 30% F, and 30% VRF). Even though exact percentages are difficult to determine due to diagenetic alteration and low-grade metamorphism, all sedimentary rocks contain much visible pyroclastic material, including devitrified and recrystallized ash-shards (Fig. 2) and occasional euhedral feldspar and quartz. The grains are often surrounded by a matrix of authigenic and metamorphic clay (including quartz, sericite, and chlorite) that we interpret to have origi- nated from the alteration of unstable volcanic grains. Because of this Figure 2. Photomicrograph of angular tuffaceous sandstone from alteration, we cannot determine the exact percentage of pyroclastic versus the 4.5 m interval of the Moel Siabod measured section. Scale bar is epiclastic material present in the sediments. Because pyroclastic material 1 mm long. generally makes up more than 25% of the grains, however, we refer to them as tuffaceous sediments, following the usage of Fisher and Schmincke (1984). Where pyroclastic material is less than 25%, we refer caldera. The intracaldera tuffaceous sandstones are very poorly sorted, fine to the sediments as sandstones and siltstones, and where pyroclastic grains to coarse grained, with sand-sized angular to subrounded grains; many make up more than 75% of the rock, as tuffs. contain lenses of pebbly sandstone. Conglomerate lenses include pumice and clasts of a welded tuff at the base of the LRTF, showing that the latter INTRACALDERA SEDIMENTARY ROCKS was reworked. Common sedimentary structures in the intracaldera sedimentary Two stratigraphic sections illustrate sedimentation within the caldera. rocks include hummocky and swaley cross-stratification (HCS), wave rip- The first (Fig. 3), high on the east flank of Moel Hebog southwest of ples, trough cross-beds, and plane beds. Many of the plane beds exhibit Beddgelert, is on the southwest margin of the caldera (Fig. 1). The second heavy mineral concentrations and occur in coarse-grained tuffaceous section, Llyn Gwynant, is exposed on the west side of modern lake Llyn sandstone. Gwynant (Fig. 1) and illustrates sedimentation near the center of the Moel Hebog Section

The stratigraphic section on Moel Hebog (located at SH 567 469 on

TABLE 1. LITHOFACIES CODES ASSOCIATED WITH REWORKED SEDIMENTS IN AND AROUND the British National Grid) lies just inside the western margin of the caldera THE LOWER RHYOLITIC TUFF FORMATION IN NORTH WALES (Figs. 1,3) as defined by Howells and others (1986). The reworked facies of the Lower Rhyolitic Tuff Formation is well exposed with -90+ m of Facies Lithofacies Sedimentary Interpretation code structures vertical section. Near its base, large blocks and rafts of the Pitts Head Tuff

Gms Massive, matrix- Nonstratified Debris flow supported gravel

Sm Massive coarse sand Nonstratified, Grain flow, crude horizontal turbidity flow TABLE 2. SEDIMENTARY FACIES. ENVIRONMENTS, PROCESSES, AND LITHOFACIES stratification, (mass flow) ASSOCIATED WITH MARGINAL MARINE VOLCANICLASTIC SEDIMENTATION IN ORDOVICIAN normal grading ROCKS OF THE LOWER RHYOLITIC TUFF FORMATION, NORTH WALES

Sh Sand, medium to Horizontal Upper-flow-regime very coarse laminations plane beds Dominant Common processes Lithofacies codes depositional St Medium to very coarse Solitary or grouped Lower-flow-regime environment grained sand; may trough cross-beds dunes be pebbly with volcanic grains Intracaldera Wave swash, storm waves, Sm, Sh, Shcs, St, oscillation currents, tidal Sr, Fh, T Shcs Medium to very coarse Hummocky cross- Storm waves Beach, tidal currents, debris flow, grained sand with stratification channel, shelf suspension deposition volcanic grains (MH. LG) Sr Medium to very coarse Ripples, cross- Wave ripples grained sand laminations Outside caldera Pyroclastic apron, suspension Gms, Sm, St, Shcs, Water depths deposition, turbidite, Fh, Fla, T Fh Black mudstone Horizontal Suspension storm waves, grain flow, laminations deposition below fair- weather wave base debris flow Fla Tuffaceous mudstone Horizontal Suspension (CI) laminations deposition of volcanic ash Outside caldera Wave swash, wave oscillation Sh, Sr, St, Fla, T T Rhyolitic tuff Massive to laminated Ash-flow tuff; Shallow water currents, fair-weather and (MS) storm waves, upper-flow regime

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MASSIVE MATRIX- DEBRIS FLOW OR DEBRIS FLOW SUPPORTED VOLCANI- SLUMP. INTO LAGOON CLASTIC PEBBLE FROM RIM HIGH. TO BOULDER CONGLOMERATE CLASTS UP TO 30 CM. DIAMETER.

MEDIUM TO COARSE- STORM WAVE SHALLOW GRAINED TUFFACEOUS ACTION WAVE LAGOON SANDSTONE WITH MAS- OSCILLATION ABOVE STORM SIVE MATRIX-SUPPORTED CURRENTS, WAVE BASE TO VOLCANICLASTIC PEBBLE MASS FLOW. FORESHORE CONGLOMERATE LENSES. "BEACH-

O O O o O o ° o °\ TUFFACEOUS SAND- -DEVELOPED ON °o° o°° « °J GmS STONES EXHIBIT TROUGH CALDERA RIM. Figure 3. Measured section of the CROSS-BEDS, HUMMOCKY CROSS-STRATI FITION reworked facies of the Lower Rhyolitic AND PLANE BEDS. Tuff Formation at Moel Hebog illustrating ZONES OF RIPPLE St, Sr intracaldera sedimentation. Right margin 60 ' CROSS-LAMINATED reflects grain size. Base of section located TUFFACEOUS SANDSTONE AND STRAIGHT-CRESTED at SH 567 469 on the British National SYMMETRICAL RIPPLES. Grid. SHELL LAG DEPOSITS.

MASSIVE MATRIX- MASS FLOW AND DEBRIS FLOW Gm SUPPORTED VOLCANI- UPPER FLOW REGIME FROM CALDERA CLASTIC PEB8LE PLANE BEDS. RIM HIGH INTO AND COBBLE SHALLOW CONGLOMERATE LAGOON. 40 AND PLANE BEDDED Gm TUFFACEOUS SANDSTONE.

HORIZONTAL PLANE UPPER FLOW REGIME FORESHORE BEDS OF MEDIUM- PUNE BEDS AND PLANE BEDS Sh GRAINED TUFFACEOUS WAVE PROCESSES. AND SHOREFACE 30 SANDSTONE, TROUGH WAVE WORKED CROSS BEDDED TUFFACEOUS AND HUMMOCKY SANDS ABOVE CROSS-STRATIFIED FAIR WEATHER TUFFACEOUS SAND- WAVE BASE. STONE. Sh

20 WATER DEPTHS BELOW FAIR Shcs WEATHER AND ABOVE STORM St WAVE BASE.

HORIZONTAL LAMINAE ASH-FLOW AND SUSPENSION OF SILTSTONE AND SUSPENSION DEPOSITION T,Fh,Ra TUFFACEOUS SILTSTONE DEPOSITION OF BELOW STORM IN 10-50 CM. BEDS. ASH. WAVE BASE. Sm OCCASIONAL MASSIVE BEDS OF FINE- T.FIa GRAINED SANDSTONE.

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Figure 4. Plane beds (Sh lithofacies) in medium-grained tuffaceous sandstone on Moel Hebog.

within the LRTF presumably slid as large slump blocks from the rim of the very coarse-grained tuffaceous sandstone (0.8-1.0 mm). These are very caldera (Reedman and others, 1987). Low in the section, horizontally straight-crested ripples with no sinuosity or bifurcations observed along bedded, fine-grained tuffs in 20- to 50-cm-thick beds within tuffaceous 2 m of exposed crest length. These straight-crested ripples represent wave siltstone are very well indurated and may represent either small pyroclastic oscillation in shoreface depth water. The HCS observed within the section flow deposits or highly cemented ash layers deposited from suspension. probably represents deposition and reworking of ash and sand during Structureless to horizontally bedded tuffaceous sandstones are also in- episodic storms in shelf-depth water. cluded within the tuffaceous siltstone layers. Approximately 10-20 m Throughout the Moel Hebog Section, many contorted beds and soft- above the base of the section, these grade upward into horizontally lami- sediment deformational structures were observed. These are interpreted as nated tuffaceous sandstones in plane beds less than 1 cm thick (Fig. 4). having resulted from instability within the caldera and from rapid sedi- These plane beds are interpreted to represent upper-flow regime plane mentation rates. The section reflects a combination of ash flows and beds characteristic of wave swash in very shallow water. Alternatively, turbidity-type mass flow at the base that were then reworked by high- they may represent plane beds in a tempestite deposit caused by storm energy, upper-flow regime or tempestite storm currents in shallow water, processes in a shallow lagoon as described by Aigner (1985). The basal possibly on the foreshore of beaches along the rim of the caldera. Even part of the section (12-35 m interval, Fig. 3) also contains numerous zones though exposures do not permit the observation of shallow dipping beds, of HCS sandstone that increase in occurrence toward the top of the unit. numerous low-angle truncation surfaces, plane beds, and heavy mineral These form from storm waves and suggest that water depth was decreasing concentrations suggest that some of the plane beds indicate deposition in for this interval. the swash zone of a beach. Because of the high sedimentation rates and Overlying the plane-bedded and hummocky cross-stratified sand- local source of pyroclastic material, the beach sand remained texturally stone, there is a thick sequence of massive, tuffaceous, matrix-supported, and compositionally very immature. polymictic pebble and cobble conglomerate (35-48 m interval, Fig. 3). As instability in the caldera continued, coarse-grained debris (sand The bases of the conglomerate units undulate, indicating both erosion and and pebble- and cobble-sized material) was shed as mass flow (debris flow loading into underlying beds. The tops of the conglomerate beds are or high-density turbidity flow) into the lagoon formed by the caldera mostly flat if overlain by sandstone beds and irregular where eroded by the represented by the matrix supported conglomerate near the top of the base of overlying conglomerate. These conglomerates probably represent section. At no point, however, was the water inside this portion of the debris flows or high-concentration turbidity flows of volcaniclastic mate- caldera deeper than wave base, as shown by the numerous wave-formed rial derived from a shoreline close to the western edge of the caldera. ripples and hummocky and swaley cross-stratification throughout the sec- Above the massive, debris-flow conglomerates to near the top of the tion. Thus, subsidence/instability within the caldera and sedimentation section, volcaniclastic sandstones are interbedded with matrix-supported, were in balance. cobble and boulder conglomerate beds and lenses. Sedimentary structures are well exposed, and these include small-scale, trough cross-stratification; Llyn Gwynant herringbone cross-beds; horizontal laminations; plane beds; and low-angle (< 10°), hummocky cross-stratification. Near the 60-m interval (Fig. 3), an A 38-m-thick section of reworked sediments and ash-flow tuffs along exposed bedding plane displays symmetrical wave oscillation ripples in the western shore of Llyn Gwynant (SH 643 522 on the British National

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BIOTURBATED TUFFA- UPPER FLOW REGIME LOWER FORESHORE/ CEOUS SANDSTONE WITH PLANE BEDS, TIDAL UPPER SHOREFACE WORM BURROWS,SHELL CURRENTS, WAVE ABOVE FAIRWEATHER FRAGMENTS AND SWASH, WAVES WAVE BASE. REMNANT PLANE BEDS. OSCILLATORY CURRENTS.

HERRINGBONE CROSS- BEDS, PLANE BEDS, TROUGH CROSS-BEDS AND HUMMOCKY CROSS- STRATI FICTION.

MASSIVE SANDSTONE. MASS FLOW Figure 5. Measured section illustrating facies characteristic of shallow- RHYOLITIC TUFF. ASH-FLOW. ERUPTION OF 'I -J ^ ^ 1/ ASH AND water and high-energy environments EMPLACEMENT within the Lower Rhyolitic Tuff For- OF ASH-FLOW 25- M, ^ mation caldera at Llyn Gwynant. Right INTO SHALLOW i LAGOON. margin reflects grain size. Base of sec- /v i tion located at SH 643 522 on the Brit- ish National Grid. A

20- 1 v

w v-

FINE TO COARSE- MASS FLOW, UPPER SHALLOW WATER GRAINED TUFFACEOUS FLOW REGIME PLANE LAGOON ABOVE Sm SANDSTONE WITH LENSES BEDS, SUSPENSION FAIR-WEATHER OF LAPILLI TUFFACEOUS DEPOSITION, INFLUX WAVE BASE. SANDSTONE AND LAYERS OF VOLANIC ASH, Gm OF ASH. ALTERNATING WAVE GENERATED LAYERS OF PLANE CURRENTS. Fh, Sh BEDDED TUFFACEOUS SANDSTONE, HUMMOCKY CROSS-STRATIFIED TUFFACEOUS SANDSTONE AND LAMINATED Shcs PERIODIC QUIET TUFFACEOUS SILTSTONE. PERIODS OF IMTERVALS OF HIGHLY T SUSPENSION SILICIFIED TUFFS, DEPOSTION AND BEDDED TUFF, TROUGH St, Sh TIMES OF CROSS-BEDS AND MASS FLOW MASSIVE TUFFACEOUS TEMPESTITE T SANDSTONE BEDS. SEQUENCE Sh BELOW FAIR- St WEATHER AND ABOVE STORM WAVE BASE.

Fh FINE-GRAINED SUSPENSION LAGOON BELOW TUFFACEOUS MUDSTONE. DEPOSITION WAVE BASE.

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Shcs

Sm 90 — « COARSE-GRAINED MASS FLOW (GRAIN PYROCLASTIC TUFFACEOUS SAND- FLOW AND TURBIDITY APRON MOSTLY STONE IN MASSIVE FLOW PROCESSES), ABOVE STORM Shcs 10-50 CM THICK SUSPENSION WAVE BASE AND TABULAR BEDS, BASES DEPOSITION, BELOW FAIR FLAT TO SCOURED, REWORKED BY WEATHER WAVE INTERVALS OF GRADED STORM WAVES. BASE. TOP OF AND MASSIVE SECTION MAY TUFFACEOUS SANDSTONE INDICATE A BEDS OCCASIONAL PLANE SHALLOWING OF BEDS. INTERVALS OF WATER TO ABOVE HUMMOCKY CROSS- FAIR WEATHER 70 STRATIFIED TUFFACEOUS WAVE BASE. SANDSTONE. Figure 6. Measured section illustrating sedimentation on a pyroclastic apron below fair-weather wave base

60 outside of the Lower Rhyolitic Tuff Formation caldera at Cwra Idwal. Shcs Right margin reflects grain size. Base of section located at SH 645 589 on the British National Grid.

50 —

. Q o . to" -or. - - 0.^0 -p. i

40 -

FINE-GRAINED WHITE TUFFACEOUS SILTSTONE SUSPENSION OUTER MARGIN WITH IRREGULAR DEPOSITION OF A PYROCLASTIC 20 WHISPS OF BLACK OF VOLCANIC ASH APRON DEEP WATER MUDSTONE, HIGHLY AND BACKGROUND. MOSTLY BELOW SILICEOUS, TUFFACEOUS RE MOBILIZATION STORM WAVE BASE, SILTSTONE OFTEN OF SEDIMENTS BY OCCASIONALLY OCCURS IN GRADED THIN STORM WAVES. AFFECTED BY BEDS AND LAMINAE. BEDS A FINE GRAINED STORM WAVES. OF URGE SECONDARY DISTAL TURBIDITY 10 —I CONCRETIONS. INTERVALS FLOWS. OF HUMMOCKY CROSS- STRATIFIED TUFFACEOUS SANDSTONE BEDS _1.-J£LCM.JH1CK. BLACK MUDSTONE, SUSPENSION DEEP WATER HORIZ. LAM. DEPOSITION OF BELOW STORM NON-VOLCANIC WAVE BASE. SILT AND CLAY.

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Grid) illustrates a slightly different type of intracaldera sedimentation mudstone represents background, non-volcanic-influenced sedimentation, (Figs. 1, 5). This section consists of coarse-grained tuffaceous sandstone in water below even storm-wave base, on the extreme distal portions of a and lesser amounts of horizontally laminated tuffaceous mudstone and pyroclastic apron outside the caldera. Overlying the black mudstone, there fine-grained tuffaceous sandstone that occur mostly near the base of the is a fine-grained white laminated tuffaceous siltstone with wispy layers of section. Sedimentary structures in the tuffaceous sandstone include dune black mudstone which reflects the initial incursion of fine-grained volcanic trough cross-stratification, wave oscillation ripples, plane beds, and hum- ash into the depositional environment. Near the top of this unit (-12-20 mocky cross-stratification. m), zones of undulating hummocky and swaley cross-stratification indicate Near the base of the section, isolated pods as much as 1 m in diameter that at this time the environment had shallowed to above storm-wave base, and thin continuous layers of highly indurated tuff occur. The pods appear possibly on the order of -100 m of water depth. Large secondary carbon- to be secondary concretions. Because concretions generally represent very ate concretions representing an early cement are included in this unit. early cement, the bending of the laminae around the pods represents the The upper 75 m of the Cwm Idwal section is of coarse-grained amount of compaction in these sedimentary rocks during burial and dia- volcaniclastic sandstone with interbeds of fine-grained tuffaceous siltstone. genesis much in the manner as described by Raiswell (1971). Some of the The sandstone layers include massive, 10- to 50-cm-thick graded beds with layers and pods, however, appear as possible detached lobes and load casts flat to scoured bases and plane beds near the top. These may represent (ball-and-pillow structures), because sedimentary layers below the tuffs are turbidity-current deposition on the pyroclastic apron. The tops of some of often contorted and disrupted. Tuffaceous sandstone beds in this lower the graded sandstone beds have been reworked into hummocks and swales part of the section are contorted; they also contain convolute beds, flame with a height of 5-30 cm and a wavelength of more than a meter. Trunca- structures, and small clastic dikes that were probably formed by soft- tion angles are less than 10° and no current direction can be determined. sediment deformation caused by very rapid depositional rates. This hummocky cross-stratification formed by episodic reworking by A 12.5-m-thick ash-flow tuff 15 m above the base of the section is storm waves, and they resemble the tempestite sequence described by overlain by a very coarse grained, angular, tuffaceous, lithic sandstone with Aigner (1985) as resulting from storm waves. This upper part of the Cwm hummocky cross-stratification, horizontal laminations, trough cross- Idwal section formed as the pyroclastic apron prograded, and it represents stratification, herringbone cross-beds, and many reactivation surfaces. Tuf- deposition of the proximal to middle part of the apron that was at least faceous sandstone layers at the top 3 m of the section are extensively above storm-wave base. bioturbated in zones, contain shell debris, burrows, and relict plane beds. The Cwm Idwal section represents a pyroclastic apron that was fed The Llyn Gwynant section represents fluctuating water depths and an from the caldera margin. It is finer grained than the intracaldera sequences, over-all decrease in depth from low-energy tuffaceous siltstone at the base and it contains a lower diversity of sedimentary structures and interbedded to overlying higher energy deposits. Most of the section, however, repre- ash-flow tuffs. Background sedimentation was of a fine-grained, black silt sents sedimentation in shallow water as shown by the herringbone that was quickly overwhelmed at the onset of reworking of the intracal- cross-beds and reactivation surfaces characteristic of tidal processes. The dera sequence following resurgence of the caldera 10-12 km to the south. plane-bedded and bioturbated sandstone layers with associated shell lags The reworked LRTF facies in Cwm Idwal is overlain by a siltstone formed by upper-flow regime and were generated by wave swash on the sequence again reflecting the local background sedimentation. These silt- foreshore of a beach. Shallow water above fair-weather wave-base is also stone layers are in turn overlain by a rhyolitic lava, as much as 75 m thick, indicated by the numerous instances of wave-ripple cross-stratification. at the edge of a thicker dome to the south. The lava is in turn overlain by The hummocky cross-stratification reflects the occurrence of storm waves 35 m of plane- and cross-laminated, wave-rippled, rhyolitic tuffaceous and deeper water than that over the planes beds. The sequence possibly sandstone. These tuffaceous sandstones are compositionally comparable, accumulated on a topographic high above a rhyolitic resurgent dome due to a characteristic trace-element signature of as much as 1,780 ppm Zr, within the caldera. to the underlying rhyolite lava (Campbell and others, 1987).

EXTRACALDERA SEDIMENTARY ROCKS Moel Siabod

Both the Cwm Idwal and Moel Siabod sections (Fig. 1) lie outside The Moel Siabod section (SH 718 537 on the British National Grid) the caldera margin (Howells and others, 1986) and are of markedly con- reflects a shallow-water environment that existed outside of the caldera trasting sedimentological character. Composition and textural maturity of margin (Figs. 1, 7). The 20-m-thick section is composed of very coarse the grains are similar to those of the intracaldera sedimentary rocks and, grained, poorly sorted, volcaniclastic sandstone with angular grains. The except for near the base of the Cwm Idwal section, are tuffaceous sandstone layers are interbedded with beds of fine-grained, silicified tuff sediments. containing unreworked ash shards (Fig. 8). The bases of the sandstone units commonly erode into these tuffs. The tuffs contain a high proportion Cwm Idwal of air-fall volcanic ash, which, because it was very cohesive, resisted resus- pension in the high-energy conditions that scoured and deposited the sand. The main phase of LRTF primary outflow tuff in Cwm Idwal (SH The sandstone and tuffaceous sandstone layers contain plane beds; low- 645 589 on the British National Grid) is overlain by an ~100-m-thick angle, hummocky, cross-stratification; and small-scale, wave-ripple, cross- sequence of reworked tuffaceous sediments which represent turbidity- lamination (micro hummocky cross-stratification) (Fig. 9). The entire current deposition on a pyroclastic apron (Figs. 1, 6). This interpretation section reflects high-energy deposition in shallow water above normal was also suggested by Orton (1987). The top of the primary tuffs is wave base (possibly on the order of less than 9-18 m) and the plane- marked by a 2-m-thick, laminated, black mudstone with a strong tectonic bedded sandstones may represent upper-flow regime wave swash on the cleavage. The absence of sedimentary structures other than parallel lami- foreshore of a beach. Beach-type, or at least high-energy, deposition sug- nations, and its very low volcanic ash content suggest that the black gests accumulation on a topographic high outside the caldera. The entire

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LITHOFACIES DESCRIPTION PROCESSES ENVIRONMENT

VERY COARSE-GRAINED SUSPENSION SHALLOW TUFFACEOUS SANDSTONE DEPOSITION WATER ABOVE WITH HUMMOCKY CROSS- OF ASH AND SILT. FAIR WEATHER STRATIFICATION AND UPPER FLOW REGIME WAVE BASE, PLANE BEDS. SILICEOUS PLANE BEDS, FORESHORE TUFF IN 3-19 CM BEDS WAVE OSCILLATORY "BEACH" Figure 7. Measured section illus- WITH FLAT BASES CURRENTS. ZONE. trating shallow-water and high-energy AND SCOURED conditions outside the caldera at Moel TOPS. SCOURS FILLED Siabod. Right margin of column re- WITH MICRO HUMMOCKY CROSS-STRATIFIED flects weathering and degree of cemen- TUFFACEOUS SANDSTONE tation. Base of section located at SH TUFFACEOUS SILTSTONE 718 537 on the British National Grid. BEDS CONTAIN INTERNAL PLANE LAMINATIONS.

reworked facies of the LRTF is very thin here (on the order of 20 m), but in the reworked fades indicates depositional processes ranging from below its original thickness is not known due to restricted outcrops in the core of storm-wave base to high-energy environments above fair-weather wave the synclinal outlier. base. Specific environments of deposition may include small, shallow lagoons with associated surrounding beaches to suspension and turbidity- SUMMARY OF ENVIRONMENTS OF DEPOSITION current deposition on a pyroclastic apron of debris shed from the caldera rim. Locally, the rim of the caldera and resurgent domes within the caldera The primary ash-flow tuffs of the LRTF were reworked in a shallow- existed in very shallow water and were probably subaerially exposed. marine environment (Fig. 10). Interpretation of the sedimentary structures Small beaches were established around these highs. Sedimentation within the caldera consisted of suspension deposition of air-fall ash, pyroclastic flows, and mass flows (debris flows and turbidity currents) from sediment shed into the lagoon from the caldera rim and probably a shoreline close to the western edge. These deposits were reworked and remobilized by storm waves, normal "fair-weather" wave oscillation currents, and tidal currents. Facies within the lagoon are reminiscent of the shoreface to offshore transition described by Howard and Reineck (1981) and Bourgeois (1980). If this interpretation is correct, water depths varied from intertidal close to the rim and resurgent dome highs to ~20 m in the lagoon. Outside the caldera, a large pyroclastic apron developed from heads on the north side of the outer caldera rim. These aprons extended north-

Figure 8. Backscatter SEM image of white, fine-grained tuff at Moel Siabod shown in Figure 8. This image shows angular glass shards and a possible collapsed bubble in the center of the photo. These angular shards show little evidence of sedimentary transport and may indicate direct air-fall ash deposits. Image from polished, carbon-coated thin section using JEOL JSM-840 SEM. Beam conditions and scale on image.

Figure 9. Horizontal plane beds and wave-ripple cross-lamination (micro-hummocky cross-stratification) at Moel Siabod. White layers of tuffaceous siltstone at top of photo.

ward into deeper water, to below storm-wave base; their construction was existed in very shallow water. Characteristically these shallow-water sec- controlled by turbidity current deposition of reworked pyroclastics, back- tions contain wave-ripple cross-lamination, small scoured surfaces, upper- ground suspension deposition, and the suspension deposition of fine- flow-regime plane beds, and hummocky cross-stratification. These grained rhyolitic ash. The aprons prograded through time as seen in the sediments accumulated in foreshore- to shoreface-depth water over locally section at Cwm Idwal where fine-grained, suspension-dominated deposits developed highs. Our model of the environments of deposition is summa- grade upward into zones of storm-reworked sandstone layers. This inter- rized in the cross section shown in Figure 10. pretation suggests very approximate depths on the order of 30 to 100 m Unlike sediments deposited on beaches and reworked by waves in or more near the base. shallow high-energy lagoons, the LRTF sandstones are texturally and In contrast to the pyroclastic apron, other areas outside of the caldera compositionally very immature with a high percentage of volcanic and

PRIMARY TUFF REWORKED TUFF RHYOLITE LAVA FLOW OR DOME

PRE-CALDERA SEDIMENTARY PRE-CALDERA ASH-FLOW TUFF ROCKS (PITTS HEAD TUFF)

Figure 10. Paleogeographic reconstruction of depositional environments in and around the Lower Rhyolitic Tuff Formation caldera during the time of reworking of the primary LRTF volcanics. A, Cwm Idwal; C, Llyn Gwynant; D, Moel Hebog. Ash-flow cloud illustrates the eruption of an ash flow from a vent along the caldera rim.

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feldspar grains and a very angular texture. Values for a Mineralogical terpreted as having resulted from tectonism or climatic changes (periods of Maturity Index as defined by McBride and Picard (1987) average ~43, aridity). The example from the LRTF presented in this paper also suggests typical of terrestrial alluvial sediments. This textural and compositional that explosive volcanism produced major alteration in the sedimentation immaturity resulted from the rapid sediment influx from the nearby vol- patterns in the Ordovician Welsh marginal marine basins. Without the canic source area together with rapid subsidence of both the caldera basin influence of volcanism and the topographic features associated with cal- and basins devolving around the caldera, which inhibited them from ex- dera development, the sedimentary patterns throughout this region may tensive reworking. Thus, sediments in a marine volcaniclastic environment have been similar to the suspension sedimentation of the black mudstone may have maturity characteristics similar to terrigenous and alluvial sedi- at the base of the Cwm Idwal section. Thus, the entire area would have ments. This difference is very important to note when reconstructing vol- been one of low relief, below storm-wave base, and with no source for caniclastic environments, and it illustrates that caution should be used in influx of coarse-grained sediment. applying standard fades models to volcaniclastic deposits. The onset of rhyolitic explosive volcanism profoundly altered the geometry of the marginal basin and the patterns of sedimentation by PALEOECOLOGY produdng large topographic highs of the main caldera and a depositional lagoon-type basin that formed by caldera collapse. Resurgence within the Shallow-marine faunas of Longvillian (Caradoc) age occur within the caldera created local shallow-water conditions. Not only did volcanism reworked facies of the LRTF at many localities both within and outside significantly alter the geometry of the depositional basin, but it provided an the caldera and are useful for interpreting environments of deposition. The abundant source of unconsolidated sediment directly as ash flows, and faunas are dominated by brachiopods and trilobites with associated echi- intermittently as ash-suspension deposition. These volcaniclastic deposits noderm (crinoid and cystoid) fragments; bryzoa (cushion and stick types); were also readily available for reworking due to active and rapid uplift, occasional tentaculitids, gastropods, and ostracods; and solitary corals. presumably before thorough lithification could take place. As seen in the Generally, the faunas are of low diversity; low articulation ratios of the Cwm Idwal section, this effect on the sedimentation patterns could be faunal elements indicate that few, if any, are in life position. Even though reasonably expected to exist for at least tens of kilometers away from the they are not in situ, these shells do not indicate extensive reworking and caldera margin. were derived locally. Thus, they reflect paleoenvironments in and around the caldera system. ACKNOWLEDGMENTS In terms of the recent environmental interpretations of Caradoc faunas in Wales (Pickerill and Brenchley, 1979; Lockley, 1980), two distinct Cathy Busby-Spera, R. Michael Easton, and Roger Suthren provided faunal associations are represented within the caldera and particularly in numerous suggestions to improve both content and readability of the the vicinity of Moel Hebog. One association, dominated by the brachio- manuscript. We thank Roger Suthren and Anton Kearsley for assistance pod Dinorthis berwynensis, is considered to have colonized shifting coarse- with SEM images. This paper is published with the permission of the sand substrate in high-energy, nonturbid, well-oxygenated environments in Director, British Geological Survey. It results from work undertaken as very shallow conditions. This environment is consistent with the environ- part of the multidisciplinary Snowdonia Regional Geological Survey. mental interpretations based on the sedimentary structures on Moel Hebog. REFERENCES CITED The other association, containing the brachiopods Nicolella actonial Aigner, T., 1985, Storm depositional systems—Dynamic stratigraphy in modern and ancient shallow-marine sequences: obesa and its associates Howellites antiquior and Dolerorthis sp., occurs Lecture Notes in Earth Sciences, Volume 3: Berlin, West Germany, Springer-Verlag, 174 p. Bourgeois, J., 1980, A transgressive shelf sequence exhibiting hummocky cross-stratification: The Cape Sebastian Sand- locally within the caldera (for example, north of Moel Hebog). A compar- stone (Upper Cretaceous), SW Oregon: Journal of Sedimentary Petrology, v. SO, p. 681-702. Campbell, S.D.G., Reedman, A. J., Howells, M. F., and Mann, A. C., 1987, The emplacement of geochemically distinct able fauna with a significantly more diverse trilobite fauna (Dean, 1965), groups of rhyolites during the evolution of the Lower Rhyolitic Tuff Formation caldera (Ordovician), North including Brongniartella cf. bisulcata, Atractopyge celtica, and others, Wales, U.K.: Geological Magazine, v. 124, p. 501-511. Dean, W. T., 1965, A shelly fauna from the Snowdon Volcanic Series at Twl Ddu, Caernarvonshire: Geological Journal, occurs to the north of the caldera, and stratigraphically toward the top of v. 4, p. 301-314. Fisher, R. V., and Schmincke, H.-U., 1984, Pyroclastic rocks: New York, Springer-Verlag, 472 p. the LRTF near its upper contact with the overlying Bedded Pyroclastic Fritz, W. J., and Harrison, S., 1985, Early Tertiary volcaniclastic deposits of the northern Rocky Mountains, in Flores, Formation. The Nicolella association has been interpreted (Pickerill and R. M., and Kaplan, S. S., eds., Cenozoic paleogeography of the west-central United States: Society of Economic Paleontologists and Mineralogists Rocky Mountain Section Symposium, v. 3, p. 383-402. Brenchley, 1979) as having inhabited a variety of substrates but having Howard, J. D., and Reineck, H. E., 1981, Depositional facies of high energy beach to offshore sequence: Comparison with low energy sequence: American Association of Petroleum Geologists Bulletin, v. 65, p. 807-830. developed most typically on calcareous silt and fine-grained sand. The Howells, M. F., Leveridge, B. E., and Evans, C.D.R., 1973, Ordovician ash-flow tuffs in eastern Snowdonia: Report of the faunas from the LRTF occur in such sediments, and the facies analysis Institute of Geological Sciences, 73/3, p. 1-33. Howells, M. F., Reedman, A. J., and Campbell, S.D.G., 1986, The submarine eruption and emplacement of the Lower described above suggests low-energy and low-sedimentation rates in water Rhyolitic Tuff Formation (Ordovician), N Wales: Geological Society of London Journal, v. 143, p. 411-423. depth of the order of 30 m or greater. Lockley, M. G., 1980, The Caradoc faunal associations of the area between Bala and Dinas Mawddwy, North Wales: British Museum of Natural History (Geology) Bulletin, v. 33, p. 165-235. McBride, E. F., and Picard, M. D., 1987, Downstream changes in sand composition, roundness, and gravel size in a short-headed, high gradient stream, northeastern Italy: Journal of Sedimentary Petrology, v. 57, p. 1018-1026. VOLCANIC CONTROL OF SEDIMENTATION Miall, A. D., 1978, Lithofacies types and vertical profile models in braided river deposits: A summary, in Miall, A. D., ed.. Fluvial sedimentology: Canadian Society of Petroleum Geologists Memoir 5, p. 597-604. Orton, G., 1987, Discussion on the submarine eruption and emplacement of the Lower Rhyolitic TulT Formation (Ordovidan), North Wales Journal, v. 143, p. 411-424: Geological Society of London Journal, v. 144, Facies analysis of the reworked volcaniclastic sediments of the Ordo- p. 523-525. vician Lower Rhyolitic Tuff Formation in North Wales provides a model Pickerill, R. K-, and Brenchley, P. J., 1979, Caradoc marine benthic communities of the south Berwyn Hills, North Wales: Palaeontology, v. 22, p. 229-264. for examining the type of control that explosive rhyolitic volcanism places Raiswell, R., 1971, The growth of Cambrian and Liassic concretions: Sedimentology, v. 17, p. 147-171. Reedman, A. J., Howells, M. F„ Orton, G, and Campbell. S.D.G., 1987, The Pitts Head Tuff Formation: A subaerial to on major environments of deposition. The control of volcanism on sedi- submarine welded ash-flow tuff of Ordovician age. North Wales: Geological Magazine, v. 124, p. 427-439. mentation patterns in terrestrial environments has been discussed by var- Smith, G. A., 1987, The influence of explosive volcanism on fluvial sedimentation: The Deschutes Formation (Neogene) in central Oregon: Journal of Sedimentary Petrology, v. 57, p. 613-629. ious authors (for example, Fritz and Harrison, 1985; Smith, 1987) and was Smith, G. A., and Fritz, W. J., 1989, Penrose Conference report on "Volcanic influences on terrestrial sedimentation": the topic of a 1988 Geological Society of America Penrose Conference Geology, v. 17, p. 375-376. (Smith and Fritz, 1989). These works suggest that rapid influx of sediment MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 9, 1989 REVISED MANUSCRIPT RECEIVED JANUARY 18,1990 from explosive volcanism produce sedimentation patterns previously in- MANUSCRIPT ACCEPTED JANUARY 25, 1990

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